Make POT bitmap from NPOT bitmap using bilinear interpolation

make_pot_bitmap_from_npot.cpp

template <typename T>
static T RoundToPowerOfTwo(T x)
{
    T n = 1;
    while (n < x)
        n <<= 1;
    return n;
}
static int InterpolateColor(int c1, int c2, float alpha)
{
    int a1 = (c1 & 0xff000000) >> 24;
    int b1 = (c1 & 0xff0000) >> 16;
    int g1 = (c1 & 0x00ff00) >> 8;
    int r1 = (c1 & 0x0000ff);
    int a2 = (c2 & 0xff000000) >> 24;
    int b2 = (c2 & 0xff0000) >> 16;
    int g2 = (c2 & 0x00ff00) >> 8;
    int r2 = (c2 & 0x0000ff);
    int r = r1 + (int)((r2-r1)*alpha);
    int g = g1 + (int)((g2-g1)*alpha);
    int b = b1 + (int)((b2-b1)*alpha);
    int a = a1 + (int)((a2-a1)*alpha);
    return (a << 24) | (b << 16) | (g << 8) | r;
}
static void MakePotTextureFromNpot(int& width, int& height, std::vector<unsigned char>& vdata)
{
    if (((width & (width-1)) != 0) || ((height & (height-1)) != 0))
    {
        // Need to make POT
        unsigned int * data = reinterpret_cast<unsigned int*>(&vdata[0]);
        int w2 = RoundToPowerOfTwo(width);
        int h2 = RoundToPowerOfTwo(height);

        // Rescale image to power of 2
        int new_size = w2 * h2;
        std::vector<unsigned char> new_vdata;
        new_vdata.resize(new_size*4);
        unsigned int * new_data = reinterpret_cast<unsigned int*>(&new_vdata[0]);
        for (int dh2 = 0; dh2 < h2; ++dh2)
        {
            float rh = (float)dh2 / (float)h2;
            float y = rh * (float)height;
            int dh = (int)y;
            int dh1 = std::min<int>(dh+1, height-1);
            float ry = y - (float)dh; // fract part of y
            for (int dw2 = 0; dw2 < w2; ++dw2)
            {
                float rw = (float)dw2 / (float)w2;
                float x = rw * (float)width;
                int dw = (int)x;
                int dw1 = std::min<int>(dw+1, width-1);
                float rx = x - (float)dw; // fract part of x

                // We will use bilinear interpolation
                int sample1 = (int) data[dw +width*dh ];
                int sample2 = (int) data[dw1+width*dh ];
                int sample3 = (int) data[dw +width*dh1];
                int sample4 = (int) data[dw1+width*dh1];

                int color1 = InterpolateColor(sample1, sample2, rx);
                int color2 = InterpolateColor(sample3, sample4, rx);;
                int color3 = InterpolateColor(color1, color2, ry);
                new_data[dw2+w2*dh2] = (unsigned int)color3;
            }
        }
        // Finally
        width = w2;
        height = h2;
        vdata.swap(new_vdata);
    }
}