uniform sampler2D texture;
uniform float edge_threshold;
void main()
{
const float offset = 1.0 / 512.0;
vec2 offx = vec2(offset, 0.0);
vec2 offy = vec2(0.0, offset);
vec4 hEdge = texture2D(texture, gl_TexCoord[0].xy - offy) * -2.0 +
texture2D(texture, gl_TexCoord[0].xy + offy) * 2.0 +
texture2D(texture, gl_TexCoord[0].xy - offx - offy) * -1.0 +
texture2D(texture, gl_TexCoord[0].xy - offx + offy) * 1.0 +
texture2D(texture, gl_TexCoord[0].xy + offx - offy) * -1.0 +
texture2D(texture, gl_TexCoord[0].xy + offx + offy) * 1.0;
vec4 vEdge = texture2D(texture, gl_TexCoord[0].xy - offx) * 2.0 +
texture2D(texture, gl_TexCoord[0].xy + offx) * -2.0 +
texture2D(texture, gl_TexCoord[0].xy - offx - offy) * 1.0 +
texture2D(texture, gl_TexCoord[0].xy - offx + offy) * -1.0 +
texture2D(texture, gl_TexCoord[0].xy + offx - offy) * 1.0 +
texture2D(texture, gl_TexCoord[0].xy + offx + offy) * -1.0;
vec3 result = sqrt(hEdge.rgb * hEdge.rgb + vEdge.rgb * vEdge.rgb);
float edge = length(result);
vec4 pixel = gl_Color * texture2D(texture, gl_TexCoord[0].xy);
if (edge > (edge_threshold * 8.0))
pixel.rgb = vec3(0.0, 0.0, 0.0);
else
pixel.a = edge_threshold;
gl_FragColor = pixel;
}
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