Detecting mostly empty images using imagemagick

Question

I'd like to use imagemagick or graphicsmagick to detect whether an image has basically no content.

Here is an example:

https://s3-us-west-2.amazonaws.com/idelog/token_page_images/120c6af0-73eb-11e4-9483-4d4827589112_embed.png

I've scoured Fred's imagemagick scripts, but I can't figure out if there is a way to do this:

http://www.fmwconcepts.com/imagemagick/

Answer 1

Easiest way would be to use -edge detection followed by histogram: & text:. This will generate a large list of pixel information that can be passed to another process for evaluation.

convert 120c6af0-73eb-11e4-9483-4d4827589112_embed.png \
        -edge 1 histogram:text:- | cut -d ' ' -f 4 | sort | uniq -c

The above example will generate a nice report of:

50999 #000000
  201 #FFFFFF

As the count of white pixels is less then 1% of black pixels, I can say the image is empty.

This can probably be simplified by passing -fx information to awk utility.

convert 120c6af0-73eb-11e4-9483-4d4827589112_embed.png \
        -format '%[mean] %[max]' info:- | awk '{print $1/$2}'
#=> 0.00684814

Answer 2

If you are talking about the amount of opaque pixels vs the amount of transparent pixels, then the following will tell you the percentage of opaque pixels.

convert test.png -alpha extract -format "%[fx:100*mean]\n" info:

39.0626

Or if you want the percentage of transparent pixels, use

convert test.png -alpha extract -format "%[fx:100*(1-mean)]\n" info:

60.9374