[magick-users] Problems with Overlay

[Wolfgang Hugemann]

> convert rect.jpg -threshold 99.8% -fill "rgb(75%,75%,75%)" -opaque white 
> mask.jpg
> 
> Note: you should also be able to use gray75 or grey75 instead of rgb(...)
> See: http://imagemagick.org/script/color.php

These two alternatives give deviant results: in the first case, the gray 
background is somewhat transparent, while it is completely opaque when I 
make use of gray75.

 > convert backgroundimage overlayimage maskimage -composite resultimage

Yes, this works in principle, but the functioning of "mask" is 
inversively to what I would have expected: black is transparent and 
white is opaque! Thus I arrive at the correct mask by

convert rect.jpg -threshold 99.8% -negate -fill gray50 mask.jpg

The opaque option seems not to be needed.
......

I though have to admit that I don't quite understand how the fill option 
exactly functions over here (?).

Thanks for your help on this.
Wolfgang Hugemann