Question

I’ve been searching Google for a while now to find pseudo-code of a decently efficient 8×8 (or nxn) DCT algorithm, and I can’t find anything!

I implemented the naive approach, and it took far too long to execute.

If you could post some pseudo-code or reference a good book/document/website, that would be helpful.

C or C++ examples would be better yet!

Answer 1

As requested in the comments, source (be slightly warned, it’s in C#, but the difference with C++ should be minimal, and yes I know the code is lame):

Main loop (A = result, B = input):

for (int y = 0; y < 8; y++)
{
    for (int x = 0; x < 8; x++)
    {
        A[y * 8 + x] = 0;
        for (int u = 0; u < 8; u++)
            for (int v = 0; v < 8; v++)
                A[y * 8 + x] += alpha(u) * alpha(v) * B[u, v] *
                    cosine[u, x] * cosine[v, y];
    }
}

Support stuff:

static double alpha(int i)
{
    if (i == 0)
        return SQRT2o2 * 0.5;
    return 0.5;
}
const double SQRT2o2 = 1.414213562373095048801688724209 * 0.5;
cosine = new double[8, 8];
const double inv16 = 1.0 / 16.0;
for (int i = 0; i < 8; i++)
{
     for (int j = 0; j < 8; j++)
     {
         cosine[j, i] = Math.Cos(Math.PI * j * (2.0 * i + 1) * inv16);
     }
}

edit: I timed it – for 512 by 512 pixels (single channel) it takes half a second. Sure that’s slow, but nowhere near “forever”.