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Hydra Derivatives
Hydra Derivatives is an important component of our app, which we don't currently override in any way. We may have to fork it or override parts of it at some point down the road.
It is dependent on several command-line utilities which are often configured differently in dev vs production. This is a place to record issues with these components.
ImageMagick's identify command is pivotal to the actions taken by Hydra Derivatives.
If identify cannot anayze PDFs then no thumbnail will be produced for a PDF file, for example.
In my macOS dev space, I found that I had to install GhostScript and then reinstall ImageMagick with GhostScript enabled to get Hydra Derivatives to make thumbnails for PDF files.
This assumes you've been using Homebrew as mentioned in the macOS Setup Guide, and that you've already installed ImageMagick. If not, then I guess just install rather than reinstall.
brew install ghostscript
brew reinstall imagemagick --with-ghostscript
You can check if identify is returning information after doing this (beforehand it just came back blank, which also meant no details were available in Resque's failed jobs listing):
$ identify pdfs/fig_x.pdf
pdfs/fig_x.pdf PDF 612x792 612x792+0+0 16-bit sRGB 367433B 0.000u 0:00.009
The production machines use Debian-provided ImageMagick, and didn't seem to have this problem.