Creating a semi-transparent PNG with ImageMagick/convert

Question

I have PNG files that I would like to convert the whole image to be semi-transparent.

The image will be referenced in a KML file as an icon overlay for use in Google Ear

Answer 1

Resolution has been found:

convert input.png -alpha set -channel A -evaluate set 50% output.png

The above command makes the entire image (all colors) semi-transparent.

Another problem I had was that the latest version of ImageMagick was compiled from source without all the most recent image libraries installed (in particular libpng). Pay close attention to the output of configure to ensure libpng is found if compiling from source. It also appears that versions of ImageMagick earlier than 6.6 and/or older versions of libpng may not support transparent png generation.