How to make grayscale png images with GraphicsMagick

If you have a color PNG image with transparency don't do this:
gm mogrify -type grayscale image.png
as it will lose the alpha channel.


You can get a similar result by just desaturating it:
gm mogrify -modulate 100,0 image.png

Photo Geotag

One program to write them

http://www.geosetter.de/en/

and one program to read them

http://www.photome.de/