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