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/* 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;
}
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