GLSL with glDrawPixels

Hi, i’m trying to use a fragment shader with glDrawPixels, but i wasn’t able to realize how to access the pixel data.
For instance i can draw a blue square putting:

gl_FragColor = vec4(0,0,1,0);

in the shader, but it is not what i wanted to do, as i would like to process the color that is being written :slight_smile:

Try this fragment shader:

void main(void)
 gl_FragColor = gl_Color + vec4(0.2, 0.3, 0.4, 1.0);

It works on NV hardware but seems it is not accelerated :frowning: