GLSL : accessing framebuffer to get RGB and change it

Question

I'd like to access framebuffer to get RGB and change their values for each pixel. It is because the glReadPixels, and glDrawPixels are too slow to use, so that i should use shaders instead of using them.

Now, I write code, and success to display three-dimensional model using GLSL shaders.

I drew two cubes as follows.

.... 
glDrawArrays(GL_TRIANGLES, 0, 12*6);
....

and fragment shader :

varying vec3 fragmentColor;
void main()
{
    gl_FragColor = vec4(fragmentColor, 1);
}

Then, how can I access to RGB values and change it? For example, If the pixel values at (u1, v1) on window and (u2, v2) are (0,0,255), then I want to change them to (255,0,0)

Nicol Bolas Nicol Bolas · Accepted Answer · 2017-02-03T04:31:07

With the exception of an OpenGL ES-only extension, fragment shaders cannot just read from the current framebuffer. Otherwise, we wouldn't need blending.

You also can't just render to the image you're reading from in a shader. So if you need to do some sort of post-processing, then that is best done by rendering to a separate image. That is, you do your rendering to image 1, then bind that as a texture and change the FBO so that you're rendering to image 2.

Alternatively, if you have access to OpenGL 4.5/ARB/NV_texture_barrier, then you can use texture barriers to handle this. This permits you a single read/modify/write pass, if you bind the current framebuffer's image as a texture. You'd issue the barrier before doing your read/modify/write, then bind that texture to a sampler while still rendering to that framebuffer.

Also, this requires that the FS read from the exact texel that it would write to. Assuming a viewport anchored at 0,0, the code for this would be texelFetch(sampler, ivec2(gl_FragCoord.xy), 0). You can't read from someone else's texel and modify it.

Obviously you must be rendering to a texture; you cannot use the default framebuffer for this.

Texture barrier could be used for cases where you read from different texels than you write to. But that would require doing something similar to the first case of switching bound images. Though you wouldn't need to change the FBO exactly; you could change the region of the FBO that you render to. That is, so long as you're reading from a different area than you're rendering to, and you use barriers appropriately when switching between those regions, everything is fine.

GLSL : accessing framebuffer to get RGB and change it

1 Answers