Function DCT

deps/opencv/modules/core/src/dxt.cpp:2076–2116

   http://www.ece.utexas.edu/~bevans/courses/ee381k/lectures/09_DCT/lecture9/:
*/
template<typename T> static void
DCT( const T* src, int src_step, T* dft_src, T* dft_dst, T* dst, int dst_step,
     int n, int nf, int* factors, const int* itab, const Complex<T>* dft_wave,
     const Complex<T>* dct_wave, const void* spec, Complex<T>* buf )
{
    static const T sin_45 = (T)0.70710678118654752440084436210485;
    int j, n2 = n >> 1;

    src_step /= sizeof(src[0]);
    dst_step /= sizeof(dst[0]);
    T* dst1 = dst + (n-1)*dst_step;

    if( n == 1 )
    {
        dst[0] = src[0];
        return;
    }

    for( j = 0; j < n2; j++, src += src_step*2 )
    {
        dft_src[j] = src[0];
        dft_src[n-j-1] = src[src_step];
    }

    RealDFT( dft_src, dft_dst, n, nf, factors,
             itab, dft_wave, n, spec, buf, 0, 1.0 );
    src = dft_dst;

    dst[0] = (T)(src[0]*dct_wave->re*sin_45);
    dst += dst_step;
    for( j = 1, dct_wave++; j < n2; j++, dct_wave++,
                                    dst += dst_step, dst1 -= dst_step )
    {
        T t0 = dct_wave->re*src[j*2-1] - dct_wave->im*src[j*2];
        T t1 = -dct_wave->im*src[j*2-1] - dct_wave->re*src[j*2];
        dst[0] = t0;
        dst1[0] = t1;
    }

    dst[0] = src[n-1]*dct_wave->re;
}


template<typename T> static void