[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:  feature request  |      Status:  new
 Priority:  normal           |   Milestone:  Future Release
Component:  Media            |     Version:
 Severity:  normal           |  Resolution:
 Keywords:                   |     Focuses:
-----------------------------+-----------------------------

Comment (by abitofmind):

 Again an example where the smaller sized lossy version is larger in file
 size than original WebP lossless with alpha:

 || 836 × 426 || original || lossless || 24.598 bytes ||
 || 768 x 391 || thumbnail || lossy || **35.324 bytes (!)** ||
 || 300  ×  153 || thumbnail || lossy || 9.056 bytes ||
 || 150 ×  76 || thumbnail || lossy || 3.346 bytes ||

 I compared them with Exiftool:
 - They are all: 24 Bit Color, 8 Bit Alpha
 - But although the sized versions use compression, their **alpha channel
 is saved uncompressed**!
 - I cannot explain why this is larger than the original which supposedly
 has both composite and alpha channel uncompressed, but I do not know the
 details of the WebP format.

 But hopefully this gives you an idea what is going on and how to improve
 it.
 - Attached is a screenshot of the image file comparison + ZIP archive
 containing original and 3 sized versions
 - Below you find the relevant ExifTool output (irrelevant stuff like file
 permissions, dates, etc removed)


 {{{
 ExifTool Version Number         : 12.50

 ======== Style-Guide.webp
 File Name                       : Style-Guide.webp
 File Size                       : 25 kB
 File Type                       : Extended WEBP
 File Type Extension             : webp
 MIME Type                       : image/webp
 WebP Flags                      : Alpha
 Image Width                     : 836
 Image Height                    : 426
 Image Size                      : 836x426
 Megapixels                      : 0.356

 ======== Style-Guide-768x391.webp
 File Name                       : Style-Guide-768x391.webp
 File Size                       : 35 kB
 File Type                       : Extended WEBP
 File Type Extension             : webp
 MIME Type                       : image/webp
 WebP Flags                      : Alpha
 Image Width                     : 768
 Image Height                    : 391
 Alpha Preprocessing             : Level Reduction
 Alpha Filtering                 : Horizontal
 Alpha Compression               : Lossless
 VP8 Version                     : 0 (bicubic reconstruction, normal loop)
 Horizontal Scale                : 0
 Vertical Scale                  : 0
 Image Size                      : 768x391
 Megapixels                      : 0.300

 ======== Style-Guide-300x153.webp
 File Name                       : Style-Guide-300x153.webp
 File Size                       : 9.1 kB
 File Type                       : Extended WEBP
 File Type Extension             : webp
 MIME Type                       : image/webp
 WebP Flags                      : Alpha
 Image Width                     : 300
 Image Height                    : 153
 Alpha Preprocessing             : Level Reduction
 Alpha Filtering                 : Horizontal
 Alpha Compression               : Lossless
 VP8 Version                     : 0 (bicubic reconstruction, normal loop)
 Horizontal Scale                : 0
 Vertical Scale                  : 0
 Image Size                      : 300x153
 Megapixels                      : 0.046


 ======== Style-Guide-150x76.webp
 File Name                       : Style-Guide-150x76.webp
 File Size                       : 3.3 kB
 File Type                       : Extended WEBP
 File Type Extension             : webp
 MIME Type                       : image/webp
 WebP Flags                      : Alpha
 Image Width                     : 150
 Image Height                    : 76
 Alpha Preprocessing             : Level Reduction
 Alpha Filtering                 : Horizontal
 Alpha Compression               : Lossless
 VP8 Version                     : 0 (bicubic reconstruction, normal loop)
 Horizontal Scale                : 0
 Vertical Scale                  : 0
 Image Size                      : 150x76
 Megapixels                      : 0.011

 }}}

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Ticket URL: <https://core.trac.wordpress.org/ticket/57238#comment:4>
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