Prevent unnecessary whitespace around thumbnails when processing cropped PDFs

[michel.weimerskirch]

When generating thumbnails from cropped PDF pages, Imagemagick uses the uncropped area - resulting in unnecessary (and ugly) whitespace - unless the option "use-cropbox" is set.

I attached a patch that solves the issue by setting a parameter in ImageMagick. Note that this doesn't in any way affect uncropped PDFs.

I also attached a demo.pdf for you to reproduce the issue as well as two thumbnails generated before and after the patch.

[michel.weimerskirch]

Patch

[michel.weimerskirch]

Demo file to reproduce the issue

[michel.weimerskirch]

Thumbnail generated BEFORE the patch (with whitespace around the cropped image not visible in the PDF itself)

[michel.weimerskirch]

Thumbnail generated AFTER the patch

[michel.weimerskirch]

Replying to michel.weimerskirch:

Not that this doesn't in any way affect uncropped PDFs.

Small typo. This should say "Note that this doesn't in any way affect uncropped PDFs." (unfortunately, Trac doesn't allow editing tickets)

[joemcgill]

Thanks @michelweimerskirch, this looks like a nice improvement. I'm adding this to the 5.3 milestone for testing.

[joemcgill]

Patch applies cleanly and works great without introducing any noticeable performance problems (in fact, if can improve performance because Imagick is generating smaller images).

Also wanted to reiterate that this change will only affect PDFs that have been created with a tool that includes a crop-box. For more info about the CropBox setting, I found this article helpful: ​https://www.w3pedia.com/uk/the-pdf-page-boxes-cropbox-bleedbox-trimbox.

[kirasong]

In 46238:

Media: Prevent unnecessary whitespace around PDF Thumbnails.

Stops unintended whitespace from showing around PDF thumbnails
when the PDF creation tool utilized a CropBox.

Props michelweimerskirch, joemcgill.
Fixes #45598.