[size="5"]What
Sub-texel accuracy is used to clean up texturing quality, and solve other accuracy problems (such as wrap-around in textures, and overflow for Gouraud shading).
This document relies on the fact that you have already read WWH1 (sub-pixel accuracy) as they are interrelated, and especially since you can't achieve sub-texel accuracy without first achieving sub-pixel accuracy.
Considering what we learned from WWH1, sub-texel accuracy is a very simple extension of sub-pixel accuracy. In the same way that we need to "slide" polygons across the screen to align with pixel boundaries, sub-texel accuracy simply "slides" the texture across the polygon to align to the polygon edges.
[size="5"]Why
For better looking renders, of course. (See WWH1 for more details).
[size="5"]How
Considering what we learned about sub-pixel accuracy from WWH1, here's what we need to do to achieve sub-texel accuracy.
In our polygon scan-conversion model explained in WWH1, we "slide" our pixels down the top of each edge. For sub-texel accuracy, we'll need to "slide" our texture at the beginning of each scan-line. Due to the frequency at which this code gets called (once per scan-line vs. once per edge) this code may cause a noticeable speed hit, however, with pipeline optimizations that speed-hit can be minimized.
Consider the following code fragment:
start_u += delta_u * SUB_PIX(start_x); start_v += delta_v * SUB_PIX(start_x); What this does is to simply "slide" the starting U/V coordinates along its scan-line by the distance that the starting X is from the right-edge of its pixel.
This process must be done for each value that gets interpolated along the edges (i.e. any U/V for texturing or Gouraud color values, etc.)
This is sub-texel accuracy. To test your modifications, try rotating a quad polygon perpendicular to the screen very slowly, with a checkerboard pattern textured onto it.
