float mirror ( float p )
{
    return abs ( p - 2.0f * floor ( ( p + 1.0f ) / 2.0f ) );
}

vector [ ] computeSamplePoints ( int quality; int xres; int yres )
{
    vector samples [ ];
    int nsamples = quality;
    int tsamples = nsamples * nsamples;
    float n = 1.0f / ( 1.0f + tsamples );
    for ( int i = 1; i <= tsamples; ++i )
    {
        float y = i * n;
        float x = frac ( y * nsamples );
        samples [ i - 1 ] = set ( ( x - 0.5f ) / xres, ( y - 0.5f ) / yres, 0 );
    }
    return samples;
}

cop2 swirl ( float angle = 0; float r = 1; vector2 t = { 0.5, 0.5 }; int quality = 3; int filter = 1; int outside = 0 )
{
    float rx = 0.5 * r;
    float ry = 0.5 * r;
    float maxr = rx;
    float x = X - t.x;
    float y = Y - t.y;
    float sx = max ( 1, 1.0f * XRES / YRES );
    float sy = max ( 1, 1.0f * YRES / XRES );
    
    if ( XRES > YRES )
        rx = 0.5 * r * min ( XRES, YRES ) / max ( XRES, YRES );
    else
        ry = 0.5 * r * min ( XRES, YRES ) / max ( XRES, YRES );
    
    int setSample = 0;
    vector sample = 0;
    vector samplePoints [ ] = computeSamplePoints ( quality, XRES, YRES );
    foreach ( vector p; samplePoints )
    {
        if ( ( x * x ) / ( rx * rx ) + ( y * y ) / ( ry * ry ) <= 1 )
        {
            float xx = x * sx + p.x;
            float yy = y * sy + p.y;
            float amount = ( maxr - sqrt ( x * x * sx * sx + y * y * sy * sy ) ) / maxr;
            float theta = amount * amount * radians ( angle );
            float s = sin ( theta );
            float c = cos ( theta );
            xx = c * x - s * y + t.x;
            yy = s * x + c * y + t.y;
            
            if ( outside == 0 )
            {
                xx = mirror ( xx );
                yy = mirror ( yy );
            }
            
            if ( filter == 0 )
                sample += cinput ( 0, PL, AI, xx % 1, yy % 1, F, "wrap", "streak" );
            else if ( filter == 1 )
                sample += binput ( 0, PL, AI, xx % 1, yy % 1, F, "wrap", "streak" );
            else if ( filter == 2 )
                sample += finput ( 0, PL, AI, xx % 1, yy % 1, F, "wrap", "streak" );
            
            setSample = 1;
        }
    }
    
    if ( setSample )
        assign ( Cr, Cg, Cb, sample / arraylength ( samplePoints ) );
}

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