Graphics Reference
In-Depth Information
Coverage. The coordinate system is always defined over the entire image.
These properties of the st coordinate system are true independent of the resolution
of the image. For example,
Position in an image
st coordinate value
top-left corner
(0.0, 0.0)
bottom-left corner
(0.0, 1.0)
middle
(0.5, 0.5)
xy=(0,1)
uv=(0,0)
xy=(1,1)
uv=(1,0)
top-right corner
(1.0, 0.0)
V a
V d
bottom-right corner
(1.0, 1.0)
y
In this way, depending on the resolution of the texture image, the same st coordi-
nate position may identify very different texel positions. For example, indepen-
dent of the resolution of a texture image, st
rectangle
primitive
=(
0
.
5
,
0
.
5
)
indentifies the texel in the
center: texel
(
5
,
5
)
for an 11
×
11 pixel image; or texel
(
50
,
50
)
for a 101
×
101
x
pixel image; or texel
(
50
,
5
)
for a 101
×
11 pixel image.
xy=(0,0)
uv=(0,1)
xy=(1,0)
uv=(1,1)
V b
V c
The Explicit uv Texture Coordinate System
We, as the programmers, are responsible for defining and assigning uv texture co-
ordinate values to each vertex position on the primitives. Figure 12.14 shows an
example of texture coordinate assignments at the vertices of a rectangle primitive.
We observe that in addition to the xy geometric position, each vertex also has a
uv texture position. These two coordinate positions identify points in two com-
pletely independent domains, and yet together they serve a complementary role
in computing final colors for the pixels: the xy geometric positions assist in deter-
mining which of the pixels are covered, whereas the uv texture positions assist in
determining the color for these pixels. The process of defining and assigning uv
texture coordinate values to vertex positions is referred to as texture placement.
In general, high-quality texture placement is a laborious process involving highly
experienced artists spending long hours manipulating the uv values at individual
vertex positions to accomplish specific artistic effects.
Figure 12.14.
The uv
coordinate system.
12.3.3
Texel Value Look-up
As illustrated in Figure 12.15, during rendering, for each pixel inside a primitive,
the following happens.
• Compute uv coordinate values based on interpolation from the surrounding
vertices.
• Reference st coordinate with the computed uv values to identify a texel.
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