ImageMagick v6 Examples --
Masks

Index

ImageMagick Examples Preface and Index

Alpha (Matte) Channel

• Internal Matte Channel
• Alpha Channel Operator

  Off  (or +matte),  Set  (or -matte),  On, 
  Opaque,  Transparent,  Extract,  Copy, 
  Shape,  Remove,  Background
  

• Remove Transparency of of an Image
• Outline or Halo Transparency

Using Masks with Images

• Editing a Image Mask
• Masks as Colored Shapes
• Mathematical Composition
• Masked Alpha Composition
• Aligning Two Masked Images (under construction)

Clipping Masks (under construction)

Regions and Region Sub-Images

• Warping a Local Region
• How Regions Work, and its Problems

Background Removal

• Simple Backgrounds (floodfill)
• Masking Bordered Objects
• Removing a Known Background
• Difference Image Masking and Feathering
• Recovering Semi-Transparent Edges
• Background Removal using Two Backgrounds

Hole_Filling (under construction)

In these examples we look at the special handling of transparency, the transparency channel, using masks, and ultimatally the removal of unwanted backgrounds, or other elements, such as signs, text, spam.

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Alpha (matte) Channel

The transparency (alpha) channel of an image is completely optional, and often requires special handling separate to the normal 'color' channels. See Image Color Space above.

The existance of a transparency channel can also effect how the various operators treat the other color channels, generally because a fully-transparent color should often be completely ignored by an operation. If this was not the case you get 'Black Halos' around images, such as was seen in major IM Bugs in the early days of IM v6. For example the Resize Halo Bug, and the Blur with Transparency Bug.

To make matters worse, this channel is also sometimes referred to at an image's 'transparency' or 'opacity' channel, or even the image's 'mask'. All however refer to the same, special, fourth channel of the image.

To explain the difference we need a working example image and for this I'll use a PNG image of a 'crescent moon' image (from a CopyOpacity Composition example). Now as you can see this image has a lot of areas which are fully transparent. Not only that I needed to save the image using the 'PNG' image format which is one of the small number of image formats that properly understands and handles transparent and semi-transparent colors.

I can demonstrate this transparency by overlaying the image onto the IM built-in checkerboard pattern, using Alpha Composition. composite -compose Dst_Over -tile pattern:checkerboard \ moon.png moon_background.jpg

Internal Matte Channel

Now internally IM saves the transparency information in a 'matte' channel, which just like the color channel is just a plain grey scale image of values which range from white, for fully-transparent (or clear), to black for fully-opaque. It is sort of like what you would get if you look at a silhouette of the original image.

Here is the normal way to 'separate' the matte layer from the image. convert moon.png -channel matte -separate moon_matte.png

You can also use "channel" options 'A' or 'alpha' the result will be the same, an extraction of the internal 'matte' channel of the image. As you more typically want a 'alpha' channel, this is not the recommended way of extracting an 'alpha channel'. See Alpha Extract below.

There is another way of other extracting the internal matte channel too, typically by saving the channel as a special 'matte' image file format.

# Here is a IM version 5 way of extracting the matte (or other) channel # Note how this required two separate steps, and commands. convert moon.png matte:moon.matte convert MIFF:moon.matte moon_matte2.png # You can join those two steps in a pipeline as well... convert moon.png matte:- | convert - moon_matte3.png

This technique for extracting the 'matte' of an image was common when IM v5 was in use. Basically it was the the only method provided to get access to the transparency of an image. It is now very rarely used.

Now while the image transparency data is stored internally as 'matte' values, almost all the operators will deal with transparency as 'alpha'. As such on very low level operators such as "-level" and "-threshold" actually handle the data as 'matte' rather than alpha. Check the Official Option Reference if you are unsure.

Controlling Image Transparency

There are two operators that give you low-level control of the transparency channel of an image in memory. The newer operator "-alpha" methods are now the recommended method of control, though many IM Examples still show and use the older and now obsolete "-matte" operator.

An image can not only have alpha channel data, but it also has a 'switch' that defines if the channel data is viewable or valid. This means images can have three states with regards to the alpha channel.

Switch		Channel Data
alpha off		no alpha data (no memory has been allocated)
alpha off		old alpha data present (but not in use)
alpha on		alpha data that is currently in use

This needs to be remembered as how the various methods behave depends on which of the above three states the image was in.

If the 'switch' is off operators will not touch the alpha data, as it may not actually exist at all. In such a case old alpha could still be present, unmodified, and thus out-of-date. As you will see this is actually sometime useful in some situations.

Note however that some operators may automatically turn on, or turn off the alpha switch for one reason or another.

For example, "-compose CopyOpacity -composite" will always turn on the alpha channel in the resulting image, as the operators job is to actually copy data into an alpha channel. as such it must exist in the final result. However its existance in the input data can have other consequences. See Copy_Opacity Composition Method for more details.

Similarly creating a canvas using the color 'None' will also automatically create and enable the transparency channel, so as to ensure the blank image really is transparent. On the other hand, creating a canvas using some other Color Name will generally not create any transparency channel as images are opaque by default.

Here are the various "-alpha" methods and examples of how they effect images and there transparency.

Alpha Off or "+matte"

This is just a simple switch on the image, which turns off any effect the transparency has on the image. It does not actually delete or destory the alpha channel attached to the image, it just turns off any effect that channel has on the image. Similarly no operator will effect the attached alpha channel while it has been turned off.

For example lets the 'crescent moon' image (from a CopyOpacity Composition example), and simply turn the image alpha channel off.

Note that the moon shape completely vanished when the transparency was turned off, though that is actually rarely the case. Basically even the 'transparent' areas have color, which is just not normally visible, in this case the hidden color was the fractal canvas image that was used to create the moon image.

This hidden color could be anything, from a simple GIF Transparency Color, that the GIF format uses to represent transparency in its color table, to garbage colors left behind during the images creation, as above. More typically the transparency color is simply pure-black for any pixel that was fully-transparent. Note that pixels close to the edge may be semi-transparent, and thus still have a valid color that is only partially visible.

Note that while the transparency channel has been 'deactivated' or 'turned off' like a simple switch, the data itself has not been cleared or removed from the image data stored in-memory. It is still present, just unavailable at this time.

Be warned that saving the image with the transparency data turned off, will not save any transparency data to the image file format. In such a case the turned-off alpha data will then be lost.

Also as many file formats do not allow transparency (such as JPEG), these file formats automatically do the equivalent of a "-alpha Off" when the image is saved. Generaly resulting in all transparent areas turning unexpectantally, black. See Alpha Remove - Removing Transparency below for the corect way to remove transparency before saving to a JPEG file format.

The "+matte" operator is an older command that is exactly the same as "-alpha Off". That is it just turns off the transparency channel.

Note that this method is often required when using a gray-scale mask image with the CopyOpacity Alpha composition method, so it copies the grayscale colors, rather than some existing alpha or opacity channel.

Alpha Set or "-matte"

The 'Set' alpha method is the same as the older "-matte" option.

This ensures that the image has a 'transparency' or alpha/matte channel, but if it was not present or turned off, that it be initialised to be fully-opaque (See the Alpha Opaque method below).

However if the image already has an alpha channel present and enabled, it will do nothing.

In other words this operator ensures an alpha channel is present, without modifying the look of the image in memory, as it was before the option was run. As such on its own this operator does not show any change to the image, but has real effects when combined with other operators.

So if you turn off the alpha channel using Alpha Off, and then enable it again using Alpha Set, the image will have an alpha channel, but it will be fully-opaque, just as the image looked, before 'Set' operation was requested.

convert moon.png -alpha off -alpha set alpha_set.png

If applied to an image that has an enabled alpha channel, no change is made.

convert moon.png -alpha set alpha_noset.png

In summery, this operator should never change the appearance of the image, at the time the operator is applied. It just ensures the alpha channel set up such that the image is left as it was.

This is typically used after reading images of an unknown image file format or source, which may or may not have an alpha channel present. This operator will then ensure that the image does have an alpha channel (for image formats like JPEG), but leaving any enabled and existing alpha channel alone (such as for GIF or PNG formats).

This is the recommended way of ensuring an image has an alpha channel after reading it into memory, or more importantly, after an image has been processed and you want to re-enable a clean alpha channel.

Alpha On

The "-alpha On" is the exact opposite to the previously looked at Alpha Off method. Typically this is to simplistic for the purpose you are wanting and as such should be very RARELY used. You should use "-alpha Set" in almost all cases.

Basically 'On' method just flips the switch so that the image transparency data is visible again. Any existing transparency data is not modified, so if the in-memory image still has some old alpha channel data, that data will suddenly be visible again.

For example here we turn 'Off' the transparency data, then immediatally turn it back 'On, reproducing the original image.

convert moon.png -alpha off -alpha on alpha_on.png

However if the image does not have any previous alpha data (yet) it will initialize it to be fully-opaque. Which is the logical thing to do. As such for new images just read into memory, it is equivalent to Alpha Set, but it should not be used for this purpose.

The only time Alpha On should be used is when you previously, and purposefully turned off alpha for some reason, and not wish to restore that data.

For example turning the alpha channel off then on can be used to preserve the alpha channel data before applying some very specific operators, such as "-shade".

For an example of this special usage see Shaded Shape Images.

Alpha Opaque

This method not only ensures the alpha channel is 'active' but that it is also completely opaque, regardless of if the image had transparency 'activated/on' or 'deactivated/off'. For example...

convert moon.png -alpha opaque alpha_opaque.png

On older versions of IM, this was equivalent to using both "+matte" to turn off the alpha channel, then using "-matte" to turn it on, while resetting it to be opaque.

convert moon.png +matte -matte alpha_opaque_matte.png

The original 'shape' of the image can no longer be recovered after this operation as the original alpha channel data has been overwritten.

Of course that is also equivalent to using "-alpha off -alpha set", though you may as well use "-alpha opaque" in that case.

Alpha Transparent

Similarly this ensures the alpha channel is 'active' but also fully transparent.

convert moon.png -alpha transparent alpha_transparent.png

The color data of the image is still present, so turning off transparency afterward will again show the images existing colors.

convert moon.png -alpha transparent -alpha off alpha_transparent_off.png

Of course the original 'shape' of the image was actually destroyed, so it can no longer be recovered after this operation.

Other ways of making an image fully transparent is presented in Transparent Canvas.

Alpha Extract

The 'Extract' method will simply copy the 'alpha' mask of the image as a gray-scale channel mask.

Note that fully-opaque is white, while fully-transparent is pure black.

As image contained some semi-transparent pixels along the edges (for anti-aliasing providing the images shape with a smoother look), this image is not pure black and white, but also contains some gray colored pixels around the edges.

If your imagemagick is old, this is a (near) equivelent technique, using channel extraction.

The 'Extract' method will also turned 'Off' the alpha, but it is not cleared, so turning the alpha channel back 'On' will re-create a shape mask of the original image.

convert moon.png -alpha extract -alpha on alpha_extract_on.png

Note that all the original colors will have been replaced with white with various shades grays around the edges. We can see this if we remove the transparency with a white background, (See Alpha Remove method below)

convert alpha_extract_on.png -background white -alpha remove alpha_edge.png

These 'gray' pixels are actually used to good effect in Edge Outlines from Anti-Aliased Shapes to generate a smooth edge or outline from a image shape.

This side-effect of saving the alpha channel, has particular benefits when Using the Shade Operator, which does not understand or use the alpha channel of an image. See the sub-section, Masking Shaded Shapes.

Alpha Copy

The 'Copy' method is the reverse of 'Extract', and essentially performs a CopyOpacity against itself. That is it will turn a gray-scale image (regardless if its alpha channel is enabled or not) into a shape mask image.

It does not matter if the image had an existing alpha channel or not, all it does is create the images transparency from the image gray-scale values.

Once you have a shape mask, you can use various Color Tinting or Duff-Porter alpha composition methods, to color it. For examples of using a shape mask see Masks as Colored Shapes.

Alpha Shape

To make use of a gray-scale image easier, the 'Shape' method not only creates a shape mask (as per Alpha Extract, but will also color it using the current background color.

This means you can very quickly color a gray-scale mask simply by shaping the image, then flattening it onto a different background color

convert alpha_extract.png -background Yellow -alpha shape \ -background Blue -alpha remove alpha_colormask.png

Of course a faster and better way to map a black and white image, directly to specific colors is by using the more specialised Level Adjustment by Color. This will avoid the need to enable or even modify the existing images transparency channel.

convert alpha_extract.png +level-colors Blue,Yellow level_color.png

Alpha Remove

The "-alpha Remove" method (added to IMv6.7.5) is designed to remove the transparency from an image, using the current "-background".

convert moon.png -background tan -alpha remove alpha_remove.png

Note that while transparency is 'removed' the alpha channel will remain turned on, but will now be fully-opaque. If you no longer need the alpha channel you can then use Alpha Off to disable it.

This operation is simple and fast, and does the job without needing any extra memory use, or other side effects that may be associated with alternative transparency removal techniques. It is thus the prefered way of removing image transparency.

For other techniques, or if your Imagemagick is older that v6.7.5, then look at the larger discussion Removing Transparency from Images) below.

Alpha Background

As of IM v6.5.2-10, a 'Background' method was made available that will set the hidden color of fully-transparent pixels to the current background color.

Normally this color is of no consequence, as it can only be seen if the alpha channel is turned off. However the color of fully-transparent pixels is saved in PNG Image file format, and for large images random unknown colors can significantly effect its compression handling. See the IM Forum Discussion Eliminating alpha channel garbage.

No color mixing is applied, only a direct color assignment to the fully-transparent colors. The pixels however will still remain fully-transparent.

For example here I use it to set all fully-transparent pixels to 'HotPink'. convert moon.png -background HotPink -alpha Background moon_hotpink.png

As you can see this made no change to the actual look of the image. But if we now turn off the alpha channel, you can see that any pixel that was fully-transparent will now be the color specified. convert moon_hotpink.png -alpha off moon_hotpink_off.png

This process of replacing the colors is actually almost the same as doing a "-channel RGB -fill color -opaque None +channel". See Direct Color Replacement.

Note that many other image processing operators will also convert any fully-transparent pixels, to fully-transparent black (color 'None'), as this is the color equivalent of a mathematical zero. Here is a summery of some image operations that are known to do this, though none are as direct or as fast as using this operator.

That last method (see Fuzz Factor and Transparent colors is particularly useful as you can not only set all transparent colors to full-transparent-black ('None'), but also all near-fully-transparent colors (which otherwise does have a valid but practically invisible color), simply by specying a fuzz factor. It will produce some data loss, but may improve compression in images with lots of near-fully-transparent colors.

See PNG with Better Compression for more deatils.

Removing Transparency from Images

While the Alpha Off will simply flip a switch and turn off the transparency channel. You can also get the same effect if you attempt to save the image into a file format that does not allow the use of transparency. For example saving an image to the JPEG File Format...

As you can see all transparent parts just became black (a typical result). But depending on the image source the transparent areas could have just as easily become some other random, or other inappropriate color.

Also in many cases semi-transparent pixels can have some very odd colors that is typically not visible because they are almost completely transparent. Simply turning off transparency will make these pixels stand out like a sore thumb, making the result look even worse than you may have expected. See for example the top-left edges of 'A' in the above.

In either case simply turning off transparency is typically NOT what is wanted.

The best solution is to use the Alpha Remove method to quickly and simply replaces the transparency with a background color underlay...

convert a.png -background skyblue -alpha remove \ -alpha off a_remove.jpg

Strictly speaking the Alpha Off is not needed in this case, as the save to JPEG does this automatically.

Alternative technqiues of removing transparency, is to somehow generate a new 'background' or 'canvas' image and Over Compose your image onto that background so that the transparency is replaced. Preferably while preserving the riginal images meta-data, such as profiles, labels, captions and comments that may be present.

Methods for generating such a canvas is exampled in Creating Image Canvases of Same Size. Here is one such method...

convert a.png \( +clone -alpha opaque -fill SkyBlue -colorize 100% \) \ +swap -geometry +0+0 -compose Over -composite \ -alpha off a_compose.jpg

Other simplier ways to do this, is to use a operation that internally creates a 'cloned background canvas' for you, generating it as part of the larger image processing operation that the operator is performing.

The most common method is to Flatten the image. This operator is so often used for this purpose that the process of removing transparency has often erroneously been called 'flattening'. For example...

convert a.png -background skyblue -flatten -alpha off a_flatten.jpg

However this will not work with "mogrify" or with a sequence of multiple images, basically because the "-flatten" operator is really designed to merge multiple images into a single image.

The other common method that does work with multiple images is to give the image a zero sized Border with the appropriate "-bordercolor". For example...

convert a.png -bordercolor skyblue -border 0 -alpha off a_border.jpg

Other image processing operators which are related to the above two methods, can also remove transparency from an image include, the layering image operators Mosaic and Merge, as well as Frame.

The Extent operator can also be used, and allows you to expand or crop images at the same time as you remove the transparency, but only if you know the size of final image desired.

You do not have to replace transparency with a solid color. If you use a DIY composition (as shown above) you can use any image for the replacement background. One simple example of this is to use the "composite" command to Tile an image 'under' the original, (using Dst_Over). This compose method ensures the original images meta-data and size is preserved.

composite -compose Dst_Over -tile pattern:checkerboard \ a.png a_undertile.jpg

Many of the above methods, are either effected by, or may destroy any virtual canvas information an image may have, as part of its processing. When the virtual canvas is involved, you may need to look at the details of individual operators more closely. In many cases the virtual canavs effects can be useful to your overall image processing.

Outline or Halo Transparency

Sometimes you will like to add a outline around an image containing transparency.

One way is to use EdgeOut Morphology to quicked get all the neighbouring pixels to the original image, color them, and then Under (DstOver) Compose it with the original image.

This can be particularly useful when creating GIF format images from PNG images containing semi-transparent edge pixels. It provides a minimal amount of background color, but leave the rest of the image fully-transparent. For more about this see GIFs on a Background Pattern for more about this problem.

An alturnative method is to generate a soft semi-transparent halo around the shape. To do this we Blur and recolor the image, then again Under (DstOver) Compose it with the original.

convert knight.png \( +clone \ -channel A -blur 0x2.5 -level 0,50% +channel \ +level-colors red \ \) -compose DstOver -composite knight_halo.png

This last is actually similar to using a Soft Outline Compound Font effect, but using a shaped image rather than annotated text.

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Using Masks with Images

Masking An Image

As shown previously there are a couple of ways to mask an image, so as to make part of the image transparent. And which method you choose depends on the weather your image mask is a grayscale mask, or a shaped mask.

Editing a Image Mask

The mask of an image is a really useful thing to have.

We can for example erase parts of an image very easily by modify a mask of the original image. Remember the "-draw" operator can not draw nothing, and currently has no erase option.

Here we create a image, then by extracting and modifying its mask, before restoring it to the original image.

Remember "black" in a mask is transparent, while white is opaque, so all we need to do is draw black over anything we don't want visible.

Don't forget the "-alpha Off" operation in the above as it is vital to ensure the grayscale image does not contain unneeded transparent channel.

And Presto we took a bite out of the original image.

We can also re-add a part of the image we removed. For example here I re-add part of the 'bite' I removed from the original image, by drawing white area onto the mask. The mask is then again returned to the original image using CopyOpacity Channel Composition.

Just a word of warning about re-adding parts. Typically ImageMagick replaces any fully-transparent color with black, usually because that is how the mathematics behind operators work. It is after all fully-transparent and thus its color should not normally matter. That means that if we make a part of the image we haven't drawn before opaque, then it will generally be black, since that is the color under the image's transparency.

However in the above example you will have noticed that the PNG image file format correctly preserved the original (made transparent) color of the image. As such the color of the re-added part remained the original 'SteelBlue' color of the original image. You should not count on this if the image was saved to some other file format or further modified.

Here is an alternative method of erasing parts out of an image but rather than extracting and modifying a Grayscale Mask, we instead use a Shape mask as a sort of 'erase' tool using DstOut Composition Method.

As you can see sometimes Shape Masks are easier to handle, as you avoid the need to extract and restore the alpha channel.

However the Duff-Porter Alpha Composition Methods, which is what I am using, will never allow you to restore colors that have been made transparent. With these methods, anything that has been made transparent (and thus undefined in color), stays transparent.

In actual fact erasing parts of an image using Alpha Composition Methods will actually destroy the underlying color of fully-transparent pixels. It will not preserve it. After all, a transparent color is actually not a real color!

Masks as Colored Shapes

An alternative to just using the mask to add or re-add transparency to an image is to actually combine the mask directly with images in various ways.

For example suppose we just want to use a mask as a symbol or shape we want to overlay onto an image in various colors. For this we need a mask, which I'll extract from a special 'symbol' font.

convert -font WebDings -pointsize 24 label:Y \ +trim +repage -negate heart_mask.gif

Note that I negated the label image to make it proper mask image, consisting of a white foreground (opaque) on black background (transparent).

As of IM v6.4.3-7 the simplest way to convert a grayscale mask, into a colored shape is to use the Alpha Shape operator, This is exactly like Alpha Copy, but with a extra step to color the final shape.

convert heart_mask.gif -background Red -alpha Shape heart_red.png

Note the use of 'PNG' image format for generated shaped image rather than GIF so as to avoid problems with GIF Boolean Transparency.

Before this the simplist solution was to negate the alpha mask into a matte Channel Image then use Combine to generate the shaped image.

convert heart_mask.gif -negate \ -background Gold -channel A -combine heart_gold.png

The color of the shaped mask in this case is defined by the "-background" color which Combine used to fill in the undefined channels of the new image.

An older but more complicated way is to use 'CopyOpacity' composition method to set an image's transparency to the given mask, then use Uniformly Color Tinting to color the resulting shape. This works and for a long time was the best technique to use, but is no longer recommended.

convert heart_mask.gif \( +clone \) -alpha off \ -compose CopyOpacity -composite \ -fill HotPink -colorize 100% heart_hotpink.png

Now that you have a 'shaped' image, you can just simply overlay the image on any background we want, such as the built-in rose image, using one of the many Image Layering Techniques and Alpha Composition Methods.

convert rose: -page +2+2 heart_gold.png \ \( +clone -repage +7+29 \) \ \( +clone -repage +52+14 \) \ -flatten rose_with_love.gif

This is fine if we want all our symbols the same color, but would require multiple intermediate images if we want to use multiple colors, making it impractical for overlaying lots of symbols with lots of different colors.

One way you can make multi-colored overlays is to re-color the shaped image immediately after reading in the image.

convert rose: \( heart_gold.png -repage +2+2 \) \ \( +clone -fill Red -colorize 100% -repage +7+29 \) \ \( +clone -fill HotPink -colorize 100% -repage +52+14 \) \ -flatten rose_colored_love.gif

Note we only read in one shaped image, then recolored a Clone of that image for each new 'layer' to be overlaid. For more examples of re-coloring a base image, see the whole section on Color Modifications.

See also Drawing Symbols for an alturnative method of marking specific locations in an image. As well as a Pinning Maps Laying Example for a more automated layering techniques.

Mathematical Composition

Rather than overlaying the mask onto some background, you may only be interested in coloring the image with just the white or black parts of the mask itself. This is relatively straight forward, simply by using some Mathematical Alpha Composition Methods to change the color of the mask to match a color, tile or other image.

For example the 'Multiply' compose method will replace the white areas (multiply value of 1) with the overlay image, while leaving the black areas (multiply value of 0), black.

The 'Screen' operator is exactly the same as 'Multiply' but with the images negated so it effectively replaces the black areas of the image.

For example, lets use the larger mask image from above, to overlay a larger image generated with a tile pattern.

The 'Multiply' alpha composition method is especially useful for replacing the background of text images (IE: black text on white background), such as images generated from Postscript Documents.

Masked Alpha Composition

The special three image form of Masked Alpha Composition allows you use the same mask to directly merge two images together.

convert -size 100x100 tile:tile_water.jpg tile:tile_disks.jpg \ mask_bite.png -composite compose_masked.png

The first image will replace the black background parts of the mask, while the second image replaces the white foreground parts of the mask. The mask itself is given as the third image.

The mask is used to select and mix two different images together to generate the final result. It is actually rather like a mapped Blend of the two images.

Remember the final size and meta-data of the resulting image will come from the first 'background' image of the above operation (black parts), so swap images and Negate the mask if you want it the other way around.

And finally remember that if you use the "composite" command instead of "convert", the 'overlay' image (white parts) is given first with the 'background' image (black parts) second. In other words the first two images need to be swapped for that command.

Aligning Two Masked Images

On aligning two masked images...

The best idea is to use the same mask (negated) for BOTH pieces, rather than
attempting to draw the two masks separately. Otherwise you have the two masks
overlap, OR leave a gap, exactly as you have seen.

correct methods of mask joining..

    * use mask to set transparency on one piece
      use negated mask to set transparency of other piece
      'Plus' the two pieces together.

    * Use mask to Add transparency to just one piece, then
      'Over' compose that piece over a complete image.

    * use a three image masked composition
      see http://www.imagemagick.org/Usage/compose/#mask
      and http://www.imagemagick.org/Usage/masking/#masked_compose
      Which uses the mask to select results from two different images.

Remember, 'Over' only needs the 'source' or 'overlay' image masked,  'plus'
however needs both images masked with a negative mask of each other.

The only time you get different results is if BOTH images already contain an
existing shape mask!  In that case the 'Plus' with 'Dst-In' and 'Dst-Out'
performed on each image (to multiply the alpha channels correctly), or
a 3-image blend method works better.

For more on the difference between 'over' and 'plus' see 'Blend' (plus) vs
'Dissolve' (over) http://www.imagemagick.org/Usage/compose/#blend_dissolve



For an example of DIY image joining, using 'Dst-In', 'Dst-Out', and 'Plus'
composition, see examples in...
   http://www.imagemagick.org/Usage/compose/#dstin

I also go though this in detail in the bug report of 3 image alpha composition
Composite Mask Bug - Fixed.


You need to be careful with Over as merging the two masked images with 'Over'
will probably leave a slightly transparent join.   That is Over is only truly
useful, if the destination image is a complete (opaque) image.  Do not use
while you are overlaying a 'part' image.

This transparency problem is also seen in examples in
3d Cubes - Affine
   http://www.imagemagick.org/Usage/distorts/#cube3d
and again in
3d Boxes - Perspective
  http://www.imagemagick.org/Usage/distorts/#box3d
and in Isometric Cube using Shears
  http://www.imagemagick.org/Usage/warping/#sheared_cube

The Major problems in these examples is that the individual parts were NOT
generated using the same mask, but distorted to their final positions.
As such they do not quite fit together properly and joined together.

These examples need to be updated to use a 'Plus' composition method.  To
generate improved results, but even then they will still probably not be quite
'right'.



WARNING:  Draw does NOT currently allow you to generate two shapes that will
fit together properly without overlap!!!!

See Draw Fill Bounds for details.

I have not checked SVG to see if it has the same problem.

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Write Masks

A 'write' or 'clip-mask' is a special greyscale image that is added to an existing image of the size size. It defines areas of the image which are to be classed a 'immutable' or 'not-writable' by most image processing operators.

The operator "-mask" takes an external image to be linked to the images in memory. The 'plus' form of the operator "+mask" removes the mask from the image.

For example here I apply a circle 'write-mask' to a red image, and then try to rotate the color hues of the image.

Write or clip masks are designed to work when the pixels in an image are being directly modified. EG: negate, level, color tinting, modulute, drawing, composite, morphology, convolutions.

For operators that generate NEW images (resize, distorts, extent, etc) it will probably fail to preserve original pixels, as the mask will not be able to correspond to the new image size. Such operations will also have the side-effect of removing or unsetting the images 'write-mask'.

A practical example of using a 'write mask' is Chroma Key Masking.

This is actually exactly how Composition Masking actually works! For example.

Is equivelent to...

convert write_mask.png -negate -write MPR:mask +delete \ red_image.png -mask MPR:mask \ green_image.png -composite +mask masked_composite_equiv.png

That is the mask is negated, then applied to the first 'destination' image. the second is then composed over the first image, modifying only the 'white' areas of the original mask image.

The 'write-mask' is a blending mask in that 'grey' pixels in the mask will produce a blending of the new pixels with the old image values, by the amount of gray that is present. This produces very smooth edges, and also allows you to generate a gradient across the image between modified and un-modified areas.

Of course operators like morphology, which may result in writing to the same image multiple times (in a internal loop) will not handle this 'pixel blending' very well. In such a situation a purely boolean write mask is the better option, or simply generate the final result and mask compose it over the original.

For a morphology example, a 'conditional dilation' effect was discussed in the IM Discussion forum, Cleaning up noise around text.

FUTURE: -crop should be able to preserve the image mask of individual images.

Clip Mask

The "-clip-mask" form of this operator, is almost exactly the same as the above but only provides a boolean (all-or-nothing) style of masking. As a result you can not achieve a 'blended' or smoothed result.

For example...

convert red_image.png -clip-mask write_mask.png \ -modulate 100,100,33.333 +clip-mask clipped_modulate.png

As you can see the result is highly aliased (with stair-cased edges), as a "-clip-mask" does not produce a blended result as "-mask" does.

The only good thing about this is that it is slightly faster (though not very much). It was provided originally to allow the handling of Clip Paths in TIFF image files, and is a very old operator (IMv5). The newer "-mask" operator should be used instead.

In IMv6, a 'write-mask' and 'clip-mask' are implemented side-by-side, even though they techniqually do exactly the same function. As such you could techniqually apply both masks simultaniously. However using both at the same time is not recommended, and will not be avialable in IMv7.

Clip Paths for TIFF images

A 'clip path' is part of TIFF image file format, and defines a vector path that is used define a 'shaped area' within the TIFF image.

In IM the operators "-clip" and "-clip-path", reads this 'clip-path' and converts it into a Clip mask (above). As such it defines a 'write mask' that will protect the shape from modification.

A clip-path stored in the TIFF image is defined as a SVG Path Drawing, which you can extract from a TIFF image file format using...

The biggest problem people often have is making everything that is not clipped transparent. Which requires you to write the areas the mask write protects!

This is one solution, which converts the whole image to transparency, then turned on the 'clip path' and makes the now non-wriatable parts opqaue (visible) again.

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Regions and Region Sub-Images

Regions are another way of limiting the effects of operations to a smaller area of an image.

For example here I color tint the whole rectangular region red...

convert koala.gif -region 40x33+15+5 -fill red -colorize 50% \ koala_region_red.gif

You can also make a region transparent...

convert koala.gif -alpha set \ -region 40x33+15+5 -alpha transparent koala_region_trans.gif

Note that I needed to ensure the original image has an alpha channel enabled before making the 'region image' transparent. If that was not done, IM would make any transparency in the 'region image', see-thru, and you would see no change. See How Regions Work below for details.

Before IM v6.6.9-5 transparency preservation was broken, and the results of transparency in a region was always "see-thru to original". As such the result of the above would not include any transpart pixels, even though the image allowed the use of transparency.

The biggest reasons for using Regions is that it doesn't just simply limit its effect to a small area, it actually extracts that rectangular area of the image, and applies all the Simple Operations that follow to that smaller area.

This means that if your are only modifying one very small area of a very large image, say for example doing red-eye removal, then you not only limit the scope of the operations to that area, but perform it much faster too, and the extracted region image is itself smaller.

In summary... Clipping Masks will perform operations over the whole image. But Regions only apply it to a smaller extracted sub-image.

Note that there is nothing preventing you from using both of these methods together. Though if you apply a clipping mask to a region, the clipping mask should match the size of the region image that was extracted.

Warping a Local Region

As a 'image region' actually extracts a 'small sub-image' of the original for processing, you can make use of the special 'localised' Circular Distortions to warp small regions of the original image.

For example here we have a line of stripes.

Now by defining regions we can distort the line in different ways in different areas.

Note that the "-implode" and "-swirl", fit into the use of regions very well, as they have the property that the outside edge of the distorted image matches up to the rest of the image outside the defined region. That is they are actually designed to perform 'localized image warping'.

Note that when I used the Wave Distortion, I had to crop the size of the resulting 'wave' image so that it would again fit into the original area from which it was extracted.

Remember Regions only work when used with Simple Image Processing Operators. Any other operator including another "-region" operator will cancel the region processing, before that operation is applied.

How Regions Work, and its Problems

In reality the way regions work is...

• Extract from the image a smaller image according to the "-region" operator, using a simple crop with the region argument.
• Apply any Simple Image Processing Operators, that follow, to the smaller image.
• When a non-Simple Image Operator is seen, OR another "-region" operator is found, OR the region is turned off using "+region", then the extracted region is overlaid on the original image at its extracted location.

Region works in a way that is similar to using Image Stack Operators, though it existed in ImageMagick a long time before those operators. It was for example a integral part of IM version 5.

For example, if you have this Region Operation...

the result is equivalent to this (for a single image)...

Or this (for multiple images)...

How the 'region image' is actually overlaid onto the 'original image' is a little tricky...

If the original image has the no Transparency Channel enabled, the 'region image' is composed using Over Composition. That means that transparent areas in the region image will become see-thru, allowing you to see the original image behind it.

For example here I purposefully turned off the transparency in the original image, but then Rotate the region so as to produce some areas of transparency in the corners.

convert koala.gif -alpha off -region 30x30+10+10 \ -alpha on -background None -rotate 30 koala_region_rotate_1.gif

As you can see the corners of the rotated area (which was transparent in the 'region image') shows the 'original image'. Basically as the original image can not handle transparency, the region image is simply overlaid, with see-thru corners.

If the original image does contain active Transparency then the transparency in the modified region image can also be modified, so transparency is just 'copied' as is.

convert koala.gif -alpha set -region 30x30+10+10 \ -background None -rotate 30 koala_region_rotate_2.gif

As you can see IM uses a Copy Composition, so that any transparency that exists in the region image, will also be copied to the original image.

If for some reason you want the original image to preserve its original transparency, Turn off the Alpha first, then after the region image has been restored, Turn it on again so as to restore it.

Region Images which are enlarged or shrunk, may not 'fit' back into the original. For example here I resize (and color) the region image so it becomes smaller...

convert koala.gif -region 30x30+10+10 \ -resize 75% -fill red -colorize 30% koala_region_shrink.gif

As you can see the original region was not covered by the region image that was restored. As such those parts not covered were not replaced.

In a similar way, if the region becames larger, more of the original image may become covered by the overlaid region image.

convert koala.gif -region 30x30+10+10 \ -resize 150% -fill red -colorize 30% koala_region_enlarge.gif

In both cases the top-left offset of the region, does not move. You can not just shrink an region image and center it within the region area, nor can you position the region image in another position.

Caution should be exercised to prevent region images from changing size.

Like "mogrify" you can not merge multiple sub-images as that requires the use of a non-simple image operation. However you can use "-draw" as an alternative composition method. See Alpha Composition in Mogrify for an example.

At the time of writing, the 'region image' still contains the Crop Virtual Canvas Offset from its extraction from the original image. This may, or may not be regarded as a bug, depending on if you find this information useful or not. The offset is currently not used when a region image is restored. If the offset is not wanted (as it interferes with an operator such as Distort), follow the "-region" option by a "+repage" operator to remove the offset, from the region images. Its removal or modification will not effect its restoration back onto the original image.

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Background Removal

One of the most common problems in image processing is mask generating from a existing fully-opaque image. Such images are commonly downloaded from the World Wide Web, or generated by programs, or in image formats that don't provide any form of transparency.

It may also be that you have a photo of some object, and want to remove the background. Remember photos do not have any understanding of transparency, so you need to remove the unwanted parts yourself.

Unfortunately there is no general solution to this problem, especially when you also want to retain any semi-transparent edging to the image. Consequentially their are hundreds of ways and variations on doing this task, all dependant on the exact situation.

Closely related to image masking is transparency adjustments to match a background that an image is going to be overlaid on. This is talked about in detail as part of saving to the GIF Image File Format which only allows Boolean transparency.

Masking Simple Backgrounds (floodfill)

When an images background is a simple single solid color, you can often generate simple masks (and background removal) by just doing Replacing Colors in Images.

For example here is a direct floodfill masking of a image with a solid color background.

convert cyclops.png -alpha set -channel RGBA \ -fuzz 1% -fill none -floodfill +0+0 white \ cyclops_flood_1.png

Well that did not work, as the floodfill 'seed' point in the top-right corner does not actually reach all parts of the image!!!

The solution to this is to enlarge the image slightly, so as to provide a path for the floodfill to reach all the outside edges of the image. However for this you need to know the color of the background.

convert cyclops.png -bordercolor white -border 1x1 \ -alpha set -channel RGBA -fuzz 1% \ -fill none -floodfill +0+0 white \ -shave 1x1 cyclops_flood_2.png

Of course we did not specify a very good Fuzz Factor. The problem with this is that you get a halo around the object within the image. This is because most images contain special pixels along the edges which smooths the look of the image.

However as this image has a good black border to it, relative to the background, using a nice large fuzz setting can be used to nicely separate the image from the background.

convert cyclops.png -bordercolor white -border 1x1 \ -alpha set -channel RGBA -fuzz 20% \ -fill none -floodfill +0+0 white \ -shave 1x1 cyclops_flood_3.png

This technique has some problems with it. First it is an all or nothing masking of the image, producing edges that are aliased, staircase-like and often horrible looking. This is fine for the limited GIF image file format, but not very good if you plan to overlay that image onto another background.

It is also very very difficult to get every anti-aliasing edge pixel. As such if I overlay the above image on a black background, you may see some pixels that are much whiter that normal.

convert cyclops_flood_3.png -background black -flatten \ cyclops_flood_3_over.png

Also if you do manage to use a high enough fuzz factor, you are likely to have the problem of having very little edging pixels left, or 'leaking' into the center of the image.

Finally a direct flood fill like this does not work for a background that isn't a simple single solid color.

Cutting Out Bordered Objects

Images with a existing single color border has a distinct advantage for these methods of background removal, as the border provides a definite boundary between what is 'inside' and what is 'outside' the image, and that in turn allows use a better method of specifying the boundary of the background image.

That is rather than specifying what colors should be regarded as background, we can instead specify what colors mark the border of the object being masked.

Further more, as the border color is known, only two specific colors will have been mixed together around the edges of the image. That is both colors are known, and so exactly how transparent the edges should be is also very well known.

Removing a Known Background

While removal of a simple background to a 'Boolean' mask, is relatively straight forward, things get more complicated when the background is not so simple. However if the background itself is known. you can use that to help in its removal from other images.

As of IM v6.3.4 a special Alpha Composition method was added called 'ChangeMask' which allows for the direct removal of a known background from an image.

For example here we have a unaltered background image, and one that has been overlaid by a GIF image with a simple Boolean (straight on/off) transparency. By using 'ChangeMask' we can recover that original overlaid image (if it is very different to the background).

Basically what this does is determine how 'different' the pixels are from one image to the other, and is the difference is less than the current Fuzz Factor, then make that pixel transparent.

Only fully transparent pixels are added to the image, otherwise the original image is left as is, transparency and all.

We can simulate the operator by using the older 'Difference' composition method to generate a Comparison Difference Image...

As you can see the difference image is black for all the unchanged parts and a mix of colors for the parts which has changed.

By separating and adding the individual color channels together and thresholding we get a mask of any difference in any channel between the two images. convert overlay_difference.png -channel RGBA -separate +channel \ -evaluate-sequence add -threshold 0 overlay_mask.png

Using this mask we can set anything that has not changed to transparency. convert overlay_figure.gif overlay_mask.png \ -alpha off -compose CopyOpacity -composite \ overlay_removed.png

As you can see the 'ChangeMask' composition method makes this process a lot easier.

However this only presents a 'on/off' style of background masking. It does not allow for fuzzy or anti-aliased edges, or transparent feathering of the result.

Difference Image Masking and Feathering

The above can be taken further to images that have aliased edges. as well as non-simple backgrounds.

For example, Here we have a 'Cyclops' on a white background, which we want to extract. We then generate gray-scale image of the differences between this image and the background color (as defined by top-left most pixel).

convert cyclops.png \( +clone -fx 'p{0,0}' \) \ -compose Difference -composite \ -modulate 100,0 -alpha off difference.png

Of course this difference image is no good as a mask directly. If you did use it you will effectively make most of your image semi-transparent, instead of just the surrounding background.

However from this difference image, huge number of different transparency masks can be created, depending on exactly what you are trying to achieve.

We can adjust the above difference image to produce a mask of all pixels that are even the smallest amount different from the background color.

convert difference.png -threshold 0 boolean_mask.png convert cyclops.png boolean_mask.png \ -alpha off -compose CopyOpacity -composite \ cyclops_boolean.png

As you can see a boolean 'any difference' resulted in a good amount of the original background being included. This is because the original image is either 'anti-aliased' or blurred slightly with the background (in this case it was caused by the original image being resized from a JPEG format image).

This would not be a problem if the original image was itself a Boolean overlay (EG a GIF format image, overlaid on a background). In that case your result will be perfect (see the 'ChangeMask' example above).

By varying the "-threshold" you can add a 'fuzz factor' to the boolean (on/off only) mask, so as to get the mask closer to the image proper.

convert difference.png -threshold 15% threshold_mask.png convert cyclops.png threshold_mask.png \ -alpha Off -compose CopyOpacity -composite \ cyclops_threshold.png

Notice that the eye of the cyclops image is now also regarded as being a transparent hole!

This 'hole' highlights the biggest drawback with this whole technique. Parts of the image object which are close to the background color, or worse still, exactly matches the background, will be thought of as being the same as the background.

Of course this may be desirable in images of 'holey' object, such as a donut, but for our cyclops, a 'holey eye' is definitely a mistake.

The original 'halo' effect can also be desirable for some things like text to make it more readable when you want to overlay it again on some other 'noisy' background. You can enhance the halo effect by blurring the mask a little before applying it, so that the resulting 'halo' becomes diminished by distance.

convert difference.png -bordercolor black -border 5 \ -threshold 10% -blur 0x3 halo_mask.png convert cyclops.png -bordercolor white -border 5 halo_mask.png \ -alpha Off -compose CopyOpacity -composite cyclops_halo.png

This 'halo' effect can be further modified by using more Histogram Adjustments on the mask image, giving you very precise control of the results for specific images.

A small amount of blurring (say "-blur 0x0.5") is actually recommended in the threshold masking, just to smooth out the edging of the mask. Of course the result will not be boolean, so don't try to save it to a GIF format image file.

Recovering Semi-Transparent Edges

The Difference Masking technique that we used above can be used with the previous FloodFill Masking technique to solve most of the problems we have seen with simpler masking techniques.

Here we look at a more complex multi-layered masking technique, but one that should produce near ideal removal of the images background. However for it to work you will need to know either the color of the background, or be able to generate that background from a known pattern, or other image.

For this example I decided to use something that was very hard to separate. A patterned letter on a textured background. On top of this I also added a shadow that I want to also be extracted.

convert -size 70x60 xc:none -font Candice -pointsize 50 -stroke black \ -fill black -annotate +12+42 'A' -channel RGBA -blur 0x3 \ -fill tile_disks.jpg -annotate +10+40 'A' \ tile_water.jpg -compose DstOver -composite letter.png

First we will need to generate a difference image, and lucky for us we do have a background image. Of course it will work just as well for a plain colored image as well, such as we used above in our previous difference image.

Basically by using a difference image we can remove any influence of the background image, as we generate the various masks needed.

convert letter.png tile_water.jpg \ -compose Difference -composite \ -modulate 100,0 -channel B -evaluate set 0 \ -alpha Off diff_mask.png

Note that this time I processed the difference image slightly, by clearing out the blue channel, and producing a black-yellow difference image. This is tricky as it frees 'blue' channel to allow the generation of a clean floodfill mask, separate to the difference image itself.

Now we need to generate two masks: an outside mask of all areas that will definitely be transparent; and a mask of the definite inside of the object in the image, such that it does not contain any 'holes'. So lets flood fill the image from the outside inward, using a number of different fuzz factors.

The blue areas in the above images is the area being masked.

Now we need to select two of the floodfill masks, to define the area in which the semi-transparent pixels will lie.

The first mask should mask the areas of the image we definitely want to make full-transparent. That is the parts we definitely expect to be fully transparent on the final image. The area inside the mask should still contain most of the black halo shadow of the image.

In this case we have a lot of interaction between the image proper and the rest of the background so I chose a Fuzz Factor of '1%' which still contained a large area surrounding the image. In a more typical non-shadowed case this area can be even smaller, down to a non-percentile value such as 5 or 10.

The second mask should have large enough 'fuzz' so as to eat up all the semi-transparent pixels that is present. That is right up to and preferably actually into border of the image without completely removing the border, or 'leaking' into the image proper (see last image above). The negative of this mask will actually represent all the pixels that will be definitely fully-opaque in the final image.

This selection can be difficult and may require a lot of trial and error to figure out the best value to use. For this image a very high fuzz '32%' was able to be chosen without any major problems. Basically you want to try and get it high enough that the final image will not contain any of the original 'background' pixels in it, but without the mask eating away the insides of the image. It may even require a little hand editing to get the mask just right when it leaks.

We can now use this mask to extract the 'core' or inside of our image. That is the parts we are sure does not contain any semi-transparency through to the background pattern we are removing.

convert diff_mask_32.png -channel blue -separate +channel -negate \ letter.png +swap -alpha Off -compose CopyOpacity -composite \ letter_inside.png

Note how I extracted the blue mask from the flood-filled masked images. Also due to the all-or-nothing nature of flood-filling, the mask will show heavy staircasing or alias effects around the edges. This is the problem the second mask will allow us to fix.

Remember this image is only of the pixels that we know does not interact with the original background, and will be left as is, in the final image. It does not include any of the shadow effects, and anti-aliasing pixels that I am specifically attempting to recover. Recovering those pixels is where the real work lies.

By negating and subtracting (multiplying) the masks we can generate a new mask which defines the area where we want to extract semi-transparent edging or shadowing pixels...

convert diff_mask_01.png -negate diff_mask_32.png \ -channel blue -separate +channel -compose multiply -composite \ mask_aliasing_area.png

This area is then used to extract the anti-aliasing pixels from the difference mask, which defines how transparent the pixels should be. We normalize those pixels to get a smooth transition from opaque to transparency.

convert diff_mask.png -channel red -separate +channel \ mask_aliasing_area.png -alpha Off -compose CopyOpacity -composite \ -background gray30 -compose Over -flatten -normalize \ mask_antialiased_pixels.png

The lighter the color in the above mask, the more opaque the pixel will be. Similarly the darker the color, the more transparency it will be.

Note that I used a gray background here to ensure that the transparent colors that is present in the image will not interfere with the Normalization of the image. Without this this normalization will fail. The flat gray color itself is not important as they are outside the mask area, so will be ignored later.

Now that we have the right transparency level, we need to know what color should be used for these semi-transparent pixels. This color will usually be the same as the edge color of the image, in this case simply, black. However because of the interaction of the original background I decided to go for a dark grey color for the shadow

For an image, such as from a photo, you may need to somehow replicate the varying colors of the edge of inside area to set the correct color for the semi-transparent anti-aliasing pixels. This is a process which I have not figured out (yet).

While we are at it lets also re-mask the image to leave just these special edging pixels.

convert mask_antialiased_pixels.png mask_aliasing_area.png \ -compose multiply -composite -negate \ -background '#444' -channel A -combine letter_edging.png

All that is needed is to now layer the inside 'core' of the image with the semi-transparent edging pixels.

convert letter_inside.png letter_edging.png \ -background none -flatten letter_recovered.png

And hey presto, we have a image with the background removed to produce a perfect anti-aliased image, with correctly recovered semi-transparent edging and shadowing

You can even overlay it onto a completely different background.

convert letter_recovered.png tile_aqua.jpg \ -background none -compose DstOver -flatten letter_on_aqua.png

The image I used for this example is very difficult with a large 'edge' region. Most images are not nearly so bad, but this method is probably the best and most universal background removal technique.

This has now been placed into a shell script called 'bg_removal", which uses a single command, no temporary files, and has a number of extra options on the methods by which the masking is performed.

Background Removal using Two Backgrounds

The major problem with the previous techniques is that you really do not have enough information to completely recover all the information about the foreground object.

You really need to recover two pieces of information, how transparent each pixel in the foreground object is, and what is its original color. And you can not perfectly recover both pieces of information from just one image.

Even when you know exactly what the background image looks like, you can not just subtract it from the foreground object, unless the two are very different and known colors.

The problem is you can be sure if the color is really the color given (opaque), or it is some blending of color and the background (semi-transparent). You just can not be sure of the color/alpha separation, unless you have a source of secondary information.

The one situation in which you can completely recover all the details of a foreground object, is when you have two images containing two very different but known background colors. With two images on different but constant background colors, then you get all the information you need for perfect recovery.

The important factor in selecting two images is that the background colors are as different as possible over the whole image. That is the colors are not only complementary, they are preferably opposite in intensity as well.

For example...

While a different background color is used, both images contain the same object. The object shown is not simple, but contains lots of semi-transparent colors. You can see this in the way the dark blue background is visible in the flames of the image.

By using two colors, the semi-transparent pixels of the overlaid object will become mixed with two very different colors, as a result be slightly different colors in the two images. By measuring how different each pixel is, you can determine exactly what pixels are semi-transparent, and by how much. Essentially there is enough information to allow you to perfectly recover the transparency of the overlaid object perfectly.

Recovery the transparency or 'mask' is of course the first step, and is actually a very straight forward step. Generate a difference image, then merge and maximize the differences found in each channel.

convert match_navy.gif match_gold.gif \ -compose difference -composite -separate \ -evaluate-sequence max -auto-level -negate \ match_alpha.png

This resulting image is a perfect map of how transparent each pixel is. It is essentially the 'alpha mask' of the original object in the source images.

However it will only work if the overlay image contains both areas of full-transparency, and full-opacity. If that is not the case instead of the normalization step ("-evaluate-sequence max -auto-level") in the above , you will need to divide each channel by the difference of the two background colors. That is divide by a value between 0.0 and 1.0, the larger the difference the better. If the two background colors is pure black and pure white, then no normalization, is needed, just the difference of the two images.

The difference is then Negated so tha a maximum difference produces zero alpha or full-transparency, and no difference produces maximum alpha or full opacity.

The next task is harder, as the colors of each semi-transparent pixel is modified by the background, you can not just use the alpha mask to extract the object from one of the source images. For example...

convert match_navy.gif match_alpha.png \ -alpha Off -compose Copy_Opacity -composite \ match_bad_colors.png

Basically what we got was a horrible halo of the background color in the semi-transparent 'flame' of the image. Not a nice result at all. This is known as 'color spill' (a term from Chroma Key Masking, also known as the Blue or Green Screen technique) and this can be a major problem.

What we need to do is remove the background color from the semi-transparent pixels. However as we have already recovered the original images alpha channel, we know exactly how much color needs to be removed from the each pixel, so as to restore the original color overlaid.

To do this we not only need one one the source images, and the alpha channel we just extracted, but we also need to know the original color of the background. A relative easy problem when using a solid color background, as in these examples.

For example here I restore the original colors...

convert match_navy.gif match_alpha.png -alpha Off \ -fx "v==0 ? 0 : u/v - u.p{0,0}/v + u.p{0,0}" \ match_alpha.png -compose Copy_Opacity -composite \ match_recovered.png

I used top-left corner pixel (FX formula 'u.p{0,0}') in the source image as the background color to remove from semi-transparent pixels. Adjust this or directly substitue the color to remove if needed.

The key to the color restoration, is the complex FX blended subtration operation in the above. This will enhance the original color ('u') of the source image according to the alpha mask ('v'), then subtract the background color (u.p{0,0} or the top-left corner pixel) from the final result.

The formula is not straight forward, and major thanks does to HugoRune, in the IM Forum discussion Undo a Composite -dissolve for determining the mathematics needed. The discussion also goes on to exactly how all the steps work, were derived, and even how you can also extract the overlay from any two known but different background patterns.

Here is the whole sequence all in one command.

convert match_gold.gif match_navy.gif -alpha off \ \( -clone 0,1 -compose difference -composite \ -separate -evaluate-sequence max -auto-level -negate \) \ \( -clone 0,2 -fx "v==0?0:u/v-u.p{0,0}/v+u.p{0,0}" \) \ -delete 0,1 +swap -compose Copy_Opacity -composite \ match_recovered_2.png

In IM v6.6.8-3, if FX references a transparent pixel using 'p{}' it gets zero values rather than the actual fully-transparent color values! This is a bug and was reported and fixed in IM v6.6.8-5. It is not known when the bug was introduced. This was only a problem, if you decide to merge the alpha image into the source image first, then try to fix semi-transparent or 'spill' colors, using the known background color.

This time the 'gold' background image was used for the color extraction, and was selected by the '0' in the second "-clone" operation, but either source image coul have been used.

Just one warning. The above assumes the top-left pixel is the unadulterated background color. If it isn't you may have to modify the command to specify a specific pixel color, or use a third image that does contain the correct background color information. The latter method is vital if the background color is not constant across the image, though even that complication can be fixed.

Here is the simpler sequence for images overlaid on a pure black and a pure white background color. In this case the colors are always recovered from the black background image, as it is just a simple division, and thus a faster Divide Composition can be used instead of the painfully slow FX DIY Operator.

Studio photos for background recovery

The ideal backgrounds are a matte (non-reflective) black, and simple pure (non-reflective) white. The background should also be as smooth and unvarying shade as possible. For making photos specifically for background removal, using two complementary colors may work better. Say taking photos with a green and magenta backdrop.

Basically you will need to some how replace the background color screen before taking the second photo. Note that order of the two photos does not matter in the background removal, but they should be a clean and uniform as possible, and the primary object and camera must remain perfectly steady and fixed.

A better method may be to simply place a white screen well distanced behind the object and uniformly light that screen using two different colored lamps so as not produce any shadows from the object. With this technique you can switch to the other background color, without needing any physical change in the studio to take two photos with two different backgrounds.

These two color background techniques should work well for transparent objects, but reflections, and or background warping or 'lens' effects by the object being photographed will not be recorded by the technique, only its transparency. On the other hand reflections of a constant light source on the object will be preserved!

If you try this please let us know, and give an example of your source photos and results for inclusion here. You will be named with a pointer to your site for people to look at.
Video background recovery

If you have a large enough series of images with many different but complex backgrounds (such as a video), you can try taking a minimum and maximum value of all the images to generate a near pure black and white background image for use. The more images, the better this works.

With those two images any constant logo and its semi-transparency can be extracted, and the same technique can then be used to remove it from all the frames.

However it will only work for a constant semi-transparent overlay, and may not work for logos that use color or hue distortions, or even a solid color logo. But it will will at least let you determine the exact logo shape.

For logos that are fully opaque or more difficult, hole filling (see next) can then be used to fill in missing detail from surrounding colors.

See the IM Forum Discussion for more details.

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Hole Filling

While masking, adding transparency, and removing background provide one way of dealing with unwanted elements, often a 'hole' is not what you actually want as a result. Sure you can just overlay images with holes over other images to fill them, but that may not provide a seamless result.

To erase elements from an image, you don't just want to cut them out, but replace them with colors, shades, and textures of parts that surround the hole. And techniques for doing that is what we will look at here.