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

To followup on Tony Bogdanov's comment. If you already have fully transparent pixels you may want to use the following strategy:

1.) convert the transparent pixels into a mask

convert -alpha extract input.png mask.png

2.) perform the command listed in the answer above:

convert input.png -alpha on -channel a -evaluate set 65% output.png

3.) create a blank canvas the same size as the original image

example: convert -size 700x800 xc:none blankcanvas.png

4.) composite the semitransparent image and the blank canvas together using the transparent pixel mask

composite output.png blankcanvas.png mask.png -compose Src final_output.png