Using a buffer for selectioning objects: accuracy problems

in each frame (as in frames per second) I render, I make a smaller version of it with just the objects that the user can select (and any selection-obstructing objects). In that buffer I render each object in a different color. When the user has mouseX and mouseY, I then look into that buffer what color corresponds with that position, and find the corresponding objects. I can't work with FBO so I just render this buffer to a texture, and rescale the texture orthogonally to the screen, and use glReadPixe开发者_运维百科ls to read a "hot area" around mouse cursor.. I know, not the most efficient but performance is ok for now.

Now I have the problem that this buffer with "colored objects" has some accuracy problems. Of course I disable all lighting and frame shaders, but somehow I still get artifacts. Obviously I really need clean sheets of color without any variances.

Note that here I put all the color information in an unsigned byte in GL_RED. (assumiong for now I maximally have 255 selectable objects).

Are these caused by rescaling the texture? (I could replace this by looking up scaled coordinates int he small texture.), or do I need to disable some other flag to really get the colors that I want. Can this technique even be used reliably?

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It looks like you're using GL_LINEAR for your GL_TEXTURE_MAG_FILTER. Use GL_NEAREST instead if you don't want interpolated colors.

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I could replace this by looking up scaled coordinates int he small texture.

You should. Rescaling is more expensive than converting the coordinates for sure.

That said, scaling a uniform texture should not introduce artifacts if you keep an integer ratio (like upscale 2x), with no fancy filtering. It looks blurry on the polygon edges, so I'm assuming that's not what you use.

Also, the rescaling should introduce variations only at the polygon boundaries. Did you check that there are no variations in the un-scaled texture ? That would confirm whether it's the scaling that introduces your "artifacts".

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What exactly do you mean by "variance"? Please explain in more detail.

Now some suggestion: In case your rendering doesn't depend on stencil buffer operations, you could put the object ID into the stencil buffer in the render pass to the window itself, don't use the detour over a separate texture. On current hardware you usually get 8 bits of stencil. Of course the best solution, if you want to use a index buffer approach, is using multiple render targets and render the object ID into an index buffer together with color and the other stuff in one pass. See http://www.opengl.org/registry/specs/ARB/draw_buffers.txt