[magick-users] Problems with Overlay

[Wolfgang Hugemann]

This is probably quite easy, but I can't solve it at the moment:

I would like to suerpose the attached images (i.e. rather their larger 
originals) such that I can see both of them.

composite -dissolve rect.jpg correct.jpg overlay.jpg

gives a quite what I desire, but I would like to restict the overlay to 
the non-white region of 'rect.jpg'. I already generated a mask by

convert rect.jpg -threshold 99.9% mask.png

but I don't get any further. I read that 'composite' can take a third 
image as a mask, but the 'dissolve' option somehow seems to affect that.

.......
The background behind this question is image rectification. The image 
'rect.jpg' was rectified by use of IM such that it matches with the 
perspectively correct image. This works well in the region surrounded by 
the match points, but gives poor results the further you go out of that 
surrounded area.

So in the end, I would like to use several rectified patches and map 
them on the perspectively correct image. Therefore I have to limit each 
supersposition area.

Greetings from Germany
Wolfgang Hugemann