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copied!<p>I still don't think that there is a clear answer here. So here is how I made it work through a 2D texture:</p> <pre><code>// First, create a frame buffer: glGenFramebuffers(1, &fbo); glBindFramebuffer(GL_FRAMEBUFFER, fbo); // Then generate your texture and define an unsigned int array: glGenTextures(1, &textureid); glBindTexture(GL_TEXTURE_2D, textureid); glTexImage2D(GL_TEXTURE_2D, 0, GL_R32UI, w, h, 0, GL_RED_INTEGER, GL_UNSIGNED_INT, 0); glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST); glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST); // Attach it to the frame buffer object: glFramebufferTexture(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT1, textureid, 0); // Before rendering glBindFramebuffer(GL_FRAMEBUFFER, fbo); GLuint buffers[2] = { GL_COLOR_ATTACHMENT0, GL_COLOR_ATTACHMENT1 }; // this assumes that there is another texture that is for the render buffer. Color attachment1 is preserved for the element ids. glDrawBuffers(2, buffers); // Clear and render here glFlush(); // Flush after just in case glBindFramebuffer(GL_FRAMEBUFFER, 0); </code></pre> <p>On the GLSL side the fragment shader should have (4.3 core profile code here):</p> <pre><code>layout(location = 0) out vec4 colorOut; // The first element in 'buffers' so location 0 layout(location = 1) out uvec4 elementID; // The second element in 'buffers' so location 1. unsigned int vector as color // ... void main() { //... elementID = uvec4( elementid, 0, 0, 0 ); // Write the element id as integer to the red channel. } </code></pre> <p>You can read the values on the host side:</p> <pre><code>unsigned int* ids = new unsigned int[ w*h ]; glBindTexture(GL_TEXTURE_2D, textureid); glGetTexImage(GL_TEXTURE_2D, 0, GL_RED_INTEGER, GL_UNSIGNED_INT, ids); </code></pre>

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