This took me way too long to find out, so I thought I’d blog here and hopefully save someone else some time.
ImageMagick is a great swiss-army-knife type tool, with a shed-load of options for converting and combining images. Unfortunately, the sheer number of options can make it a bit time-consuming and frustrating trying to find the one you want.
My aim was simple: given a PNG, make the whole thing semi-transparent.
Searching Google using “transparent” and “opacity” drew a blank – all I got was instructions on how to set transparency for certain colours – not what I wanted to do.
The word I was missing was “alpha”, and the magic incantation for changing the opacity of the whole image is:
convert input.png -channel Alpha -evaluate Divide 2 output.png
In my case, I wanted to set the PNG to be 50% transparent (hence “Divide 2”.) Of course, you can change that number to whatever works for you.