[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:  6.8.2
Component:  Media         |     Version:  6.8
 Severity:  major         |  Resolution:
 Keywords:  has-patch     |     Focuses:
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Comment (by siliconforks):

 The patch in [https://github.com/WordPress/wordpress-develop/pull/8810 PR
 #8810] doesn't really seem to fix the fundamental issue here...

 I think fundamental problem is that [59589] seems to be assuming that if
 `getImageDepth()` returns <= 8, the image must be using <= 256 colors (so
 it is either using indexed color or it can be converted to indexed color)
 - but I don't think this is correct.

 ImageMagick appears to use the term "depth" to refer to the depth of an
 image channel rather than the depth of the entire image.  See for example
 https://github.com/ImageMagick/ImageMagick/blob/main/MagickCore/attribute.c
 where it states that "GetImageDepth() returns the depth of a particular
 image channel."  (Note that the C function `GetImageDepth` is not the same
 thing as the PHP function `getImageDepth` but I suspect they are both
 using the same definition of "depth".)

 Similarly the ImageMagick `identify` command considers the file `vivid-
 green-bird-sample-original.png` to have "Depth: 8-bit":

 {{{
 $ identify -verbose -unique vivid-green-bird-sample-original.png
 Image:
   Filename: vivid-green-bird-sample-original.png
   Format: PNG (Portable Network Graphics)
   Mime type: image/png
 ...
   Depth: 8-bit
   Channel depth:
     red: 8-bit
     green: 8-bit
     blue: 8-bit
     alpha: 8-bit
 ...
   Colors: 830043
 }}}

 So the image has more than 800,000 colors but ImageMagick still considers
 the "depth" to be 8 bits (because the depth of the red channel, the green
 channel, the blue channel, and the alpha channel are each 8 bits).

 You can get the same result with a simple PHP program:

 {{{#!php
 <?php
 $image = new Imagick('vivid-green-bird-sample-original.png');
 $depth = $image->getImageDepth();
 echo 'depth = ' . $depth . "\n";
 $colors = $image->getImageColors();
 echo 'colors = ' . $colors . "\n";
 }}}

 This will display:

 {{{
 depth = 8
 colors = 830043
 }}}

 So I think [59589] really needs to be reworked to not use `getImageDepth`.

-- 
Ticket URL: <https://core.trac.wordpress.org/ticket/63448#comment:11>
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