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#version 130
in vec2 texcoord;
uniform sampler2D sampler;
void main(void)
{
// http://www.roborealm.com/help/Sobel.php
vec4 p1 = textureOffset(sampler, texcoord, ivec2(-1, -1));
vec4 p2 = textureOffset(sampler, texcoord, ivec2( 0, -1));
vec4 p3 = textureOffset(sampler, texcoord, ivec2( 1, -1));
vec4 p4 = textureOffset(sampler, texcoord, ivec2(-1, 0));
vec4 p5 = textureOffset(sampler, texcoord, ivec2( 0, 0));
vec4 p6 = textureOffset(sampler, texcoord, ivec2( 1, 0));
vec4 p7 = textureOffset(sampler, texcoord, ivec2(-1, 1));
vec4 p8 = textureOffset(sampler, texcoord, ivec2( 0, 1));
vec4 p9 = textureOffset(sampler, texcoord, ivec2( 1, 1));
vec4 Gx = p1+2*p2+p3-p7-2*p8-p9;
vec4 Gy = p3+2*p6+p9-p1-2*p4-p7;
vec4 G = sqrt(Gx * Gx + Gy * Gy);
gl_FragColor = G;
}
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