This is a program that uses ffmpeg to extract JPG frames from a video, and then uses a simple and well-tested algorithm to linearly change the frame rate, if requested, and take only a certain range of the frames.
I had to write this script because, when ffmpeg does frame-rate changes, it's not accurate down to the frame level. For instance, if you extract and tell it to use 1/2 the frame rate, it doesn't give you exactly half of the frames. extract.py, on the other hand, will give you exactly every other frame.
For more details and usage examples and stuff, see the documentation.