bug#59242: 29.0.50; Poor quality for WebP images

bug#59242: 29.0.50; Poor quality for WebP images

[Evgeny Zajcev]

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WebP images with alpha channel renders in poor quality in the Emacs.
See http://lgarc.narod.ru/pics/emacs-webp-poor-quality.png

If I convert webp image to png, quality is ok in resulting image.  Also, if
I add webp as native image under MacOS (requires Emacs patch) then quality
is also ok.

Looks like partial transparency is incorrectly applied for webp images.

Thanks

In GNU Emacs 29.0.50 (build 1, x86_64-pc-linux-gnu, GTK+ Version
 3.24.20, cairo version 1.16.0) of 2022-10-25 built on x1
Repository revision: 9bfe00dda6b59d9b93eb7a825f6e0868afa1cca3
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.12009000
System Description: Ubuntu 20.04.1 LTS

Configured using:
 'configure --with-modules'

Configured features:
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GTK3 ZLIB

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Load-path shadows:
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-- 
lg

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bug#59242: 29.0.50; Poor quality for WebP images

[Po Lu via Bug reports for GNU Emacs, the Swiss army knife of text editors]

Evgeny Zajcev <lg.zevlg@gmail.com> writes:

> WebP images with alpha channel renders in poor quality in the Emacs.
> See http://lgarc.narod.ru/pics/emacs-webp-poor-quality.png
>
> If I convert webp image to png, quality is ok in resulting image.
> Also, if I add webp as native image under MacOS (requires Emacs patch)
> then quality is also ok.
>
> Looks like partial transparency is incorrectly applied for webp images.

Stefan, I see the following code in image.c:

	  /* An alpha channel associates variable transparency with an
	     image.  WebP allows up to 256 levels of partial transparency.
	     We handle this like with PNG (which see), using the frame's
	     background color to combine the image with.  */
	  if (features.has_alpha || anim)
	    {
	      if (mask_img)
		PUT_PIXEL (mask_img, x, y, *p > 0 ? PIX_MASK_DRAW : PIX_MASK_RETAIN);

however, you seem to have misunderstood the meaning of the PNG code: PNG
images can either have a real alpha channel (where the PNG library does
the blending AFAIK) or a simple on-off transparency switch, which is the
only case where this kind of masking is appropriate.

As a questionable optimization, Emacs also uses masking when only alpha
channels of 0 or 255 are present in a PNG image that has a real alpha
channel.

So if the webp library does not blend itself, I think we will have to do
that for it.  Am I missing anything here?

bug#59242: 29.0.50; Poor quality for WebP images

[Stefan Kangas]

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tags 59242 + patch
thanks

Po Lu <luangruo@yahoo.com> writes:

> > Evgeny Zajcev <lg.zevlg@gmail.com> writes:
>
> > Looks like partial transparency is incorrectly applied for webp images.

Thanks for the bug report.

> Stefan, I see the following code in image.c:
[...]
> So if the webp library does not blend itself, I think we will have to do
> that for it.  Am I missing anything here?

Yes, the old code was not right.  The attached patch should add proper
alpha blending for webp images.  I have tested it with the example
images here: https://developers.google.com/speed/webp/gallery2

Evgeny, could you please test this patch and report back?  It should
apply cleanly both against master and emacs-29.

[-- Attachment #2: 0001-Fix-alpha-blending-for-webp-images.patch --]
[-- Type: application/x-patch, Size: 3808 bytes --]

bug#59242: 29.0.50; Poor quality for WebP images

[Eli Zaretskii]

> Cc: 59242@debbugs.gnu.org, Evgeny Zajcev <lg.zevlg@gmail.com>
> From: Stefan Kangas <stefankangas@gmail.com>
> Date: Mon, 14 Aug 2023 02:11:56 +0200
> 
> Po Lu <luangruo@yahoo.com> writes:
> 
> > > Evgeny Zajcev <lg.zevlg@gmail.com> writes:
> >
> > > Looks like partial transparency is incorrectly applied for webp images.
> 
> Thanks for the bug report.
> 
> > Stefan, I see the following code in image.c:
> [...]
> > So if the webp library does not blend itself, I think we will have to do
> > that for it.  Am I missing anything here?
> 
> Yes, the old code was not right.  The attached patch should add proper
> alpha blending for webp images.  I have tested it with the example
> images here: https://developers.google.com/speed/webp/gallery2
> 
> Evgeny, could you please test this patch and report back?  It should
> apply cleanly both against master and emacs-29.

Thanks, but please install this on master, not on the emacs-29 branch,
even if this patch does work.  It's too late for such changes on the
release branch.

bug#59242: 29.0.50; Poor quality for WebP images

[Stefan Kangas]

 Eli Zaretskii <eliz@gnu.org> writes:

> Thanks, but please install this on master, not on the emacs-29 branch,
> even if this patch does work.  It's too late for such changes on the
> release branch.

Noted.

bug#59242: 29.0.50; Poor quality for WebP images

[Stefan Kangas]

close 59242 30.1
thanks

> Yes, the old code was not right.  The attached patch should add proper
> alpha blending for webp images.  I have tested it with the example
> images here: https://developers.google.com/speed/webp/gallery2

No further comments within 10 days, so I've pushed this to master as
commit eb61bbacef7 and I'm closing the bug.