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

convert input.png -alpha set -channel A -evaluate Divide 2 output.png

Thank you @caw for pointing this out in your comment. This helps in the case where the image already has some transparent pixels. -evaluate set 50% ruins the already-transparent pixels that had alpha 0, setting their alpha to 50%, which is often not desirable.

-evaluate Divide 2 preserves the alpha, since fully transparent pixels are alpha 0, and 0 divided by 2 is still 0.