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POOpenGL ES 1.1 on Android: Q texture coordinate not having any effect
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copied!<p>I'm developing an OpenGL ES 1.1 app for Android (with the OpenGL calls being made from C code called via NDK). </p> <p>My problem is that I'm trying to use the Q texture coordinate as well as the usual (S, T) texture coordinates. However, nothing I put into Q has any noticable effect. </p> <p>(Aside: the reason for wanting to play with the Q coordinate is that I want to map a rectangular texture to a trapezoid quad in space with correct perspective effect (i.e. not affine) -- as described at <a href="http://www.xyzw.us/~cass/qcoord/" rel="nofollow noreferrer">http://www.xyzw.us/~cass/qcoord/</a>). </p> <p>My method for trying to set the Q coordinate (to something other than the default 1.0f) is the usual: I'm providing 4 texture coordinates instead of the usual 2. I've tried putting all sorts of values in the Q coord (and even into the typically unused R coord, in case of mixup between the two) with zero apparent effect on my textures. I would expect to see effects on my textures from having Q != 1.0 -- for example, Q = 0.5 for every texture coordinate should just double every derived (S, T) value -- in effect, halving the apparent texture sizes. But no change.</p> <p>I can't really post full code, but here's the fundamentals:</p> <p>My vertex struct is defined as follows:</p> <pre><code>typedef struct { GLfloat x, y, z; // 3 x spacial coord - 12 bytes GLfloat tx, ty, tr, tq; // 4 x texture coord - 16 bytes GLfloat padding[1]; // make it up to 32 bytes (or 8 floats, in effect) } VertexData; </code></pre> <p>Creating each vertex buffer object (VBO):</p> <pre><code>// alloc space for my vertex buffer int vertexBufferSize = numVertices * sizeof(VertexData); VertexData* vertexData = (VertexData *)malloc(vertexBufferSize); ..//for each vertex I'm creating: vertex.x = (... appropriate coord) vertex.y = (... appropriate coord) vertex.z = (... appropriate coord) vertex.tx = (... appropriate coord) // AKA the 's' texture coord vertex.ty = (... appropriate coord) // AKA the 't' texture coord vertex.tr = 0.5f; // 'R' texture coord. Usually ignored, but I set in case of mixup between Q and R vertex.tq = 0.5f; // 'Q' texture coord </code></pre> <p>And here's how I give my vertex and text coord pointers:</p> <pre><code>glVertexPointer(3, GL_FLOAT, stride, (GLvoid*)((char*)NULL)); // note the 4 below - we want to give 4 tex coordinates, not 2. // 3*4 corresponds to the offset of texture coords in the VertexData struct glTexCoordPointer(4, GL_FLOAT, 32 /*stride*/, (GLvoid*)((char*)NULL + 3*4)); </code></pre> <p>It's all working beautifully, except as I say, my Q coord makes no difference. If change my <code>glTextCoordPointer</code> call to only provide 2 or 3 coordinates, there is no change in the program behaviour:</p> <pre><code>glTexCoordPointer(2, GL_FLOAT, 32 /*stride*/, (GLvoid*)((char*)NULL + 3*4)); // OR: glTexCoordPointer(3, GL_FLOAT, 32 /*stride*/, (GLvoid*)((char*)NULL + 3*4)); </code></pre> <p><strong>My question is: why are my Q texture coordinates not having any effect? If it's device or Android API level specific, can I find out on what device or API levels would it work? If I can't rely on Q coordinate doing anything useful, how else could I achieve the same effect (realistic, non-affine texture mapping to a trapezoid)?</strong></p> <p>I am testing my code on an actual Android device: the HTC Wildfire S. </p> <p>I am loading my textures using the <a href="http://www.lonesock.net/soil.html" rel="nofollow noreferrer">SOIL library</a> for convenience.</p> <p><strong>Update</strong></p> <p>I've downloaded the <a href="http://insanitydesign.com/wp/projects/nehe-android-ports/" rel="nofollow noreferrer">NeHe Android port for Lesson 6</a>. This uses the Java OpenGL ES interface (no NDK). I added random R and Q texture coordinates to that, and again, no visible effect.</p> <p><strong>Update 2</strong></p> <p>I gave up on attempting to get the Q texture coordinate to have any effect; I presume it's just not supported on all devices. Still interested in authoritative info to this effect.</p> <p>Related links:</p> <p><a href="http://www.xyzw.us/~cass/qcoord/" rel="nofollow noreferrer">http://www.xyzw.us/~cass/qcoord/</a></p> <p><a href="http://www.gamedev.net/topic/419296-skewedsheared-texture-mapping-in-opengl" rel="nofollow noreferrer">http://www.gamedev.net/topic/419296-skewedsheared-texture-mapping-in-opengl</a></p> <p><a href="http://hacksoflife.blogspot.com/2009/11/perspective-correct-texturing-q.html" rel="nofollow noreferrer">http://hacksoflife.blogspot.com/2009/11/perspective-correct-texturing-q.html</a></p> <p><a href="https://stackoverflow.com/questions/5424078/perspective-correction-of-texture-coordinates-in-3d">perspective correction of texture coordinates in 3d</a></p> <p><a href="http://sfml-dev.org/forum/viewtopic.php?t=2893" rel="nofollow noreferrer">http://sfml-dev.org/forum/viewtopic.php?t=2893</a></p> <p><a href="http://fly.cc.fer.hr/~unreal/theredbook/chapter09.html" rel="nofollow noreferrer">http://fly.cc.fer.hr/~unreal/theredbook/chapter09.html</a></p>

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