Add ImageShow support for GraphicsMagick

[latosha-maltba]

GraphicsMagick[0] is a "stable/conservative" fork of ImageMagick[1]. It provides the same commands and is almost always interchangable with the corresponding imageMagick command. The ImageMagick command <im-command> <arguments> becomes gm <im-command> <arguments>, e.g. convert file.jpg file.png becomes gm convert file.jpg file.png.

This pull request does two things:

• Update the test suite to run with either ImageMagick or GraphicsMagick (only test_file_palm.py seems to use it)
• Update ImageShow to support gm display

[0] http://www.graphicsmagick.org/
[1] http://www.graphicsmagick.org/mission.html

[hugovk]

Thank you for the PR, please could you also include this in the release notes?

https://github.com/python-pillow/Pillow/blob/master/docs/releasenotes/8.2.0.rst

[latosha-maltba]

Sure!

Sidenote: Maybe its point worth adding this suggestion (i.e. when providing a pull request also update the releasenotes) to .github/CONTRIBUTING.md?

[latosha-maltba]

The Docs fail in Test / ubuntu-latest Python 3.9 (pull_request) and I guess the culprit is the warning WARNING: py:class reference target not found: PIL.ImageShow.GmDisplayViewer.

Sadly I have no clue how to correctly reference the newly added class. The usual :py:class:`PIL.ImageShow.GmDisplayViewer` does not seem to work (might be because of autoclass:: since in the same file :py:class:`PIL.ImageShow.Viewer` does work?). Help would be appreciated.

[nulano]

Looking at the generated docs, I see that the link to the new viewer references PIL.ImageShow.UnixViewer.GmDisplayViewer: https://pillow--5349.org.readthedocs.build/en/5349/reference/ImageShow.html#PIL.ImageShow.UnixViewer.GmDisplayViewer

Therefore a quick hack would be to use :py:class:`~PIL.ImageShow.UnixViewer.GmDisplayViewer`
I'm not sure whether there is an obvious way to fix this properly without sacrificing the formating.

[latosha-maltba]

@nulano Thanks for the hint (in particular the URL to the preliminary generated docs) and solving the mystery. It seems expected and unexpected that Sphinx uses its own class hierarchy instead of the one of Python.

In the end I settled with :py:class:`PIL.ImageShow.GmDisplayViewer <PIL.ImageShow.UnixViewer.GmDisplayViewer>` so we have the full-qualified name (PIL.ImageShow.GmDisplayViewer) pointing to the correct (sub)class.

Interesting side note: Test / ubuntu-latest Python 3.9 (pull_request) failed the docs test due to the warning while docs/readthedocs.org:pillow succeeded...

[hugovk]

Sidenote: Maybe its point worth adding this suggestion (i.e. when providing a pull request also update the releasenotes) to .github/CONTRIBUTING.md?

Good idea! We already had it in https://github.com/python-pillow/Pillow/blob/master/docs/releasenotes/index.rst but makes sense to include it in the contrib guide too. Please see PR #5357. Thanks!