Directx vertex rendering: unable to get texture to display correctly for trapetzoids

Question

I'm trying to create 3d effect using vertex and index buffers in 2d (z-coord is 0) using DirectX7.

It's easier to explain with a picture:

The problem is that the lines are broken. They should be straight. To render this image it gets break up in triangles and rendered using DrawIndexedPrimitiveVB. Obviously each of the triangle is skewed a little differently and I don't see why.

Am I missing something trivial here?

I'm not sure if this will help, but the source and destination quads are as follow:

    SPoint4:= pBounds4(1, 1, W - 2, H - 2);
    DPoint4:= Point4(ProjTo2dX(i, FlyDist + DeepDist, W), ProjTo2dY(0, FlyDist + DeepDist, H), ProjTo2dX(W - i, FlyDist, W), ProjTo2dY(0, FlyDist, H), ProjTo2dX(W - i, FlyDist, W), ProjTo2dY(H, FlyDist, H), ProjTo2dX(i, FlyDist + DeepDist, W), ProjTo2dY(H, FlyDist + DeepDist, H));

Answer 1

I found a solution or at least a workaround. Instead of breaking the image up in 2 triangles, I break it up in many (several horizontal strips, each consisting of 2 triangles). In this case the image looks ok.

in this case the image is split in 10 strips (20 triangles).

I'll be happy for any comments or other solutions. Thank you.

Answer 2

You need to provide some perspective information to have a proper texture coordinates interpolation on a trapezoid, see

Problems with texture deformation in OpenGL ES 1.1 on quad made out of triangle strips

Answer 3

One way to map a square/rectangular texture to an arbitrary quad is projective interpolation. I've written an article showing how to do this (using vertex/pixel shaders).

The short version: you interpolate UVs across the quad in a way analogous to how GPUs do it for perspective-correct rendering (which, as you may have noticed, does not produce a visible seam between the two triangles). To do this, you need to calculate a false "depth" value for each vertex of the quad, and do the interpolation using homogeneous coordinates based on this "depth". Full details are in the article linked above.