Based on my comparative analysis of JPEG, WebP, and AVIF with a sample size of thirty-five PDFs, I found that WebP and AVIF require an average of 33% and 46% less bits than JPEG, respectively, to achieve a ssimulacra2 score of 80. A score of 80 falls between the "high quality" and "very high quality" brackets on ssimulacra2's scoring scale.

Given that WebP is widely supported in web browsers I propose switching DSpace's default PDF thumbnail format to WebP. Furthermore, I propose re-working the ImageMagick PDF thumbnail filter to not perform a double lossy conversion from PDF to JPEG to JPEG.

Comparison of a PDF thumbnail for an item from the CGSpace repository in JPEG and WebP formats.

JPEG Q92 (left) and WebP Q86 (right). The WebP file is 42% smaller.

See more in-depth discussion and comparisons here: https://alanorth.github.io/improved-dspace-thumbnails/evaluating-jpeg-webp-avif.html

DSpace can be configured to use ImageMagick to generate thumbnails for PDF bitstreams. Out of the box the quality of these thumbnails is underwhelming because the resolution is low and the image is often blurry. The quality can be improved by using a "supersampling" technique and by preferring the PDF CropBox over the MediaBox where possible. These two minor changes produce an image that more accurately resembles what the user would see if they opened the PDF on a screen, which is especially noticeable if the PDF contains text, gradients, or curved lines.

I propose adding the -density 144 and -define pdf:use-cropbox=true parameters to the DSpace ImageMagick PDF thumbnail operation in DSpace 6.x and 7.x.

A comparison of the default DSpace PDF thumbnail for an item on the CGSpace repository before and after adding the -density 144 parameter.

See more in-depth discussion and comparisons here: https://alanorth.github.io/improved-dspace-thumbnails/

• If we don't specify a quality ImageMagick will use 92, see: https://imagemagick.org/script/command-line-options.php#quality
  • This means that, when we do a "supersample" first followed by a thumbnail, it uses quality 92 for the first, and again for the second, which is generation loss...
• -compress Lossless is only for JPEG and is not recommended
  • Should use -quality 100 instead, as "lossless" JPEG is not widely supported (needs a patched libjpeg)
  • I'm unclear about the interaction between -quality and -define webp:lossless=true for WebP...

Copyright (C) Alan Orth

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