How do I do high quality scaling of a image?

I've found the wxWidgets implementation fairly straightforward to modify as required. It is all C++ so no problems with portability there. The only difference is that their implementation works with unsigned char arrays (which I find to be the easiest way to deal with images anyhow) with a byte order of RGB and the alpha component in a separate array.

If you refer to the "src/common/image.cpp" file in the wxWidgets source tree there is a down-sampler function which uses a box sampling method "wxImage::ResampleBox" and an up-scaler function called "wxImage::ResampleBicubic".

A fairly simple and decent algorithm to resample images is Bicubic interpolation, wikipedia alone has all the info you need to get this implemented.

Is it possible that OpenGL is doing the scaling in the vector domain? If so, there is no way that any pixel-based scaling is going to be near it in quality. This is the big advantage of vector based images.

The bicubic algorithm can be tuned for sharpness vs. artifacts - I'm trying to find a link, I'll edit it in when I do.

Edit: It was the Mitchell-Netravali work that I was thinking of, which is referenced at the bottom of this link:

http://www.cg.tuwien.ac.at/~theussl/DA/node11.html

You might also look into Lanczos resampling as an alternative to bicubic.