bicubic scaling filter

[Boudewijn Rempt]

We got a bug report recently () asking for a bicubic scaling filter. The 
report pointed me to the ImageMagick source via Gwenview and on nothing the 
similariteis between that code and our own filters, I thought I'd try my hand 
at it. Didn't work -- my filter destroys the image, just like the lanzcos 
filter. I haven't got a clue why, though. Here's my filter code:

double KisCubicFilterStrategy::valueAt(double x) const {
    if (x < -2.0)
        return(0.0);
    if (x < -1.0)
        return((2.0+x)*(2.0+x)*(2.0+x)/6.0);
    if (x < 0.0)
        return((4.0+x*x*(-6.0-3.0*x))/6.0);
    if (x < 1.0)
        return((4.0+x*x*(-6.0+3.0*x))/6.0);
    if (x < 2.0)
        return((2.0-x)*(2.0-x)*(2.0-x)/6.0);
    return(0.0);
}

Q_UINT32 KisCubicFilterStrategy::intValueAt(Q_INT32 x) const {
    if (x < 2)
        return 0;
    if (x < -1)
        return (2 + x) * (2 + x) * ( 2 + x) / 6;
    if ( x < 0)
        return (4 + x * x * ( -6 - 3 * x)) / 6;
    if (x < 1)
        return (4 + x * x * ( -6 + 3 * x)) / 6;
    if (x < 2)
        return (2 - x) * ( 2 - x) * (2 - x) / 6;
     return 0;
}

Anyone any ideas?

-- 
Boudewijn Rempt 
http://www.valdyas.org/fading/index.cgi
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