kernel Vortex
{
    // for the most part based on code by Jerry Huxtable
    
    parameter float2 size
    <
        minValue: float2(0.0, 0.0);
        maxValue: float2(600.0, 400.0);
        defaultValue: float2(300.0, 200.0);
    >;
    
    parameter float2 center
    <
        minValue: float2(0.0, 0.0);
        maxValue: float2(300.0, 200.0);
        defaultValue: float2(150.0, 100.0);
    >;
    
    void evaluatePixel(in image4 src, out pixel4 dst)
    {
        float2 coord = outCoord();
        float dx = coord.x - center[0];
        float dy = coord.y - center[1];
        float d2 = dx * dx + dy * dy;
        coord.x = center[0] + center[0] * center[0] * dx / d2;
        coord.x = mod(coord.x, size[0]);
        coord.y = center[1] + center[1] * center[1] * dy / d2;
        coord.y = mod(coord.y, size[1]);
        dst = sampleNearest(src, coord);
    }
}