Apple Color 1.0 - Manual (Page 245)

Chapter 11 Color FX 245
Math Layering Nodes Explained
The layering nodes shown in the previous example use simple math to combine two
differently modified versions of the image together. These mathematical operations
rely on the following numerical method of representing tonality in each of the three
color channels of an image:
 Black = 0 (so black for RGB = 0, 0, 0)
 Midtone values in each channel are fractional, from .00001 through .999999
 White = 1 (so white for RGB = 1, 1, 1)
Bear these values in mind when you read the following sections.
Add
The pixels from each input image are added together. Black pixels have a value of 0, so
black added to any other color results in no change to the image. All other values are
raised by the sum of both values. The order in which the inputs are connected doesn’t
matter.
Add operations are particularly well suited to creating aggressive glowing effects,
because they tend to raise levels very quickly depending on the input images. Bear in
mind that the best way of controlling which areas of the image are being affected
when using an Add operation is to aggressively control the contrast of one of the input
images. The darker an area is, the less effect it will have.
Note: By default, the Bias parameters of the Add node divide each input image’s values
by half before adding them together. If the results are not as vivid as you were hoping
for, change the Source 1 and Source 2 Bias parameters to 1.
Difference
The pixels from the image that’s connected to Source 1 are subtracted from the pixels
from the image that’s connected to Source 2. Black pixels have a value of 0, so any
color minus black results in no change to the Source 1 image. The order in which the
inputs are connected matters.
This node is useful for darkening the Source 1 image based on the brightness of the
Source 2 image.
Multiply
The pixels from each input image are multiplied together. White pixels have a value of
1, so white multiplied with any other color results in no change to the other image.
However, when black (0) is multiplied with any other color, the result is black.
When multiplying two images, the darkest parts of the images remain unaffected,
while the lightest parts of the image are the most affected. This is useful for tinting
operations, as seen previously, as well as for operations where you want to combine
the darkest portions of two images.