CODEWITHSUNDEEP
iosopengl-esfragment-shader
aledalgrande
aledalgrande

Reputation: 5217

Most efficient way of discarding pixel shader calculations with OpenGL ES

I have multiple texture reads in my fragment shader, and I am supposedly doing bad things, like using the discard command and conditionals inside the shader.

The thing is, I am rendering to a texture and I want to reuse it in following passes with other shaders, that do not have to operate on pixels that were previously "discarded". This is for performance. The thing is, I need also to discard calculations if uniforms are out of certain ranges (which I read from another texture): imagine a loop with these shaders running always on the same textures, which are not cleared.

So what I have now, is a terrible performance. One idea I thought about is using glFragDepth together with the depth buffer and use that to fire depth testing in order to discard some pixels. But this does not work with the fact I want to have ranges.

Is there any alternative?

Upvotes: 0

Views: 2112

Answers (1)

Reto Koradi
Reto Koradi

Reputation: 54592

You could enable blending, and set the alpha values of pixels you don't want to render to zero. Setup:

glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
glEnable(GL_BLEND);

Then in the fragment shader, where you previously called discard:

...
if (condition) {
    discard;
}
...

Set the alpha to zero instead:

float alpha = float(condition);
...
gl_FragColor(r, g, b, alpha);

Whether this will perform better than discarding pixels could be very system dependent. But if you're looking for alternatives, it's worth trying.

Upvotes: 2

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