[wp-trac] [WordPress Trac] #57238: Criteria for Responsive Images of WebP: Preserve Alpha + Preserve Lossless + Smaller in size than original

[WordPress Trac]

#57238: Criteria for Responsive Images of WebP: Preserve Alpha + Preserve Lossless
+ Smaller in size than original
--------------------------+-----------------------------
 Reporter:  abitofmind    |      Owner:  (none)
     Type:  defect (bug)  |     Status:  new
 Priority:  normal        |  Milestone:  Awaiting Review
Component:  Media         |    Version:  6.1.1
 Severity:  normal        |   Keywords:
  Focuses:                |
--------------------------+-----------------------------
 On WordPress 6.1.1 I uploaded different WebPs (lossless/lossy +
 with/without alpha channel) and the Responsive Images that WordPress
 creates from them should fulfill these criteria

 1. ✅ Preserve the alpha channel → To maintain layout/design capabilities
 2. ✅ Lossless original should result in lossless sized versions → To
 preserve full quality (e.g. screen designs, diagrams, etc)
 3. ✅ File size of smaller sized versions should never be larger than the
 original → Else the purpose of Responsive Images to conserve bandwidth is
 moot.

 To my shock in one case ALL 3 criteria were violated, in other cases some
 criteria were violated.

 == Input: WebP, lossless, with alpha channel
 **Original**: Axure-Prototyping-2-dynamic-title-scaled.webp • 2880 × 1750
 • 21.77 KB • 1:926 (compression factor)

 1. ❌ Without alpha channel → Looses me layout/design capabilities.
     - Found no ticket "Responsive Images of WebP should preserve alpha
 channel" - Should we file this?
 2. ❌ Lossy versions from lossless original → Looses me quality against my
 intention.
   - Possibly related to one of those:
     - #53663 another example for a failed feature detection?
     - #53669 maybe related?
 3. ❌ Larger in file size, even if smaller in dimensions → Contrary to the
 intended goal of Responsive Images.
   - Axure-Prototyping-2-dynamic-title-scaled.webp • 2560 × 1556 • 29.67 KB
 • 1:536
   - Axure-Prototyping-2-dynamic-title-2048x1244.webp • 2048 × 1244 • 23.89
 KB • 1:426
   - Axure-Prototyping-2-dynamic-title-1536x933.webp • 1536 × 933 • 16.20
 KB • 1:353
     - Is the first one to be smaller in file size than the original
   - Issue #53653 fixed larger file sizes by avoiding lossless versions
 from lossy original.
     - But here we have the reverse, but nevertheless an increase in file
 size! Also need to avoid that.


 == Input: WebP, lossless, without alpha channel
 Original: Confluence-Absence-Inspector-in-Create-Mode.webp • 2880 × 1620 •
 672.73 KB • 1:27

 1. N/A No alpha channel present in the original. So no alpha channel can
 be lost either.
 2. ❌ Lossy versions from lossless original → Undesired!
 3. ✅ File size of smaller sized versions are all smaller than the
 original:
   -  Confluence-Absence-Inspector-in-Create-Mode-scaled.webp • 2560 × 1440
 • 136.88 KB • 1:107
     - Already the first smaller version is already smaller in file size
       - Thanks to compression ofc. Note: When the version would remain
 lossless (see point 2) I still would have that expectation ofc.

 == Further notes
 - The WebP files were created from PNGs with GraphicConverter 11.7.1 (beta
 build 5672) with these metadata options:
   - Without any color profile information (the raw pixel values are sRGB
 compliant, so the default fallback sRGB applies)
   - With a [https://www.lemkesoft.info/forum/viewtopic.php?p=27608#p27608
 minimal Exif chunk which only stores resolution info] (I tag my Retina
 images as 144 PPI, something which I would like WordPress to detect as a
 @2x Retina image, but that's another issue)
    - Btw I was concerned that that Exif chunk is at the end of the file
 (at the end of the linked post), but the developer of GraphicConverter
 said, [https://www.lemkesoft.info/forum/viewtopic.php?p=27646#p27646 I
 just use the public code from Google to create the WebP and can't
 influence the order, that's how the WebP comes out.]
      - Wanted to mention it here for completeness, maybe relevant.
 - With webpinfo I confirmed the existence of alpha channel and
 lossless/lossy status of WebP for all mentioned files (originals +
 responsive image versions by WordPress) .

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Ticket URL: <https://core.trac.wordpress.org/ticket/57238>
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