/* HorizontalMinimumEdgeDistanceShader
 * Scans along a horizontal line in bound rectangle texture zero.
 * Detects the first steep increase in luminance between two
 * consecutive texels and writes its value and position into
 * the output fragment color.
 *
 * This shader computes the minimum distance to an intensity edge.
 * (w)2006 by Mario Kleiner. Licensed under MIT license.
*/

#extension GL_ARB_texture_rectangle : enable

const float threshold = 0.05;
const vec4 ColorToGrayWeights = { 0.3, 0.59, 0.11, 0.0 }; 
uniform sampler2DRect Image;

void main()
{
    vec4  leftcolor;
    vec4  rightcolor;
    float rlum, llum;
    int pos;
    float minposition = 1;

    for(pos = 0; pos<255; pos++) {
      leftcolor  = texture2DRect(Image, gl_TexCoord[0].st + vec2(pos, 0.0));
      rightcolor = texture2DRect(Image, gl_TexCoord[0].st + vec2(pos + 5.0, 0.0));
      llum = dot(leftcolor, ColorToGrayWeights);
      rlum = dot(rightcolor, ColorToGrayWeights);
	if ((llum - rlum) > threshold) minposition=min(minposition, pos/255.0);
    }

    gl_FragColor.a = 255.0;
    gl_FragColor.rgb = minposition;
}