All appears well in texture mapping a quad checkerboard with four points to a rectangle using GL_LINEAR. (e.g. the checkerboard sample in the Red Book.) The OpenGL pipeline tessellates the GL_QUAD to two triangles under the hood but this is not seen. Interestingly, when the rectangle is converted to a trapezoid by moving one vertex, the result is poor. The tesselation is done in an implementation specific way and generates a diagonally appearing artifact or streak. That is, the trapezoid’s diagonal artifact is apparent in one of two possible directions depending on the board: Wildcat, nVidia Quadro, HP fx6, Matrix G400 etc., but is always there.
Aside from tessellating this myself, are there any suggestions as to generating a better texture mapped result to the trapezoid? It would be helpful if all 4 points were included in a bilinear interpolation so the texture appeared smooth. Are their any nVidia extensions?
[This message has been edited by iss (edited 06-02-2001).]