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Quad texture distortion?

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I'm drawing polygons in an orthogonal projection, but I'm experiencing some texture distortion when I try to draw a quadrilateral that isn't a parallelogram. For example: here's my test texture: Now here's what happens when I try to draw this texture as a trapezoid, using two triangles: I can see what's happening. When the first primitive is drawn, it doesn't know about the other one; so it just assumes that it's part of a parallelogram, and stretches the texture accordingly. I'm drawing the quad as a triangle strip, but it still distorts. My vector format is D3DFVF_XYZ | D3DFVF_TEX1. There must be some way of making the texture stretch according to each vertex of the quad. Do I need to add anything to my vertex format? Or is there some other special way of rendering the quad? Thanks!

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You need to use perspective correct texture mapping to solve this problem, which basically means setting the correct Z value for each vertex. Other than that, there is not much you can do, although you can try subdividing your quadrilateral: for instance, take the middle point and render the quadrilateral as four triangles instead of two, or divide each edge by two to get six triangles.

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There *is* a fix for this. I saw a paper on it once, which I can't seem to find right now. It involved always using the W coordinate so that a texture coordinate divide was happening per pixel instead of being baked into the UVs of each vertex.

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