Question

Say I have a texture sitting in video memory and somewhere hidden inside this texture maybe one or more pixels with a particular color. In c++ I want to call a function on this gltexture to check for the presence of this color and simply return true or false. What is the fastest way to do this?

Answer 1

I guess the best performance/simplicity ratio would be to keep a copy on the CPU and test each texel, optionnaly with SSE.

You can also do it in OpenGL :

• Create a render target of the same size than your texture
• Do a fullscreen render of your texture in it – that is, each texel will match one fragment of the render target. Disable any fancy texture sampling
• When rendering, use a shader which discards any texel except the one with the wanted color
• Count the number of rendered fragments with an occlusion query.

I’m not sure of the performance of this, but it’s likely to by the fastest you’ll get on the GPU side. Note that while fast, this method has lag : you usually only get the occlusion query’s result 1 or 2 frames later (you can also busy wait, though).