How to make java genreated thumbnail sharper

I have thumbnails generated in Java, but not sharp enoug开发者_如何学编程h, why?

I'm getting excellent results with the java-image-scaling library.

Possibly because of the kernel you've used.

A lanczos filter, for instance, gives relatively sharp results.

I found the best results are produced with Lanczos scale for thumbnails and unsharp mask after that. Most scale algorithms apply as a first step a low pass filter (blur) and sub-sampling after that, this makes scaled down images to look not sharp enough.