// fast painter for composition modes which can be implemented with blendfuncs

uniform sampler2D mask_texture;

uniform vec2 inv_mask_size;

uniform vec2 mask_offset;

uniform vec4 mask_channel;

float mask()

{

    return dot(mask_channel, texture2D(mask_texture, (gl_FragCoord.xy + mask_offset) * inv_mask_size));

}

void main()

{

    // combine clip and coverage channels

    float mask_alpha = mask();

    gl_FragColor = brush() * mask_alpha;

}