Comment 7 for bug 679974

> If I have a full person on a picture I might want to mark it as:
>"please include all this", because otherwise enblend might cut
> of a head or a foot. These are the red and green areas in "ptgui".

This can also be done easily by masking.

> What I was suggesting is that we use the alpha mask for both of these cases.
> 0.5 means: blend allowed, 0.7 means prefer this image, 0.3 means
> prefer other picture, 1.0 means This image data HAS to be used
> (i.e. it's an error to have another image with 1.0 alpha mask at that pixel).

First, that will break existing functionality.
Second, then will need to generate accurate masks if you want to force to include a specific area. This can be done more easily by masking.