I seem to remember some years ago learning about a matte that takes 2 video files and compares the motion in them, creating a matte from the pixels that are different (moving). Is there such a thing? Or is my memory making things up that I wish were possible?

it is not possible since even between 2 frames, the same pixel will likely not give the same value.
Despite this fact , there are some programs that are able to track an object with or without reference background.
for a rough mask, this can work nicely, but the quality of a mask is in the borders, so from that point of view, it is most of time very difficult to obtain good results.
i nice program to use is MOKEY
http://www.mokey.com/products/mokey/

It is possible, and it's called Difference Matte. With some tweaking of the color space, denoising and blurring the clean plate and so on you can pull a somewhat usable mask. I did this in Fusion when I needed to key out a dancing person from a garage for additional color correction. Your shot has to be really static, with same lighting conditions and noise-free. Even then I had to make an animated garbage matte around the moving person, because the key wasn't too clean.

Try to avoid this technique.

I've never gotten the difference matte to work. Ever.

intresting. Well i filmed it today to try so we`ll see what happens later when i get to editing. :D Any tips are helpful

What do you use as FX app? I did this only in Fusion, but Diff Matte exists even in Premiere (but I cannot imagine ever getting anything usable from it without extensive cleaning).

I`ll e editing on CS3 eventually, premiere and after effects

I just recently learned how to use the "matte choker" effect in AE. Use with (any of ?) the keying effects; adjust those keys so that the maximum amount of your background keys-out, what usually happens is at some point parts of the foreground start to fade too. The choker then goes in and magically locates those little transparent islands and gets rid of them. Does a nice job on edge blending as well.

At least on my miniDV-painfully-cheap-green-screen footage...


andy

I've never gotten the difference matte to work. Ever.
I've gotten it to work for me... shot HD resolution to an Avid (DNxHD codec/compression), downres to SD and deinterlaced by discarding a field.

The camera was a full-sized Panasonic HD camera... HDC-900 I think (it is not a very common camera; it is not the HDX900).