[wp-trac] [WordPress Trac] #63448: Image quality significantly degrades for resized PNGs with transparency in WordPress 6.8.1

[WordPress Trac]

#63448: Image quality significantly degrades for resized PNGs with transparency in
WordPress 6.8.1
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 Reporter:  elvismdev                            |       Owner:
                                                 |  adamsilverstein
     Type:  defect (bug)                         |      Status:  reopened
 Priority:  normal                               |   Milestone:  6.8.2
Component:  Media                                |     Version:  6.8
 Severity:  critical                             |  Resolution:
 Keywords:  has-patch has-test-info has-unit-    |     Focuses:
  tests changes-requested                        |
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Comment (by siliconforks):

 Replying to [comment:87 SirLouen]:
 > @siliconforks got your point
 >
 > Anyway, can you refresh to me the reason of quantizing and then forcing
 `png8` afterwards, specifically for greyscale images? I remember there was
 a reason, but I've read through the thread, but I can't completely remind
 why this was useful.

 The problem appears to be that when ImageMagick sees an image with only
 gray in it, it seems to think, "PNG has a format specifically for
 grayscale images - I'll use that."  But it's usually a smaller file when
 the image is saved as an indexed PNG, not as a grayscale PNG.  So that's
 what the original purpose of the `setOption( 'png:format', 'png8' )` line
 was - to force ImageMagick to save it in indexed format rather than
 grayscale format.

 For grayscale images, I'm not sure if it makes a difference if you call
 `quantizeImage()` and then call `setOption( 'png:format', 'png8' )`, or if
 you skip the `quantizeImage()` call and just call `setOption(
 'png:format', 'png8' )`.

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Ticket URL: <https://core.trac.wordpress.org/ticket/63448#comment:88>
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