1 float quad_aa()

     2 {

     3     float top = min(gl_FragCoord.y + 0.5, gl_TexCoord[0].x);

     4     float bottom = max(gl_FragCoord.y - 0.5, gl_TexCoord[0].y);

     5 

     6     float area = top - bottom;

     7 

     8     float left = gl_FragCoord.x - 0.5;

     9     float right = gl_FragCoord.x + 0.5;

    10 

    11     // use line equations to compute intersections of left/right edges with top/bottom of truncated pixel

    12     vec4 vecX = gl_TexCoord[1].xxzz * vec2(top, bottom).xyxy + gl_TexCoord[1].yyww;

    13 

    14     vec2 invA = gl_TexCoord[0].zw;

    15 

    16     // transform right line to left to be able to use same calculations for both

    17     vecX.zw = 2.0 * gl_FragCoord.x - vecX.zw;

    18 

    19     vec2 topX = vec2(vecX.x, vecX.z);

    20     vec2 bottomX = vec2(vecX.y, vecX.w);

    21 

    22     // transform lines such that top intersection is to the right of bottom intersection

    23     vec2 topXTemp = max(topX, bottomX);

    24     vec2 bottomXTemp = min(topX, bottomX);

    25 

    26     // make sure line slope reflects mirrored lines

    27     invA = mix(invA, -invA, step(topX, bottomX));

    28 

    29     vec2 vecLeftRight = vec2(left, right);

    30 

    31     // compute the intersections of the lines with the left and right edges of the pixel

    32     vec4 intersectY = bottom + (vecLeftRight.xyxy - bottomXTemp.xxyy) * invA.xxyy;

    33 

    34     vec2 temp = mix(area - 0.5 * (right - bottomXTemp) * (intersectY.yw - bottom), // left < bottom < right < top

    35                     (0.5 * (topXTemp + bottomXTemp) - left) * area,    // left < bottom < top < right

    36                     step(topXTemp, right.xx));

    37 

    38     vec2 excluded = 0.5 * (top - intersectY.xz) * (topXTemp - left); // bottom < left < top < right

    39 

    40     excluded = mix((top - 0.5 * (intersectY.yw + intersectY.xz)) * (right - left), // bottom < left < right < top

    41                    excluded, step(topXTemp, right.xx));

    42 

    43     excluded = mix(temp, // left < bottom < right (see calculation of temp)

    44                    excluded, step(bottomXTemp, left.xx));

    45 

    46     excluded = mix(vec2(area, area), // right < bottom < top

    47                    excluded, step(bottomXTemp, right.xx));

    48 

    49     excluded *= step(left, topXTemp);

    50 

    51     return (area - excluded.x - excluded.y) * step(bottom, top);

    52 }

    53 

    54 void main()

    55 {

    56     gl_FragColor = quad_aa().xxxx;

    57 }

    58