Photoshop offers more
than one type of layer. Some of
them are specially endowed to aid
specific types of editing, such as
changing colors, filling areas with
gradients, or creating shadows. The layer
here lets us create a shadow for the frog so
that it looks more natural in its new setting.4
Separately, we have
four images that look like these.
Because few graphics formats
support layers when we save the edited
image, we tell Photoshop to flattenall
the layers into a single image. Photoshop
uses a technique called Z-bufferingby
giving each pixel a Z value that indicates its
third dimensional value in the composite photo.
Based on the order in which the layers are
arranged, the Z values identify which pixels would
be covered by pixels in other layers. Photoshop
eliminates all the pixels it decides would be
obscured and replaces them
with the topmost pixel for that
location. This is the result.5
CHAPTER 9 HOW SOFTWARE CHANGES PIXELS BY THE NUMBERS^161