[wp-trac] [WordPress Trac] #39216: PDFs with non-opaque alpha channels can result in previews with black backgrounds.

[WordPress Trac]

#39216: PDFs with non-opaque alpha channels can result in previews with black
backgrounds.
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 Reporter:  gitlost                              |       Owner:
     Type:  defect (bug)                         |      Status:  new
 Priority:  normal                               |   Milestone:  Awaiting
Component:  Media                                |  Review
 Severity:  normal                               |     Version:  4.7
 Keywords:  dev-feedback 2nd-opinion has-patch   |  Resolution:
  has-unit-tests                                 |     Focuses:
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Comment (by gitlost):

 Hi, thanks for responding. Yes you're right the
 `Imagick::unsharpMaskImage()` CMYK issue is distinct from the alpha
 channel issue, as it applies to any jpeg with a CMYK color space, so I'll
 split it off into a new ticket and refresh the patch here without it. The
 noticeable color change for CMYK PDFs issue still remains though.

 Was messing about with ImageMagick's "delegate.xml"
 ("/etc/ImageMagick-6/delegate.xml" on Ubuntu 16.10) and found that if on
 the `decode="ps:cmyk"` line you change `-sDEVICE=pam` to `-sDEVICE=jpeg`
 then both the CMYK and the alpha issues disappear and you get perfect
 previews.

 Similarly if on the `decode="ps:alpha` line you change `-sDEVICE=pngalpha`
 to `-sDEVICE=jpeg` and add `-dUseCIEColor` then the alpha channel issue
 disappears for the "test_alpha.pdf" uploaded here.

 So there may be a way through Imagick to get the behaviour from
 ImageMagick that one gets by calling GhostScript directly (where none of
 these issues arises), but it's not easy to see. It's a bit like fishing
 through a keyhole.

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Ticket URL: <https://core.trac.wordpress.org/ticket/39216#comment:5>
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