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I am wondering whether there is a way to recognize (and possibly move/delete) B&W photos in folder containing both B&W and colored images? I am using Ubuntu Linux.

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1 Answer

If you install imagemagick, you can use the following command to test if something is greyscale:

$ convert bw.jpg -format "%[colorspace]" info:
Gray

To install:

apt-get install imagemagick

So to go through them all and move:

for i in /images/folder
do
  if [ `convert $i -format "%[colorspace]" info:` == "Gray" ] ; then mv $i /images/folder/bw ; fi
done

However, this method only tests for the colorspace that an image is using. An image might be using a full RGB colorspace, while only actually using greyscale tones (ideally these would be converted to greyscale to be optimal).

In order to work out is just using grey tones, one option is to convert the image to HSL colour, then calculate the average saturation of an image. For a true greyscale image, the average saturation will be zero. With greyscale images in jpg, you are going to get a bit of deviation from perfect greyscale due to artefacts, and generally they aren't perfect depending on how they ended up black and white.

This image for example:

black and white comics

If we convert this to HSL and get the average saturation:

$ convert black-and-white-comics.jpg -colorspace HSL -channel g -separate +channel -format "%[fx:mean]" info:
0.00781798

The figure output ranges from 0-1, so you would have to define a threshold underwhich you consider something to be grescale depending on your source files.

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'convert bw.jpg -format "%[colorspace]" info:' gives a lot of false RGB results. –  hnns Jun 6 at 2:04
    
@hnns Can you post an example somewhere? –  Paul Jun 6 at 3:46
    
For example try this one: cristinabarkerjones.files.wordpress.com/2013/09/… –  hnns Jun 6 at 4:32
    
Hi @hnns, I have updated the answer –  Paul Jun 6 at 5:19
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