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float quad_aa()
{
float top = min(gl_FragCoord.y + 0.5, gl_TexCoord[0].x);
float bottom = max(gl_FragCoord.y - 0.5, gl_TexCoord[0].y);
float area = top - bottom;
float left = gl_FragCoord.x - 0.5;
float right = gl_FragCoord.x + 0.5;
// use line equations to compute intersections of left/right edges with top/bottom of truncated pixel
vec4 vecX = gl_TexCoord[1].xxzz * vec2(top, bottom).xyxy + gl_TexCoord[1].yyww;
vec2 invA = gl_TexCoord[0].zw;
// transform right line to left to be able to use same calculations for both
vecX.zw = 2 * gl_FragCoord.x - vecX.zw;
vec2 topX = vec2(vecX.x, vecX.z);
vec2 bottomX = vec2(vecX.y, vecX.w);
// transform lines such that top intersection is to the right of bottom intersection
vec2 topXTemp = max(topX, bottomX);
vec2 bottomXTemp = min(topX, bottomX);
// make sure line slope reflects mirrored lines
invA = mix(invA, -invA, step(topX, bottomX));
vec2 vecLeftRight = vec2(left, right);
// compute the intersections of the lines with the left and right edges of the pixel
vec4 intersectY = bottom + (vecLeftRight.xyxy - bottomXTemp.xxyy) * invA.xxyy;
vec2 temp = mix(area - 0.5 * (right - bottomXTemp) * (intersectY.yw - bottom), // left < bottom < right < top
(0.5 * (topXTemp + bottomXTemp) - left) * area, // left < bottom < top < right
step(topXTemp, right));
vec2 excluded = 0.5 * (top - intersectY.xz) * (topXTemp - left); // bottom < left < top < right
excluded = mix((top - 0.5 * (intersectY.yw + intersectY.xz)) * (right - left), // bottom < left < right < top
excluded, step(topXTemp, right));
excluded = mix(temp, // left < bottom < right (see calculation of temp)
excluded, step(bottomXTemp, left));
excluded = mix(vec2(area, area), // right < bottom < top
excluded, step(bottomXTemp, right));
excluded *= step(left, topXTemp);
return (area - excluded.x - excluded.y) * step(bottom, top);
}
void main()
{
gl_FragColor = quad_aa();
}
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