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
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