Using FrameBufferObjects in shaders

Using FrameBufferObjects in shaders would go something like this:

  • Set up your FBO (see here).
  • Render scene while writing values to gl_FragColor and gl_FragDepth (in our case). You could also have more draw buffers, then you could also write to gl_FragData[0-9]. Keep in mind, that values will be clamped to [0,1] unless you use glClampColor to turn that behaviour off. e.g.
glClampColor(GL_CLAMP_FRAGMENT_COLOR, GL_FALSE);
glClampColor(GL_CLAMP_VERTEX_COLOR, GL_FALSE);
glClampColor(GL_CLAMP_READ_COLOR, GL_FALSE);
  • Use FBO texture(s) as shader input:
//set up textures for postprocessing, color int unit 0, depth in 1
glActiveTexture(GL_TEXTURE0);
glBindTexture(GL_TEXTURE_2D, fboInfo.color);
glActiveTexture(GL_TEXTURE1);
glBindTexture(GL_TEXTURE_2D, fboInfo.depth);
//sample depth like regular texture and values are in all channels (x,x,x,x)
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_COMPARE_MODE, GL_NONE);
glTexParameteri(GL_TEXTURE_2D, GL_DEPTH_TEXTURE_MODE, GL_INTENSITY);
//use compiled post-processing shader
glUseProgram(postProcHandle);
//set up texture unit uniforms / texture samplers
glUniform1i(glGetUniformLocation(postProcHandle, "color"), 0);
glUniform1i(glGetUniformLocation(postProcHandle, "depth"), 1);
uniform sampler2D color;
uniform sampler2D depth;
void main() {
    float c = texture2D(depth, gl_FragCoord.xy / vec2(SCREEN_WIDTH, SCREEN_HEIGHT)).a;
    gl_FragColor = vec4(c, c, c, 1.);
}
  • Render a full-screen quad using your shader:
glRectf(-1,-1,1,1);

Published by HorstBaerbel

Software developer by trade and interest, but I venture into the electronics- and diy-world from time to time.

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