TVP4020
Programmers Reference Manual
Graphics Programming
85
5.7
Texture Address Unit
The Texture Address unit calculates the address of the texel that maps
to the current fragment XY position. Perspective correction can be
applied as part of the operation.
The texture coordinates are referred to as S and T where S is analogous
to X and T to Y. The S and T values are generated by interpolation; a
third component, Q, may also be interpolated and is used in perspective
correction.
5.7.1
Texture Interpolation
The DDA units perform linear interpolation given a set of start and
increment values.
P
ERMEDIA
interpolates from the dominant edge of a trapezoid to the
subordinate edges. This means that two increment values are required
per texture component, one to move along the dominant edge and one
to move across the span to the subordinate edge. This is illustrated, for
the S component, in the diagram below:
dS dyD om
dS dX
Subordinate Edges
D ominant Edge
SStart = Initial S value
dSdyDom = S gradient in the Y direction along the dominant edge
dSdx = S gradient in the X direction
Figure 5.21
Texture Address Interpolation
The calculation for the delta values is the same as other parameters
such as depth values see Appendix D6.
If perspective correction is not enabled then the S and T values are the
texture coordinates of the appropriate vertex. If perspective correction is
enabled the texture coordinates are divided by the homogenous
coordinate W, and Q is formed from 1/W. S and T are then normalized
with respect to Q so that Q lies in the range 1 to 1/127. These values are
then used to calculate delta values in the same way as color or depth. If
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