[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
--------------------------+-----------------------------
 Reporter:  elvismdev     |      Owner:  (none)
     Type:  defect (bug)  |     Status:  new
 Priority:  normal        |  Milestone:  Awaiting Review
Component:  Media         |    Version:  6.8
 Severity:  major         |   Keywords:
  Focuses:                |
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 In WordPress 6.8, there's a reproducible regression where transparent PNG
 images experience noticeable quality degradation when resized by WordPress
 into additional image sizes (e.g., 768x768). This behavior does not occur
 in WordPress 6.7 under the same conditions.


 == Environment
 * **Tested on WP Engine and locally with LocalWP**
 * **Active editor**: `WP_Image_Editor_Imagick`
 * **Imagick version**: 3.7.0
 * **ImageMagick version**: 6.9.11-60 Q16 x86_64
 * **GD version**: 2.3.0
 * **Theme**: TwentyTwentyFive
 * **No plugins installed**


 == Reproduction steps
 1. Start a fresh install of WordPress 6.7.2 (vanilla install using default
 TwentyTwentyFive theme, no plugins active).
 2. Upload a high-detail PNG with transparency (example attached: color
 bird illustration, 2500×2500 px).
 3. Observe resized image versions (e.g., 768x768) — output is clear and
 color-accurate.
 4. Upgrade same install to WordPress 6.8.1.
 5. Upload the same original PNG again or regenerate image sizes.
 6. Observe the resized versions — visible degradation, color banding.


 == Visual comparison

 I'm adding image samples showing a 768x768 output from WP 6.7.2 (clear
 output and color-accurate) and WP 6.8.1 (noticeable quality loss).

 **WP 6.7.2 (expected result - no quality lost)**:
 [[Image(vivid-green-bird-sample-768x768-no-quality-lost-wp67-local.png)]]

 **WP 6.8.1 (visible degradation)**:
 [[Image(vivid-green-bird-sample-768x768-lost-quality-wp68-local.png)]]

 **Original PNG uploaded to both installs**:
 [[Image(vivid-green-bird-sample-original.png)]]


 == Notes
 This may relate to changes mentioned in the
 [https://make.wordpress.org/core/2025/03/25/miscellaneous-developer-
 changes-in-wordpress-6-8/ WordPress 6.8 Miscellaneous Developer Changes]
 post, possibly affecting image quality logic for certain file types.


 == Expected
 Image quality for resized PNGs should remain consistent across WordPress
 versions, assuming identical libraries and environment.


 == Actual
 In WP 6.8.1, the resized image shows degradation not present in 6.7.2.

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