webgcode

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// expected #define: float pointCount
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highp float encodingFactor = (exp2(24.0) - 1.0) / exp2(24.0);
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uniform sampler2D points;
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uniform vec2 gridSize;
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uniform mat4 projectionMatrix2;
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uniform mat4 viewMatrix2;
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varying vec2 vUv;
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vec2 getPoint(int index) {
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    vec4 coords = vec4(texture2D(points, vec2((0.5 + float(index)) / float(pointCount), 0.5)).ra, 0.0, 1.0);
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    // pass it though the camera matrix
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    return vec2(0.5, 0.5) + vec2(0.5, 0.5) * (projectionMatrix2 * viewMatrix2 * coords).xy;
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}
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void main() {
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    float intersectionCount = 0.0;
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    for (int i = 1; i < pointCount; i++) {
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        vec2 prev = getPoint(i - 1);
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        vec2 curr = getPoint(i);
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        float xi = curr.x, yi = curr.y;
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        float xj = prev.x, yj = prev.y;
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        //https://github.com/substack/point-in-polygon/blob/master/index.js
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        //http://www.ecse.rpi.edu/Homepages/wrf/Research/Short_Notes/pnpoly.html
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        if ((yi > vUv.y != yj > vUv.y ) && (vUv.x < (xj - xi) * (vUv.y - yi) / (yj - yi) + xi))
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            intersectionCount++;
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    }
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    bool inside = mod(intersectionCount, 2.0) == 1.0;
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    if (!inside)
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        discard;
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    gl_FragColor = vec4(1.0, 0.0, 1.0, 1.0);
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}