Red Screen Filter

This is one way to apply a filter to make the whole screen red. After rendering the scene to a texture like usual. Render a full screen quad with the following shader attached, to get the whole screen in red. This is a nice and quick effect.

By averaging the different components with the weights 0.3, 0.59, 0.11 you will get a better result than just taking equally much from each. Read more on this page about it

http://en.wikipedia.org/wiki/Grayscale

The fragment shader:

uniform sampler2D screenRT;
varying vec2 uv;
 
void main( void )
{
    // make the screen red, (but works fine for other channels too, of course)
    gl_FragColor.r = dot(texture2D( screenRT, uv ).xyz,vec3(0.3, 0.59, 0.11));
}