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<title>ImageMagick: Alpha Compositing</title>
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<p class="lead magick-description">This page descibed the Image composition methods that is used to define how
two images should be merged together in various image operations. For the
Command Line API it is typically set using the <a
href="command-line-options.html#compose" >-compose</a> setting option. </p>
<p>The description of composition uses abstract terminology in order to allow
the description to be more precise, while avoiding constant values which are
specific to a particular build configuration. Each image pixel is represented
by red, green, and blue levels (which are equal for a gray pixel). The
build-dependent value <var>QuantumRange</var> is the maximum integral
value which may be stored, per pixel, in the red, green, or blue channels of
the image. Each image pixel may also optionally (if the image matte channel is
enabled) have an associated level of opacity, ranging from <var>opaque</var> to
<var>transparent</var>, which may be used to determine the influence of the pixel
color when compositing the pixel with another image pixel. If the image matte
channel is disabled, then all pixels in the image are treated as opaque. The
color of an opaque pixel is fully visible while the color of a transparent
pixel color is entirely absent (pixel color is ignored).</p>
<p>By definition, raster images have a rectangular shape. All image rows are of
equal length, as are all image columns. By treating the alpha channel as a
visual "mask" the rectangular image may be given a "shape" by treating the
alpha channel as a cookie-cutter for the image. This is done by setting the
pixels within the shape to be opaque, with pixels outside the shape set as
transparent. Pixels on the boundary of the shape may be between opaque and
transparent in order to provide antialiasing (visually smooth edges). The
description of the composition operators use this concept of image "shape" in
order to make the description of the operators easier to understand. While it
is convenient to describe the operators in terms of "shapes" they are by no
means limited to mask-style operations since they are based on continuous
floating-point mathematics rather than simple boolean operations.</p>
<p>The following alpha blending (Duff-Porter) compose methods are available:</p>
<table class="table table-condensed table-striped">
<tbody>
<tr>
<th align="left" style="width: 8%">Method</th>
<th align="left">Description</th>
</tr>
<tr>
<td>clear</td>
<td>Both the color and the alpha of the destination are
cleared. Neither the source nor the destination are used (except for
destinations size and other meta-data which is always preserved.</td>
</tr>
<tr>
<td>src</td>
<td>The source is copied to the destination. The destination
is not used as input, though it is cleared.</td>
</tr>
<tr>
<td>dst</td>
<td>The destination is left untouched. The source image is
completely ignored.</td>
</tr>
<tr>
<td>src-over</td>
<td>The source is composited over the destination. this is
the default alpha blending compose method, when neither the compose
setting is set, nor is set in the image meta-data.</td>
</tr>
<tr>
<td>dst-over</td>
<td>The destination is composited over the source and the
result replaces the destination.</td>
</tr>
<tr>
<td>src-in</td>
<td>The part of the source lying inside of the destination
replaces the destination.</td>
</tr>
<tr>
<td>dst-in</td>
<td>The part of the destination lying inside of the source
replaces the destination. Areas not overlaid are cleared.</td>
</tr>
<tr>
<td>src-out</td>
<td>The part of the source lying outside of the destination
replaces the destination.</td>
</tr>
<tr>
<td>dst-out</td>
<td>The part of the destination lying outside of the source
replaces the destination.</td>
</tr>
<tr>
<td>src-atop</td>
<td>The part of the source lying inside of the destination is
composited onto the destination.</td>
</tr>
<tr>
<td>dst-atop</td>
<td>The part of the destination lying inside of the source is
composited over the source and replaces the destination. Areas not
overlaid are cleared. </td>
</tr>
<tr>
<td>xor</td>
<td>The part of the source that lies outside of the
destination is combined with the part of the destination that lies
outside of the source. Source or Destination, but not both. </td>
</tr>
</tbody>
</table>
<p>Any of the 'Src-*' methods can also be specified without the 'Src-' part.
For example the default compose method can be specified as just 'Over'.</p>
<p>Many of these compose methods will clear the destination image which was
not overlaid by the source image. This is to be expected as part of that
specific composition methods defintion. You can disable this by setting the
special <a href="command-line-options.html#define"
>-define</a> 'compose:outside-overlay' to a value of 'false' will turn off
this behavior. </p>
<p>On top of the above 12 Duff-Porter Alpha Composition methods, one special
related method '<code>Copy</code>' has been provided. This is equivalent to
using the '<code>Src</code>' with the special <a href="command-line-options.html#define"
>-define</a> option '<code>compose:outside-overlay</code>' set to
'<code>false</code>', so as to only modify the overlaid area, without clearing
the rest of the image outside the overlaid area. </p>
<p>The following mathematical composition methods are also available. </p>
<table class="table table-condensed table-striped">
<tbody>
<tr>
<th align="left" style="width: 8%">Method</th>
<th align="left">Description</th>
</tr>
<tr>
<td>multiply</td>
<td>The source is multiplied by the destination and replaces
the destination. The resultant color is always at least as dark as
either of the two constituent colors. Multiplying any color with black
produces black. Multiplying any color with white leaves the original
color unchanged.</td>
</tr>
<tr>
<td>screen</td>
<td>The source and destination are complemented and then
multiplied and then replace the destination. The resultant color is
always at least as light as either of the two constituent colors.
Screening any color with white produces white. Screening any color
with black leaves the original color unchanged.</td>
</tr>
<tr>
<td>plus</td>
<td>The source is added to the destination and replaces the
destination. This operator is useful for averaging or a controlled
merger of two images, rather than a direct overlay.</td>
</tr>
<tr>
<td>add</td>
<td>As per 'plus' but transparency data is treated as matte
values. As such any transparent areas in either image remain
transparent. </td>
</tr>
<tr>
<td>minus</td>
<td>Subtract the colors in the source image from the
destination image. When transparency is involved, opaque areas is
subtracted from any destination opaque areas. </td>
</tr>
<tr>
<td>subtract</td>
<td>Subtract the colors in the source image from the
destination image. When transparency is involved transparent areas are
subtracted, so only the opaque areas in the source remain opaque in
the destination image. </td>
</tr>
<tr>
<td>difference</td>
<td>Subtracts the darker of the two constituent colors from
the lighter. Painting with white inverts the destination color.
Painting with black produces no change.</td>
</tr>
<tr>
<td>exclusion</td>
<td>Produces an effect similar to that of 'difference', but
appears as lower contrast. Painting with white inverts the
destination color. Painting with black produces no change.</td>
</tr>
<tr>
<td>darken</td>
<td>Selects the darker of the destination and source colors.
The destination is replaced with the source when the source is darker,
otherwise it is left unchanged.</td>
</tr>
<tr>
<td>lighten</td>
<td>Selects the lighter of the destination and source colors.
The destination is replaced with the source when the source is
lighter, otherwise it is left unchanged. </td>
</tr>
</tbody>
</table>
<p>Typically these use the default 'Over' alpha blending when transparencies
are also involved, except for 'Plus' which uses a 'plus' alpha blending. This
means the alpha channel of both images will only be used to ensure that any
visible input remains visible even in parts not overlaid. It also means that
any values are weighted by the alpha channel of the input and output images.
This 'Over' alpha blending is also applied to the lighting composition methods
below. </p>
<p>As of IM v6.6.1-6, if the special '<code>Sync</code>' flag is not specified
(enabled by default) with the <a href="command-line-options.html#channel"
>-channel</a> setting, then the above mathematical compositions will nolonger
synchronise its actions with the alpha channel. Instead the math composition
will be applied on an individual channel basis as defined by the <a
href="command-line-options.html#channel"
>-channel</a>. This includes the alpha channel. This special usage
allows you to perform true mathematics of the image channels, without alpha
composition effects, becoming involved. </p>
<p>This special flag is not applied to the lighting composition methods (see
below) even though they are closely related to mathematical composition
methods.</p>
<p>The following lighting composition methods are also available. </p>
<table class="table table-condensed table-striped">
<tbody>
<tr>
<th align="left" style="width: 8%">Method</th>
<th align="left">Description</th>
</tr>
<tr>
<td>linear-dodge</td>
<td>This is equivalent to 'Plus' in that the color channels
are simply added, however it does not 'Plus' the alpha channel, but
uses the normal 'Over' alpha blending, which transparencies are
involved. Produces a sort of additive multiply-like result. Added
ImageMagick version 6.5.4-3. </td>
</tr>
<tr>
<td>linear-burn</td>
<td>As 'Linear-Dodge', but also subtract one from the result.
Sort of a additive 'Screen' of the images. Added ImageMagick version
6.5.4-3. </td>
</tr>
<tr>
<td>color-dodge</td>
<td>Brightens the destination color to reflect the source
color. Painting with black produces no change.</td>
</tr>
<tr>
<td>color-burn</td>
<td>Darkens the destination color to reflect the source
color. Painting with white produces no change. Fixed in ImageMagick
version 6.5.4-3. </td>
</tr>
<tr>
<td>overlay</td>
<td>Multiplies or screens the colors, dependent on the
destination color. Source colors overlay the destination whilst
preserving its highlights and shadows. The destination color is not
replaced, but is mixed with the source color to reflect the lightness
or darkness of the destination.</td>
</tr>
<tr>
<td>hard-light</td>
<td>Multiplies or screens the colors, dependent on the source
color value. If the source color is lighter than 0.5, the destination
is lightened as if it were screened. If the source color is darker
than 0.5, the destination is darkened, as if it were multiplied. The
degree of lightening or darkening is proportional to the difference
between the source color and 0.5. If it is equal to 0.5 the
destination is unchanged. Painting with pure black or white produces
black or white.</td>
</tr>
<tr>
<td>linear-light</td>
<td>Like 'Hard-Light' but using linear-dodge and linear-burn
instead. Increases contrast slightly with an impact on the
foreground's tonal values.</td>
</tr>
<tr>
<td>soft-light</td>
<td>Darkens or lightens the colors, dependent on the source
color value. If the source color is lighter than 0.5, the destination
is lightened. If the source color is darker than 0.5, the destination
is darkened, as if it were burned in. The degree of darkening or
lightening is proportional to the difference between the source color
and 0.5. If it is equal to 0.5, the destination is unchanged. Painting
with pure black or white produces a distinctly darker or lighter area,
but does not result in pure black or white. Fixed in ImageMagick
version 6.5.4-3. </td>
</tr>
<tr>
<td>pegtop-light</td>
<td>Almost equivalent to 'Soft-Light', but using a
continuous mathematical formula rather than two conditionally
selected formulae. Added ImageMagick version 6.5.4-3. </td>
</tr>
<tr>
<td>vivid-light</td>
<td>A modified 'Linear-Light' designed to preserve very stong
primary and secondary colors in the image. Added ImageMagick version
6.5.4-3. </td>
</tr>
<tr>
<td>pin-light</td>
<td>Similar to 'Hard-Light', but using sharp linear shadings,
to simulate the effects of a strong 'pinhole' light source. Added
ImageMagick version 6.5.4-3. </td>
</tr>
</tbody>
</table>
<p>Also included are these special purpose compose methods:</p>
<table class="table table-condensed table-striped">
<tbody>
<tr>
<th align="left" style="width: 8%">Method</th>
<th align="left">Description</th>
</tr>
<tr>
<td>copy</td>
<td>This is equivalent to the Duff-Porter composition method
'<code>Src,</code>' but without clearing the parts of the destination
image that is not overlaid. </td>
</tr>
<tr>
<td>copy-*</td>
<td>Copy the specified channel (Red, Green, Blue, Cyan,
Magenta, Yellow, Black, or Opacity) in the source image to the
same channel in the destination image. If the channel specified
does not exist in the source image, (which can only happen for methods,
'<code>copy-opacity</code>' or '<code>copy-black</code>') then it is
assumed that the source image is a special grayscale channel image
of the values that is to be copied. </td>
</tr>
<tr>
<td>change-mask</td>
<td>Replace any destination pixel that is the similar to the
source images pixel (as defined by the current <a
href="command-line-options.html#fuzz">-fuzz</a> factor), with transparency.
</td>
</tr>
</tbody>
</table>
<p>On top of these composed methods are a few special ones that not only require
the two images that are being merged or overlaid, but have some extra numerical
arguments, which are tabled below. </p>
<p>In the "<code>composite</code>" command these composition methods are
selected using special options with the arguments needed. They are usually,
but not always, the same name as the composite 'method' they use, and replaces
the normal use of the <a href="command-line-options.html#compose" >-compose</a>
setting in the "<code>composite</code>" command. For example... </p>
<pre>
composite ... -blend 50x50 ...
</pre>
<p>As of IM v6.5.3-4 the "<code>convert</code>" command can now also supply
these extra arguments to its <a href="command-line-options.html#composite"
>-composite</a> operator, using the special <a href="command-line-options.html#define">-define</a>
attribute of '<code class="arg">compose:args</code>'. This means you can now
make use of these special augmented <a href="command-line-options.html#compose"
>-compose</a> methods, those the argument and the method both need to be set
separately. For example... </p>
<pre>
convert ... -compose blend -define compose:args=50,50 -composite ...
</pre>
<p>The following is a table of these special 'argumented' compose methods,
with a brief summary of what they do. For more details see the equivalent
"composite" command option name. </p>
<table class="table table-condensed table-striped">
<tbody>
<tr>
<th align="left" style="width: 8%">Method</th>
<th align="left">Description</th>
</tr>
<tr>
<td>dissolve</td>
<td>Arguments:
<var>src_percent</var>[x<var>dst_percent</var>]
<br/>Equivalent to "<code>composite</code>" <a href="command-line-options.html#dissolve">-dissolve</a>
<br/>Dissolve the 'source' image by the percentage given before overlaying
'over' the 'destination' image. If <var>src_percent</var> is
greater than 100, it starts dissolving the main image so it will
become transparent at a value of '<code class="arg">200</code>'. If
both percentages are given, each image are dissolved to the
percentages given.
</td>
</tr>
<tr>
<td>blend</td>
<td>Arguments:
<var>src_percent</var>[x<var>dst_percent</var>]
<br/>Equivalent to "<code>composite</code>" <a
href="command-line-options.html#blend">-blend</a>
<br/>Average the images together ('plus') according to the percentages
given and each pixels transparency. If only a single percentage value
is given it sets the weight of the composite or 'source' image, while
the background image is weighted by the exact opposite amount. That is
a <code>-blend 30</code> merges 30% of the 'source' image with 70% of
the 'destination' image. Thus it is equivalent to <code>-blend
30x70</code>.
</td>
</tr>
<tr>
<td>mathematics</td>
<td>Arguments: <var>A, B, C, D</var>
<br/>Not available in "<code>composite</code>" at this time.
<br/>Merge the source and destination images according to the formula
<br/> <code>A*Sc*Dc + B*Sc + C*Dc + D</code>
<br/>Can be used to generate a custom composition method that would
otherwise need to be implemented using the slow <a
href="command-line-options.html#fx">-fx</a> DIY image operator. Added
to ImageMagick version 6.5.4-3.
<br/>As of IM v6.6.1-6 this method will do per-channel math compositions
if the 'Sync' flag is removed from <a
href="command-line-options.html#channel" >-channel</a>, just like all
the other mathematical composition methods above.
</td>
</tr>
<tr>
<td>modulate</td>
<td>Arguments:
<var>brightness</var>[x<var>saturation</var>]
<br/>Equivalent to "<code>composite</code>" <a href="command-line-options.html#watermark">-watermark</a>
<br/>Take a grayscale image (with alpha mask) and modify the destination
image's brightness according to watermark image's grayscale value and
the <var>brightness</var> percentage. The destinations
color saturation attribute is just direct modified by the <var>saturation</var> percentage, which defaults to 100 percent
(no color change).
</td>
</tr>
<tr>
<td>displace</td>
<td>Arguments:
<var>X-scale</var>[x<var>Y-scale</var>][!][%]
<br/>Equivalent to "<code>composite</code>" <a href="command-line-options.html#displace">-displace</a>
<br/>With this option, the 'overlay' image, and optionally the 'mask'
image, is used as a relative displacement map, which is used to
displace the lookup of what part of the destination image is seen at
each point of the overlaid area. Much like the displacement map is a
'lens' that distorts the original 'background' image behind it.
<br/><br/>
The X-scale is modulated by the 'red' channel of the overlay image
while the Y-scale is modulated by the green channel, (the mask image
if given is rolled into green channel of the overlay image. This
separation allows you to modulate the X and Y lookup displacement
separately allowing you to do 2-dimensional displacements, rather
than 1-dimensional vectored displacements (using grayscale image).
<br/><br/>
If the overlay image contains transparency this is used as a mask
of the resulting image to remove 'invalid' pixels.
<br/><br/>
The '%' flag makes the displacement scale relative to the size of the
overlay image (100% = half width/height of image). Using '!' switches
percentage arguments to refer to the destination image size instead.
<br/><br/>
Special flags were added Added to ImageMagick version 6.5.3-5.
</td>
</tr>
<tr>
<td>distort</td>
<td>Arguments:
<var>X-scale</var>[x<var>Y-scale</var>[+<var>X-center</var>+<var>Y-center</var>]][!][%]
<br/>Not available in "<code>composite</code>" at this time.
<br/>Exactly as per 'Displace' (above), but using absolute coordinates,
relative to the center of the overlay (or that given). Basically
allows you to generate absolute distortion maps where 'black' will
look up the left/top edge, and 'white' looks up the bottom/right
edge of the destination image, according to the scale given.
<br/><br/>
The '!' flag not only switches percentage scaling, to use the
destination image, but also the image the center offset of the lookup.
This means the overlay can lookup a completely different region of the
destination image.
<br/><br/>
Added to ImageMagick version 6.5.3-5.
</td>
</tr>
<tr>
<td>blur</td>
<td>Arguments:
<var>Width</var>[x<var>Height</var>[+<var>Angle</var>][+<var>Angle2</var>]]
<br/>Equivalent to "<code>composite</code>" <a href="command-line-options.html#blur-composite">-blur</a>
<br/>A Variable Blur Mapping Composition method, where each pixel in the
overlaid region is replaced with an Elliptical Weighted Average (EWA),
with an ellipse (typically a circle) of the given sigma size, scaled
according to overlay (source image) grayscale mapping.
<br/><br/>
As per 'Displace' and 'Distort', the red channel will modulate the
width of the ellipse, while the green channel will modulate the height
of the ellipse. If a single Angle value is given in the arguments,
then the ellipse will then be rotated by the angle specified.
<br/><br/>
Normally the blue channel of the mapping overlay image is ignored.
However if a second ellipse angle is given, then it is assumed that
the blue channel defines a variable angle for the ellipse ranging from
the first angle to the second angle given. This allows to generate
radial blurs, or a rough approximation for rotational blur. Or any mix
of the two.
<br/><br/>
Added to ImageMagick version 6.5.4-0.
</td>
</tr>
</tbody>
</table>
<p>To print a complete list of all the available compose operators, use <a
href="command-line-options.html#list">-list compose</a>.</p>
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