I'm coming up empty handed while trying to find out how to modify the opacity of an image using the GraphicsMagick CLI. I'm simply trying to input a standard image and convert it to the corresponding image with a reduced opacity. I'm running GraphicsMagick 1.3.14. Using ImageMagick, I can issue the command:
convert input.png -alpha set -channel a -evaluate set 50% +channel output.png
-alpha option (among others) is unrecognized as a valid GM convert option (convert option reference here). Any help would be greatly appreciated.
Note: This question is a cross post of a question on StackOverflow. After posting on SO, I thought SuperUser might be a little more appropriate.