Adobe After Effects CS3 - Manual (Page 386)

AFTER EFFECTS CS3
User Guide
381
Threshold The greatest difference between adjacent pixels for which contrast isn’t adjusted. A lower value produces
a greater result. A value that is too low causes contrast for the entire image to be adjusted and can generate noise or
cause unexpected results.
Channel effects
Alpha Levels effect
Use Alpha Levels to convert completely opaque or completely transparent areas of a matte to be semitransparent, or
to convert semitransparent areas to be completely opaque or completely transparent. Because transparency is based
on the monochrome alpha channel, the controls for this effect refer to complete transparency as black and complete
opacity as white. This effect is very similar to the Levels effect, which performs the same operations on color channels
instead of the alpha channel.
• Use Output Black Level of 0 and Input Black Level greater than 0 to convert a range of semitransparent areas to be
completely transparent.
• Use Output White Level of 1.0 and Input White Level less than 1.0 to convert a range of semitransparent areas to
be completely opaque.
• Use Output Black Level greater than 0 to convert a range of completely transparent areas to be semitransparent.
• Use Output White Level less than 1.0 to convert a range of completely opaque areas to be semitransparent.
This effect works with 8-bpc and 16-bpc color.
Input Black Level Alpha channel values at this level or lower are mapped to the Output Black Level (maximum trans-
parency).
Input White Level Alpha channel values at this level or higher are mapped to the Output White Level (maximum
opacity).
Gamma Determines the distribution of intermediate (gray, semitransparent) values. A Gamma value of 1 produces
a linear distribution of gray values. Gamma less than 1 biases the distribution toward darker gray values (more trans-
parency). Gamma greater than 1 biases the distribution toward lighter gray values (less transparency).
See also
“Levels effect” on page 399
Arithmetic effect
The Arithmetic effect performs various simple mathematical operations on an image’s red, green, and blue channels.
This effect works with 8-bpc color.
Operator The operation to perform between the value you specify for each channel and the existing value of that
channel for each pixel in the image:
• And, Or, and Xor Apply bitwise logical operations.
• Add, Subtract, Multiply, and Difference Apply basic math functions.
• Max Set the pixel’s channel value to the greater of the specified value and the pixel’s original value.
• Min Set the pixel’s channel value to the lesser of the specified value and the pixel’s original value.