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POtexture mapping a trapezoid with a square texture in OpenGL
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  1. POtexture mapping a trapezoid with a square texture in OpenGL
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    copied!<p>I've been trying to render a GL_QUAD (which is shaped as a trapezoid) with a square texture. I'd like to try and use OpenGL only to pull this off. Right now the texture is getting heavily distorted and it's really annoying.</p> <p>Normally, I would load the texture compute a homography but that means a lot of work and an additional linear programming library/direct linear transform function. I'm under the impression OpenGL can simplify this process for me.</p> <p>I've looked around the web and have seen <a href="http://hacksoflife.blogspot.com/2009/11/perspective-correct-texturing-q.html" rel="nofollow">"Perspective-Correct Texturing, Q Coordinates, and GLSL"</a> and <a href="http://www.gamedev.net/topic/419296-skewedsheared-texture-mapping-in-opengl" rel="nofollow">"Skewed/Sheared Texture Mapping in OpenGL"</a>.</p> <p>These all seem to assume you'll do some type of homography computation or use some parts of OpenGL I'm ignorant of ... any advice?</p> <p><strong>Update:</strong></p> <p>I've been reading <a href="http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.1.9091" rel="nofollow">"Navigating Static Environments Using Image-Space Simplification and Morphing"</a> [<a href="http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.1.9091&amp;rep=rep1&amp;type=pdf" rel="nofollow">PDF</a>] - page 9 appendix A.</p> <p>It looks like they disable perspective correction by multiplying the (s,t,r,q) texture coordinate with the vertex of a model's world space z component.</p> <p>so for a given texture coordinate (s, r, t, q) for a quad that's shaped as a trapezoid, where the 4 components are:</p> <pre><code>(0.0f, 0.0f, 0.0f, 1.0f), (0.0f, 1.0f, 0.0f, 1.0f), (1.0f, 1.0f, 0.0f, 1.0f), (1.0f, 0.0f, 0.0f, 1.0f) </code></pre> <p>This is as easy as glTexCoord4f (s<em>vert.z, r</em>vert.z, t, q*vert.z)? Or am I missing some step? like messing with the GL_TEXTURE glMatrixMode?</p> <p><strong>Update #2:</strong></p> <p>That did the trick! Keep it in mind folks, this problem is all over the web and there weren't any easy answers. Most involved directly recalculating the texture with a homography between the original shape and the transformed shape...aka lots of linear algebra and an external BLAS lib dependency.</p>
 

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