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                <h1>
                    Add and Subtract blending modes
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                <div><div class="section"><p>The Add blending mode is available only for
the Calculations command. The Subtract blending mode is available
only for the Apply Image and Calculations command. </p>
</div>
<div class="section"><h4 class="sectiontitle">Add</h4><p>Adds the pixel values in two channels.
This is a good way to combine non-overlapping images in two channels.</p>
<p>Because
higher pixel values represent lighter colors, adding channels with overlapping
pixels lightens the image. Black areas in both channels remain black (0
+ 0 = 0). White in either channel results in white (255 + any value
= 255 or greater).</p>
<p>Add
mode divides the sum of the pixel values by the Scale amount, and
then adds the Offset value to the sum. For example, to find the
average of the pixels in two channels, add them, divide by 2, and
enter no Offset value.</p>
<p>The Scale factor may be any number
between 1.000 and 2.000. Entering a higher Scale value darkens the
image. </p>
<p>The Offset value lets you lighten or darken the pixels
in the destination channel by any brightness value between +255
and –255. Negative values darken the image; positive values lighten
the image. </p>
</div>
<div class="section"><h4 class="sectiontitle">Subtract</h4><p>Subtracts
the pixel values in the source channel from the corresponding pixels
in the target channel. As with Add mode, the result is then divided
by the Scale factor and added to the Offset value.</p>
<p>The Scale
factor may be any number between 1.000 and 2.000. The Offset value lets
you lighten or darken the pixels in the destination channel by any
brightness value between +255 and –255.</p>
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