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I'm coding a programm in C++ with glut, rendering a 3D model in a window. I'm using glReadPixels to get the image of the scenery displayed in the windows.

And I would like to know how I can get, for a specific pixel (x, y), not directly its color but the color of the next object behind.

If I render a blue triangle, and a red triangle in front of it, glReadPixels gives me red colors from the red triangle.

I would like to know how I can get the colors from the blue triangle, the one I would get from glReadPixels if the red triangle wasn't here.

Peter O.
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Demod
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1 Answers1

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The default framebuffer only retains the topmost color. To get what you're suggesting would require a specific rendering pipeline.

For instance you could:

  1. Create an offscreen framebuffer of the same dimensions as your target viewport
  2. Render a depth-only pass to the offscreen framebuffer, storing the depth values in an attached texture
  3. Re-render the scene with a special shader that only drew pixels where the post-transformation Z values was LESS than the value in the previously recorded depth buffer

The final result of the last render should be the original scene with the top layer stripped off.

Edit:

It would require only a small amount of new code to create the offscreen framebuffer and render a depth only version of the scene to it, and you could use your existing rendering pipeline in combination with that to execute steps 1 and 2.

However, I can't think of any way you could then re-render the scene to get the information you want in step 3 without a shader, because it both the standard depth test plus a test against the provided depth texture. That doesn't mean there isn't one, just that I'm not well versed in GL tricks to think of it.

I can think of other ways of trying to accomplish the same task for specific points on the screen by fiddling with the rendering system, but they're all far more convoluted than just writing a shader.

Jherico
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  • Thank you for the answer. If I understand correctly, that would require shader programming (GLSL), VBO, etc... ? No way around this ? The truth is, my actual rendering is done using glBegin/glEnd/glVertex. I know it's bad and it's deprecated since several years, it happened because I'm reusing an older code. I would love to learn shader programming, but the truth I dont really have the time right now, and this specific piece of code is one-time thing... – Demod Jul 15 '14 at 22:03
  • @Jherico Thanks a lot for your answers. Do you have a recommended resource, tutorial, or else, to learn to do that (ie the shader programming) ? Thank you again. :) – Demod Jul 16 '14 at 18:51