[Algorithms] 2D in a 3D world

[Jason H. <jh...@st...>]

I've been trying to resolve an issue where full screen textures being 
drawn through the 3D system appear blurry.  I feel like I'm probably not 
considering something.  Any ideas would be welcome!

- I'm using an orthographic projection matrix along Z, a view matrix 
that is pretty much identity except for some negations on various axes 
to correct for LH/RH differences in the engine.
- The quad is mapped to the full screen both for vertex positions and 
UVs (0,0,1) to (1,1,1), since the ortho matrix is 0..1, not in screen 
pixels.
- The texture has the exactly the same number of pixels in both 
directions as the back buffer.
- Back buffer and window match resolution exactly.
- Resolution is 1024x600 (but problem exists in other resolutions)
- Texture sampling is bilinear or anisotropic (shouldn't matter).  Both 
show the problem.  Sometimes my 2D stuff needs to cleanly scale, so I 
can't force point sampling globally.  Texture matrix is disabled.

Steps I've taken to debug:
- I made a simple texture with regions that are alternating white and 
black columns and rows that are 1, 2, and 4 pixels wide, so I could see 
what was happening with texture sampling.  The result is an even gray in 
the 1-pixel wide areas, but can be made brighter and darker depending on 
the offset I put into the matrices.  Half-pixel adjustment didn't fix 
it.  I never get white or black lines.
- I checked to make sure that the test image wasn't being munged by the 
texture processing tools and it's perfectly clean (no GIGO).

I can understand not getting perfect rasterization vertically due to the 
height being a non-power-of-two, but I expected columns to be cleanly 
mapped.  What else should I be looking for?

Most appreciated,
JH

Jason Hughes
President
Steel Penny Games
Austin, TX