Multiply

Multiply

Multiply is another mode whose math is as elementary as it
sounds; it uses the formula
newPixel = A * B
Keep in mind that this formula uses color values between
0 and 1 to correspond to the colors on your monitor.
Multiplying two images together, therefore, actually has
the effect of reducing midrange pixels and darkening an
image overall, although pixels that are full white in both
images remain full white.
Multiply or Add has the inverse effect of Screen mode,
darkening the midrange values of one image with another.
It emphasizes dark tones in the foreground without replacing
the lighter tones in the background, useful for creating
texture, shadow, or dark fog .

Overlay and the Light Modes

Overlay uses Screen or Multiply, depending on the background
pixel value. Above a threshold of 50% gray (or .5
in normalized terms), Screen occurs, and below the threshold,
Multiply. Hard Light operates similarly, instead using
the top layer to determine whether to screen or multiply,
so the two are inverse effects.
These modes, along with Linear and Vivid Light, can be
most useful for combining a layer that is predominantly
color with another layer that is predominantly luminance,
or contrast detail (Figure 3.30). Much of the lava texturing
in the Level 4 sequence of Spy Kids 3-D was created by using
Hard Light to combine a hand-painted color heat map
with moving fractal noise patterns.

Difference

Difference inverts a background pixel in proportion to the
foreground pixel. It can help you line up two identical layers,
which is handy while working even if you rarely use it
for fi nal output.