Visual diffs for the images in a git repo

18 March 2016

My first task for new projects is to create a git repository. Whether it is a new app, or conference slides, or even an article, I prefer to keep everything in Git. It is an excellent tool, and, personally, I enjoy its command line interface.

However, there is always room for improvement. Recently I found an idea how to make my git visualize the changes that I've done to the images in a repository.

Varyas-MBP £ ~/path/to/project ⤳ master*
  › git status
On branch master
Changes not staged for commit:
  (use "git add <file>..." to update what will be committed)
  (use "git checkout -- <file>..." to discard changes in working directory)

    modified:   desktop.blocks/social-ico/social-ico__rss.png
    modified:   desktop.blocks/social-ico/social-ico__twitter.png

no changes added to commit (use "git add" and/or "git commit -a")
Varyas-MBP £ ~/path/to/project ⤳ master*

I can run git diff over each of the modified images to see the following:

diff1 diff2

For every picture displayed, I can see a previous version (on the left side), a current version (on the right side) and visually represented difference (in between). This feature is useful for tracking exact changes in the image. Now, I can easily spot if some particular details were shifted, some colors were changed, or a picture was accidentally blended with another one.

You can configure git to do the same. First, install ImageMagick to provide a visual comparison. Note that for the latest versions of OS X you need to install a package with a support of X11. To install with brew, do the following:

  › brew install imagemagick --with-x11

Then, create a script to compare two given images. Run cat ~/bin/git-imgdiff, and paste inside:

compare $2 $1 png:- | montage -geometry +4+4 $2 - $1 png:- | display -title "$1" -

Now, test the script. It should do the comparison of two processed images:

  › ~/bin/git-imgdiff img1.png img2.png

At this step, you need to show git how to use compariing script. For that, define the image files their extensions in a special file. Run cat ~/.gitattributes and paste the following:

*.gif diff=image
*.jpg diff=image
*.png diff=image

Provide git with this configuration by:

  › git config --global core.attributesfile '~/.gitattributes'

The last step is to instruct git to use your script when diff command is applied to the matching files:

  › git config --global diff.image.command '~/bin/git-imgdiff'

That's it. Now, running a git diff over your images you will get a human-friendly output. Enjoy!


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