If you want the alpha channel to contain the luMinosity yes.
make sure of your spelling ;-)
You should also be able convert a 'clone' of the image to HSL format
(-colorspace HSL) and extract the "Luminosity" channel of the image.
I tried to generate an alpha channel for my PNG but was only partially
sucessful. Finally, I came back to my original plan - I wanted to
overlay a black and white logo on JPEGs - and succeeded with
convert -density 171 logo.eps logo.png
composite logo.png photo.jpg -compose bumpmap -gravity southeast result.jpg
Proceeding such, the white parts of the logo become totally transparent
while the darker parts overlay the photograph. I took this trick from
Anthony's webpage:
http://www.cit.gu.edu.au/~anthony/graphics/imagick6/annotating/#overlay
I also found his -density trick for vector graphics on his webpage:
http://www.cit.gu.edu.au/~anthony/graphics/imagick6/formats/#vector
----------
But I was not successful in regard to generating an alpha channel in a
PNG. The '-type TrueColorMatte' option generated a grayscale image with
a 1-bit alpha channel and I just did not manage to generate a real 8-bit
alpha channel and gave up. And of course, I have to have an 8-bit alpha
channel in order to manipulate it....
-----------
Drawing a logo on a bunch of photographs is probably a task not that
seldom. I would like to give something back and provide the code
somewhere - but where? I performed this on a windows machine with VBS.
(Spare me your mockery!)
Wolfgang Hugemann
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