User guide

194
Control (UI) Default Value Function
operation (continued) The available operations (based on the Porter & Duff paper) are:
• ImageAtop - Fb+B(1-f). This shows the shape of the
background, with the foreground covering the background
where the images overlap.
• ImageAverage - (F+B)/2. This produces the average of the
two images. The result is darker than the original images, but
accentuates highlights.
• ImageDifference - abs(F-B). This shows how much the pixels
differ and is useful for comparing two very similar images.
• ImageDivide - B/F. This divides the background values by the
foreground values.
• ImageExclusion - F+B-2FB. This is a more photographic form
of ImageDifference.
• ImageFrom - F-B. This subtracts the background from the
foreground. For subtracting the foreground from the
background instead, see ImageMinus.
• ImageGeometric - 2FB/(F+B). This is another way of
averaging two images. Visually, it's close to ImageMin.
• ImageHypot - sqrt(F*F+B*B). This resembles the ImageAdd
and ImageScreen operations. The result is not as bright as
ImageAdd, but brighter than ImageScreen. ImageHypot
works with values above 1. It can be is useful for adding
reflections, as an alternative to ImageScreen.
• ImageIn - Bf. This only shows the areas of the background
that overlap with the alpha of the foreground. It can be useful
for combining mattes.
• ImageMatte - Ff*B(1-f). This is a premultiplied ImageOver.
Use unpremultiplied images with this operation.
• ImageMax - max(F,B). This takes the maximum values of both
images. This is a good way to combine mattes and useful for
bringing aspects like bright hair detail through.
4 NODES I-K | IMAGEHYPOT