ResampleScope is a command-line utility that can help you to figure out what algorithm another application uses when it resizes images.

It is written in C, and requires libgd. It’s cross-platform, but mostly intended for Unix-like systems.

Demonstrations, etc.:

Download source code: (Nov. 2011)
Also available at GitHub.

Download Windows 32-bit binary:

View the Readme file.

Author: Jason Summers (email)