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POOpenGL: Single vertex attribute for multiple vertices?
 

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  1. POOpenGL: Single vertex attribute for multiple vertices?
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    <p>I have a vertex shader that accepts the following attributes:</p> <ul> <li><code>a_posCoord</code>: vertex position</li> <li><code>a_texCoord</code>: texture coordinate (passed to the fragment shader)</li> <li><code>a_alpha</code>: transparency factor (passed to the fragment shader)</li> </ul> <p>The objects I'm rendering are all "billboards" (a pair of right triangles to make a rectangle).</p> <p>I'm using a single call to <code>glDrawArrays</code> to render <strong>many</strong> billboards, each which may have a unique alpha value. A single billboard has 6 vertices. Here's some pseudocode to illustrate how I organize the vertex attribute buffer for a single billboard:</p> <pre><code>vertexAttributes = [ px1,py1,pz1, // vertex 1: a_posCoord tx1,ty1, // vertex 1: a_texCoord alpha, // vertex 1: a_alpha px2,py2,pz2, // vertex 2: a_posCoord tx2,ty2, // vertex 2: a_texCoord alpha, // vertex 2: a_alpha px3,py3,pz3, // vertex 3: a_posCoord tx3,ty3, // vertex 3: a_texCoord alpha, // vertex 3: a_alpha px4,py4,pz4, // vertex 4: a_posCoord tx4,ty4, // vertex 4: a_texCoord alpha, // vertex 4: a_alpha px5,py5,pz5, // vertex 5: a_posCoord tx5,ty5, // vertex 5: a_texCoord alpha, // vertex 5: a_alpha px6,py6,pz6, // vertex 6: a_posCoord tx6,ty6, // vertex 6: a_texCoord alpha // vertex 6: a_alpha // ... Many more billboards not shown ... ]; </code></pre> <p>Notice how the same <code>alpha</code> value is repeated 6 times, once for each vertex.</p> <p><strong>Is there a way I can specify an attribute for all 6 vertices without repeating it for each individual vertex?</strong></p> <p>Here's what I want my vertex attribute buffer to look like, in the interest of reducing the size of the buffer:</p> <pre><code>vertexAttributes = [ px1,py1,pz1, // vertex 1: a_posCoord tx1,ty1, // vertex 1: a_texCoord px2,py2,pz2, // vertex 2: a_posCoord tx2,ty2, // vertex 2: a_texCoord px3,py3,pz3, // vertex 3: a_posCoord tx3,ty3, // vertex 3: a_texCoord px4,py4,pz4, // vertex 4: a_posCoord tx4,ty4, // vertex 4: a_texCoord px5,py5,pz5, // vertex 5: a_posCoord tx5,ty5, // vertex 5: a_texCoord px6,py6,pz6, // vertex 6: a_posCoord tx6,ty6, // vertex 6: a_texCoord alpha // vertex 1-6: a_alpha // ... Many more billboards not shown ... ]; </code></pre> <p>For what it's worth, my current solution works just fine, but makes me feel dirty. I've only just begun to get a handle on using <code>glVertexAttribPointer</code> and was wondering if it somehow supported something like this, or if there's a different method or technique I could use to achieve something less brute-force.</p> <hr> <p>This is more specifically a WebGL question, but I'm curious about OpenGL in the general sense.</p> <p>I'm aware that a geometry shader is really what I need for this particular example, but it's out of the question since they are currently unsupported in WebGL.</p>
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    1. COpossible duplicate of [Rendering meshes with multiple indices](http://stackoverflow.com/questions/11148567/rendering-meshes-with-multiple-indices)
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    2. COVery close to being a duplicate, indeed, but I'd like to keep it open for a few days to see if there's any other knowledge out there since my question is a little more specific and someone may have an entirely different approach I am unaware of. I'm guessing I'll have to stick with redundant data until WebGL utilizes ES3's geometry shader support.
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    3. COI noticed glVertexBindingDivisor isn't part of WebGL 1.0 or ES 2.0 (which I'm using)... :(
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