[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.
--------------------------+------------------------------
 Reporter:  gitlost       |       Owner:
     Type:  defect (bug)  |      Status:  new
 Priority:  normal        |   Milestone:  Awaiting Review
Component:  Media         |     Version:  4.7
 Severity:  normal        |  Resolution:
 Keywords:                |     Focuses:
--------------------------+------------------------------

Comment (by gitlost):

 There's also an issue with PDFs with CMYK color spaces, as the resulting
 "full" jpegs will also have CMYK color spaces, and it turns out that
 `Imagick::unsharpMaskImage()` (as called in
 `WP_Image_Editor_Imagick::thumbnail_image()`) messes them up. In
 particular black text becomes fuzzy and over-black, and colors change.
 This can be seen using the attached "test_cmyk.pdf", generated using
 TCPDF:

 {{{
 #!php
 <?php
 $dirname = dirname( __FILE__ );
 $tcpdf_dirname = $dirname . '/tcpdf';
 require $tcpdf_dirname . '/tcpdf.php';

 $pdf = new TCPDF();
 $pdf->AddSpotColor( 'spot_black', 0, 0, 0, 100 );
 $pdf->AddSpotColor( 'spot_dark_blue', 89, 42, 0, 48 );
 $pdf->AddPage();
 $pdf->SetFontSize( 30 );
 $pdf->SetTextSpotColor( 'spot_black' );
 $pdf->Text( 20, 20, 'Spot Black' );
 $pdf->SetFillSpotColor( 'spot_dark_blue' );
 $pdf->Rect( 20, 40, 30, 30, 'DF' );
 $pdf->SetFontSize( 18 );
 $pdf->SetTextSpotColor( 'spot_dark_blue' );
 $pdf->Text( 20, 70, 'Dark Blue' );
 $pdf->Output( $dirname . '/test_cmyk.pdf', 'F' );
 }}}

 Not calling `Imagick::unsharpMaskImage()` if the color space is CMYK fixes
 the fuzzy black text and severe color change issues, but still results in
 noticeable color change. This could be ameliorated by adding ICC profiles
 in the proposed `pdf_process()` method, but it's messy, and anyway I now
 think that a much better solution to all these issues is to use
 GhostScript directly - see #39262.

 FWIW the following is a refreshed patch not calling
 `Imagick::unsharpMaskImage()` if the color space is CMYK. The unit test
 requires "test_cmyk.pdf" to be uploaded to the test data images directory.

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Ticket URL: <https://core.trac.wordpress.org/ticket/39216#comment:1>
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