> It would be great if TP would provide this algorithm out of the box or as a pluging.
Agreed, Lanczos would be great to have! And an Mspline-like option for upsizing would be great too.
btw, for the OP: it's generally a good idea to run Image | Filter | Select Filter | Sharpen | Unsharp Mask after you've used Resize to make an image smaller. Basically, this will restore the sharpness that was lost in the resizing. Once you have used a filter, you can simply press Ctrl-F to apply the same filter again (on another image).