Graphics Reference
In-Depth Information
each pixel location. This requires that an extra row and extra column of pixels be kept in order to
provide the
z
-values for the pixels in the rightmost row and bottommost column of the image; the
rgb
and
a
values for these pixels in the extra row and column are never used.
To compute
c ¼ f
comp
b
at a pixel, one must first compute the
z
-values at the corners to see which
is larger. There are 2
4
16 possible outcomes of the four corner comparisons. If the comparisons
are not the same at all four corners, the pixel is referred to as
confused
. This means that within this
single pixel, the layers cross each other in
z.
For any edge of the pixel whose endpoints compare
differently, the
z
-values are interpolated to estimate where along the edge the surfaces actually meet
in
z.
Figure A.8
illustrates the implied division of a pixel into areas of coverage based on the relative
z
-values at the corners of the foreground and background pixels.
¼
f
f
f
f
b
b
b
A
B
C
D
b
f
b
f
b
f
b
f
f
b
E
F
G
H
b
b
f
f
b
f
f
b
f
b
I
J
K
L
b
b
b
b
f
f
f
M
N
O
P
FIGURE A.8
Categories of pixels based on
z
comparisons at the corners; the label at each corner is sign(
z
f
-
z
b
).
Search WWH ::
Custom Search