Tuesday, April 23, 2019

imagemagick fixed heights suitable for thumbnail sheet

A while back I mentioned using ImageMagick to make CSS thumbsheets.

I do this for the Somerville Porchfest website and every year it feels like a wrestling match - with all these bandphotos some are "quirky", either portrait mode when everyone else is landscape, or else just "really weirdly long landscape.

My process is a bit clunky, but I use so many command-line-ish tools I'm not sure it's worth automating fully... instead I generated a webpage that constructs shell commands to run by hand.

The first step is to take the raw JSON from the Somerville Arts Council site and parse it into a file I call "bandjson.js". My webpage shows me a list of all band IDs and the corresponding thumbnail URLs that I can inspect by hand for any weirdness.

For each of those I extract the extesnion (usually jpg or png or JPG or jpeg or JPEG)  then I construct a
`wget -O ${bandid}.${extension} "${thumbailurl}"`;
which gathers all the files locally and normalizes the filenames to bandid.ext

I make a bit of .js then that maps bandid to bandid.ext

The painful bit for this year was working out this normalization, say for 46702.jpg:
convert '46702.jpg' -resize 100x67 -background white -gravity center -extent 100x67 '46702.jpg';

the background/gravity/extent arguments are what makes sure the image is at LEAST as big as those dimensions (centered on a white background in this case). Without that, I'd have to do weird work to read and track the size of each thumbnail separately.

After that I make a single honking big Imagemagick command line argument to glue them together, using \ at EOL so I don't overrun my shell's limit for single commands
convert \
-append thumbsheet.jpg

I then build another js file mapping bandid to it's offset # in the sheet- I can then multiply that number by 67 (the height I requested on the convert operation) to generate the CSS - each image is width 100px, height 67px, and then the background for say the second one is:
background: url(thumbsheet.jpg) 0 -67px;

I'm not positive if all the work is worth it, maybe I should just have the 153 images on the page, but that feels weird to me still.

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