User Guide

GNU Image Manipulation Program
107 / 653
Figure 8.11: Example for layer mode ‘Divide’
(a) Mask 1 is used as upper layer with 100% opac-
ity.
(b) Mask 2 is used as upper layer with 100% opac-
ity.
Divide mode multiplies each pixel value in the lower layer by 256 and then divides that by the corresponding pixel value
of the upper layer plus one. (Adding one to the denominator avoids dividing by zero.) The resulting image is often lighter,
and sometimes looks ‘burned out’.
The equation is:
E = 256
I
M+1
EQUATION 8.4: Equation for layer mode Divide
Screen
Figure 8.12: Example for layer mode ‘Screen’
(a) Mask 1 is used as upper layer with 100% opac-
ity.
(b) Mask 2 is used as upper layer with 100% opac-
ity.
Screen mode inverts the values of each of the visible pixels in the two layers of the image. (That is, it subtracts each of them
from 255.) Then it multiplies them together, inverts this value again and divides by 255. The resulting image is usually
brighter, and sometimes ‘washed out’ in appearance. The exceptions to this are a black layer, which does not change the
other layer, and a white layer, which results in a white image. Darker colors in the image appear to be more transparent.
The equation is: