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OpenGL UV mapping problem

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Hi there. I have an interesting problem with uv mapping In 2D graphics. If you can take a look @ these screens http://www.codeminion.com/codeminion/files/bad.JPG http://www.codeminion.com/codeminion/files/good.JPG you will see the problem right away. The only difference is that the first quad is a rectangular figure and the second one has the two upper vertices moved a bit to the middle it seems like a uv is the mapping problem. The graphics are rendered using DirectX draw primitive, I also tried this on OpenGL and I’m having the same results. It only happens when the quad is not rectangular figure. Both directx and opengl have orthogonal projection set. So the question is if there is a way to properly stretch the texture on the quad ? Thanks in advance Jerry

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Hi Jerry,

Check this page by Cass Everitt for more details. Basically this problem appears because the geometry has different representations in object and texture space (quad vs trapezoid), and it has to do with the way quads are rendered using graphics hardware.

You can either use the solution proposed by Everitt (calculate a suitable q-coord for the vertices) or you can use this approach : Correcting Texture Mapping Errors Introduced by Graphics Hardware.

The problem with the first method is that in some situations the number of times a texture is tiled can be different from what you wanted it to be. E.g. it may be repeated more times in the V direction because of the "pespective" projection. The problem with the second method is that it may need too many subdivisions in order to look correct.

Hope that helps.

HellRaiZer

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