Using ImageMagick to Batch Convert Photos

Back when I posted my photoset from WordCamp Minneapolis 2013, rather than performing any post processing, I just uploaded the full images from my camera’s SD card. Normally, I would open up the Gimp and reduce the resolution by 50% and then crop to a 3:2 or 16:10 ratio depending on what was appropriate for the images. This produces small files that are easy for the server to handle.

Since WordPress generally does a good job generating the image sizes it needs, I didn’t worry about uploading the full, unreduced images. Normally, the end users would never see the full size images, so no harm, right? Wrong. At least if you use Jetpack.

If you use the tiled gallery feature in Jetpack (like I do on this site) you end up using the WordPress.com CDN. Unfortunately, Jetpack tries to load the full image size when caching for the tiled gallery. Trying to pull 50 or so images, at 1 MiB to 2 MiB a piece to cache didn’t work too well. Naturally, Jetpack could do things slightly more intelligently and request for the closest, already existing, image size to be used, but that’s a topic for another day.

To get things to play nicely I needed to reduce the ‘original’ file sizes. Thankfully, Weblogs.us has ImageMagick installed. Thus, fixing the issue was as simple as running:

convert P*.jpg -resize 50% \
-quality 88 \
-set filename:newsize '50_%t' '[filename:newsize].jpg';

Then, after inspecting the results, all that was left to do was to rename the 50_ prefixed files back to the original file name.

-John Havlik

[end of transmission, stay tuned]

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