You are absolutely right, I don't know what I was thinking. Thanks for spotting this!
I uploaded the fixed version, it is still v0.3, same filename.
With the unnecessary GetFrame calls are gone, the filter is a lot faster
I tried calling the hash_map destructor in the Autolevels destructor, but that crashed VDub, so I left it out for now. I need to actually read the documentation when I have some time.