[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:  (none)
     Type:  defect (bug)                         |      Status:  new
 Priority:  normal                               |   Milestone:  6.8.2
Component:  Media                                |     Version:  6.8
 Severity:  critical                             |  Resolution:
 Keywords:  has-patch needs-testing needs-unit-  |     Focuses:
  tests                                          |
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Comment (by elvismdev):

 Thanks everyone for the great input and collaborative investigation. I’ve
 just updated my [https://github.com/WordPress/wordpress-develop/pull/8810
 GitHub PR #8810] based on the recent insights shared here.


 **What changed**:

 * I removed reliance on `getImageDepth()` since it's not a reliable
 indicator of whether a PNG is palette-based.
 * Instead, I now use:
   * `getImageColors()` to check if the image has 256 or fewer unique
 colors.
   * `getImageProperty( 'png:IHDR.color_type' )` to verify it’s an
 **indexed PNG** (`color_type == 3`).
 * Quantization is only applied when **both conditions are met**, which
 ensures it won’t degrade quality on full-color PNGs or images with
 gradients/transparency.

 This logic seems to solve the issue without regressing the optimization
 intent of [59589]. Grayscale (png8) and alpha chunk logic is preserved.

 Let me know what you think, happy to iterate further if needed.

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Ticket URL: <https://core.trac.wordpress.org/ticket/63448#comment:22>
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