gistfile1.txt

bool
are_equal(float a, float b) {
    constexpr float epsilon = 0.00001f;
    return fabs(a - b) <= epsilon;
}

float
edge_test(float2 p, float2 v0, float2 v1) {
    // NOTE(christian): Skip edge testing for edges that are straight horizontal/vertical lines since they don't need anti-aliasing.
    if (!are_equal(v0.x, v1.x) && !are_equal(v0.y, v1.y)) {
        return (p.x - v0.x) * (v1.y - v0.y) - (p.y - v0.y) * (v1.x - v0.x);
    } else {
        return FLT_MAX;
    }
}

bool
on_edge(float val, float edgeMax) {
    return (val >= 0.f && val <= edgeMax);
}

fragment float4 fragment_primitive(VertexOut in [[ stage_in ]],
                                   constant Edges &edges [[ buffer(0) ]],
                                   constant RenderTarget &renderTarget [[ buffer(1) ]])
{
    float2 pos = (in.pos.xy / float2(renderTarget.width, renderTarget.height)) * 2.f - 1.f;
    pos.y *= -1.f;

    float a = in.color.a;
    float2 v0 = edges.verts[0];
    float2 v1 = edges.verts[1];
    float2 v2 = edges.verts[2];
    float et0 = edge_test(pos, v0, v1);
    float et1 = edge_test(pos, v1, v2);
    float et2 = edge_test(pos, v2, v0);
    
    float et = FLT_MAX;
    float edgeMax = 0.f;
    bool onEdge = false;
    if (on_edge(et0, edges.edgeMaximas[0]) && (et0 < et)) {
        et = et0;
        edgeMax = edges.edgeMaximas[0];
        onEdge = true;
    }
    if (on_edge(et1, edges.edgeMaximas[1]) && (et1 < et)) {
        et = et1;
        edgeMax = edges.edgeMaximas[1];
        onEdge = true;
    }
    if (on_edge(et2, edges.edgeMaximas[2]) && (et2 < et)) {
        et = et2;
        edgeMax = edges.edgeMaximas[2];
        onEdge = true;
    }
    
    if (onEdge) {
        a = smoothstep(0.f, edgeMax, et);
    }
    
    return float4(in.color.rgb, a);
}