Best way to return a value from a Fragment Shader

OpenGL
1

I need to run couple of operations on the texture pixels and return true/false based on the result.
One example would be - if any pixel exceeded a specific value? (like 0.75) - Yes/No
If the texture contains true black pixel anywhere? - Yes/No

Thanks,
Tom

2

Just draw a specific value at the current pixel location to another render target (see MRT). Another option would be to use an SSBO.

3

Assuming OpenGL 4.3 support, compute shader reads the texture and writes the result to a buffer variable. You’ll need to test whether it’s faster to use a conditional assignment or atomicMax. If you can’t assume OpenGL 4.3, you can use a fragment shader with an atomic counter or image load/store. If you can’t assume OpenGL 4.2, you’ll need to get creative (without 4.2 or extensions, the only shader output mechanisms are the framebuffer and transform feedback).

4

Unfortunately GLSL 1.20.

What I have been doing now is sending vertices to vertex shader and calling glDrawArrays with GL_POINTS for all the pixels in the image. For each vertex I would read corresponding RGB from the texture and if the condition was passed I would return vertex position at (0,0), and if it didn’t pass I would return (1,1).

But I am looking for something faster than that.
For 4K image that’s pretty slow.

5

glGetTexImage2D then performing the test in the application would probably be faster than that.

6

I will probably try the OpenCL route…

7

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