/* Shader for conversion of RGB textures into Grayscale textures.
 * Uses standard formula for conversion:
 * (w)2006 by Mario Kleiner. Licensed under MIT license.
*/

#extension GL_ARB_texture_rectangle : enable

const vec4 ColorToGrayWeights = { 0.3, 0.59, 0.11, 0.0 }; 
uniform sampler2DRect Image;

void main()
{
    vec4 leftcolor = texture2DRect(Image, gl_TexCoord[0].st);
    vec4 rightcolor = texture2DRect(Image, gl_TexCoord[0].st + vec2(1.0, 0.0));
    float leftlum = dot(leftcolor, ColorToGrayWeights);
    float rightlum = dot(rightcolor, ColorToGrayWeights);
    gl_FragColor.a = 255.0;
    gl_FragColor.rgb = max(leftlum - rightlum, 0.0) * 1;
}