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.
Download source code: rscope-1.1.zip (Nov. 2011)
Also available at GitHub.
Download Windows 32-bit binary: rscope-1.1-win32.zip
View the Readme file.
Author: Jason Summers (email)