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uniform sampler2D brush_texture;
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uniform vec2 inv_brush_texture_size;
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uniform vec3 inv_matrix_m0;
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uniform vec3 inv_matrix_m1;
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uniform vec3 inv_matrix_m2;
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vec4 brush()
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{
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mat3 mat;
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mat[0] = inv_matrix_m0;
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mat[1] = inv_matrix_m1;
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mat[2] = inv_matrix_m2;
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vec3 hcoords = mat * vec3(gl_FragCoord.xy, 1);
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vec2 coords = hcoords.xy / hcoords.z;
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coords *= inv_brush_texture_size;
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float alpha = 1.0 - texture2D(brush_texture, coords).r;
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return gl_Color * alpha;
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}
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