Creating an alpha channel in an odd way ?

My question is about the functionality of After Effects. I highly doubt it's possible but would like to run it through people who would know. My question may not be clear as I'm not sure how to describe it!
I'm wondering if I can create an alpha channel using the still background of a shot, as an alternative to green screens and making it easier outdoors without needing crazy lights.
Here's an example. Say you film yourself using a tripod (so the background is still) walking away from a building and want to add an explosion behind you without masking every frame, would you be able to have it recognize the background using a picture without you in it or a still that has the same lighting, and using that to automatically have it differentiate that background from you thus creating an alpha channel or an automatic mask...
Thanks

For a Difference Matte to work absolutely nothing can be moving in the scene. Difference Matte will not work with compressed original footage. I know I've tried many times. If the wind is blowing in the grass or the trees are moving the Difference Matte will not work.
if you are planning this kind of shot then the best thing to do is try to find some good areas of contrast in the shot so you can create a procedural matte. A little garbage matte and a procedural matte created by adjusting levels or color or hue can create a much better matte and does not rely on the pixels being exactly the same on every frame.
Even easier is a carefully planned shot and Roto brush.

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