[sh0dan]

Just writing a "speech", so it can be put up on avisynth.org:

VideoInfo is considered the "constant" video info. This can not in any way change. The information such as width, height and colorspace cannot change. So the information you get from this can be trusted to be the same from all frames you recieve. If you change the VideoInfo you have recieved it will not change the frames you recieve, but a modified 'VideoInfo vi' can be sent to env->NewVideoFrame(vi) and a new frame with the changed parameters will be created. Only your own filter can modify the VideoInfo given to your filter.

PVideoFrame contains information about a particular frame you requested. Height and Rowsize should not change (this can be considerd a bug). Pitch can change, so you cannot rely on pitch being the same for all frames you recieve.