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uniform sampler2D strokeColor;
uniform int mode;
#define MODE_REGULAR 0
#define MODE_OVERLAY 1
#define MODE_ADD 2
#define MODE_SUB 3
#define MODE_MULTIPLY 4
#define MODE_DIVIDE 5
/* Blend equation is : FragColor0 + FragColor1 * DstColor */
layout(location = 0, index = 0) out vec4 FragColor0;
layout(location = 0, index = 1) out vec4 FragColor1;
void main()
{
ivec2 texel = ivec2(gl_FragCoord.xy);
vec4 src = texelFetch(strokeColor, texel, 0).rgba;
switch (mode) {
case MODE_REGULAR:
FragColor0 = src;
FragColor1 = 1.0 - vec4(src.a);
break;
case MODE_OVERLAY:
/* Original overlay is :
* overlay = (a < 0.5) ? (2 * a * b) : (1 - 2 * (1 - a) * (1 - b));
* But we are using dual source blending. So we need to rewrite it as a function of b (DST).
* overlay = 1 - 2 * (1 - a) * (1 - b);
* overlay = 1 + (2 * a - 2) * (1 - b);
* overlay = 1 + (2 * a - 2) - (2 * a - 2) * b;
* overlay = (2 * a - 1) + (2 - 2 * a) * b;
*
* With the right coefficient, we can get the 2 * a * b from the same equation.
* q0 = 0, q1 = -1;
* overlay = (2 * a - 1) * q0 + (2 * q0 - 2 * q1 * a) * b;
* overlay = 2 * a * b;
* */
src.rgb = mix(vec3(0.5), src.rgb, src.a);
vec3 q0 = vec3(greaterThanEqual(src.rgb, vec3(0.5)));
vec3 q1 = q0 * 2.0 - 1.0;
vec3 src2 = 2.0 * src.rgb;
FragColor0.rgb = src2 * q0 - q0;
FragColor1.rgb = 2.0 * q0 - src2 * q1;
break;
case MODE_ADD:
FragColor0.rgb = src.rgb * src.a;
FragColor1.rgb = vec3(1.0);
break;
case MODE_SUB:
FragColor0.rgb = -src.rgb * src.a;
FragColor1.rgb = vec3(1.0);
break;
case MODE_MULTIPLY:
FragColor0.rgb = vec3(0.0);
FragColor1.rgb = mix(vec3(1.0), src.rgb, src.a);
break;
case MODE_DIVIDE:
FragColor0.rgb = vec3(0.0);
FragColor1.rgb = 1.0 / mix(vec3(1.0), src.rgb, src.a);
break;
default:
FragColor0 = vec4(0.0);
FragColor1 = vec4(1.0);
break;
}
FragColor0.a = 1.0 - clamp(dot(vec3(1.0 / 3.0), FragColor1.rgb), 0.0, 1.0);
FragColor1.a = 1.0 - FragColor0.a;
}
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