GNU bug report logs - #77104
31.0.50; Transparent png image background is ignored in frames with alpha-background

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Message #5 received at submit <at> debbugs.gnu.org (full text, mbox):

From: Ihor Radchenko <yantar92 <at> posteo.net>
To: bug-gnu-emacs <at> gnu.org
Subject: 31.0.50; Transparent png image background is ignored in frames with
 alpha-background
Date: Tue, 18 Mar 2025 18:03:23 +0000

[Message part 1 (text/plain, inline)]

Try the following starting from emacs -Q + the attached .png file:

1.

(let ((ov (make-overlay 1 10)))
  (set-frame-parameter nil 'alpha-background 90)
  (overlay-put
   ov
   'display
   (create-image
    "/path/to/image.png"
    nil nil :width 100)))

2. Observe image background not being transparent, but instead using
default face background.

3.

(let ((ov (make-overlay 1 10)))
  (set-frame-parameter nil 'alpha-background 90)
  (overlay-put
   ov
   'display
   (create-image
    "/path/to/image.png"
    nil nil :width 100 :mask 'heuristic)))

4. With :mask 'heuristic, the background becomes transparent (mod
"rough" edges due to clipping)

I believe that (1) is a bug.

See also https://list.orgmode.org/orgmode/7324dc0.f8517.195a224ffd1.Coremail.dybfysiat <at> 163.com/

[image.png (image/png, attachment)]

[Message part 3 (text/plain, inline)]

In GNU Emacs 31.0.50 (build 1, x86_64-pc-linux-gnu, GTK+ Version
 3.24.42, cairo version 1.18.2) of 2025-03-16 built on localhost
Repository revision: 52879c61e961cac71e7bb0ba12b73f6dd4f28877
Repository branch: scratch/markers-as-gap-array
Windowing system distributor 'The X.Org Foundation', version 11.0.12101014
System Description: Gentoo Linux

Configured using:
 'configure --with-tree-sitter --with-native-compilation 'CFLAGS=-g3
 -I/opt/mps/include -L/opt/mps/lib'
 JAVAC=/etc/java-config-2/current-system-vm/bin/javac
 PKG_CONFIG_PATH=/usr/share/guile-data/3.0/pkgconfig'

Configured features:
ACL CAIRO DBUS FREETYPE GIF GLIB GMP GNUTLS GSETTINGS HARFBUZZ JPEG
LCMS2 LIBXML2 MODULES NATIVE_COMP NOTIFY INOTIFY PDUMPER PNG RSVG
SECCOMP SOUND SQLITE3 THREADS TIFF TOOLKIT_SCROLL_BARS TREE_SITTER WEBP
X11 XDBE XIM XINPUT2 XPM GTK3 ZLIB

-- 
Ihor Radchenko // yantar92,
Org mode maintainer,
Learn more about Org mode at <https://orgmode.org/>.
Support Org development at <https://liberapay.com/org-mode>,
or support my work at <https://liberapay.com/yantar92>

Message #8 received at 77104 <at> debbugs.gnu.org (full text, mbox):

From: Eli Zaretskii <eliz <at> gnu.org>
To: Ihor Radchenko <yantar92 <at> posteo.net>, Po Lu <luangruo <at> yahoo.com>,
 Alan Third <alan <at> idiocy.org>
Cc: dybfysiat <at> 163.com, 77104 <at> debbugs.gnu.org
Subject: Re: bug#77104: 31.0.50;
 Transparent png image background is ignored in frames with
 alpha-background
Date: Wed, 19 Mar 2025 14:14:28 +0200

> Cc: gynamics <dybfysiat <at> 163.com>
> From: Ihor Radchenko <yantar92 <at> posteo.net>
> Date: Tue, 18 Mar 2025 18:03:23 +0000
> 
> Try the following starting from emacs -Q + the attached .png file:
> 
> 1.
> 
> (let ((ov (make-overlay 1 10)))
>   (set-frame-parameter nil 'alpha-background 90)
>   (overlay-put
>    ov
>    'display
>    (create-image
>     "/path/to/image.png"
>     nil nil :width 100)))
> 
> 2. Observe image background not being transparent, but instead using
> default face background.
> 
> 3.
> 
> (let ((ov (make-overlay 1 10)))
>   (set-frame-parameter nil 'alpha-background 90)
>   (overlay-put
>    ov
>    'display
>    (create-image
>     "/path/to/image.png"
>     nil nil :width 100 :mask 'heuristic)))
> 
> 4. With :mask 'heuristic, the background becomes transparent (mod
> "rough" edges due to clipping)
> 
> I believe that (1) is a bug.
> 
> See also https://list.orgmode.org/orgmode/7324dc0.f8517.195a224ffd1.Coremail.dybfysiat <at> 163.com/

Po Lu and Alan, is this a bug or a missing feature?

Message #11 received at 77104 <at> debbugs.gnu.org (full text, mbox):

From: Po Lu <luangruo <at> yahoo.com>
To: Eli Zaretskii <eliz <at> gnu.org>
Cc: dybfysiat <at> 163.com, Ihor Radchenko <yantar92 <at> posteo.net>,
 77104 <at> debbugs.gnu.org, Alan Third <alan <at> idiocy.org>
Subject: Re: bug#77104: 31.0.50; Transparent png image background is ignored
 in frames with alpha-background
Date: Wed, 19 Mar 2025 21:06:07 +0800

Eli Zaretskii <eliz <at> gnu.org> writes:

> Po Lu and Alan, is this a bug or a missing feature?

I'm tending towards "bug", but only Alan can be sure.

Message #14 received at 77104 <at> debbugs.gnu.org (full text, mbox):

From: Alan Third <alan <at> idiocy.org>
To: Po Lu <luangruo <at> yahoo.com>
Cc: dybfysiat <at> 163.com, Eli Zaretskii <eliz <at> gnu.org>, 77104 <at> debbugs.gnu.org,
 Ihor Radchenko <yantar92 <at> posteo.net>
Subject: Re: bug#77104: 31.0.50; Transparent png image background is ignored
 in frames with alpha-background
Date: Wed, 19 Mar 2025 16:23:44 +0000

On Wed, Mar 19, 2025 at 09:06:07PM +0800, Po Lu wrote:
> Eli Zaretskii <eliz <at> gnu.org> writes:
> 
> > Po Lu and Alan, is this a bug or a missing feature?
> 
> I'm tending towards "bug", but only Alan can be sure.

I'd lean more towards missing feature, but it's a toss up really.

I think the problem is that Emacs has never handled image
transparency, so it sets the transparent pixels of the image to the
frame's background colour. There's quite a bit of fiddling to manage
this with changing backgrounds and so on.

I'm not sure what would be required for drawing transparent images to
the glass. On NS I think it would be near trivial as the toolkit will
handle it for us, but I don't know about others. For example I have a
memory of being told that some terms don't clear under images so
drawing a transparent image might show the previous pixels.
-- 
Alan Third

Message #17 received at 77104 <at> debbugs.gnu.org (full text, mbox):

From: Ihor Radchenko <yantar92 <at> posteo.net>
To: Alan Third <alan <at> idiocy.org>
Cc: Po Lu <luangruo <at> yahoo.com>, dybfysiat <at> 163.com, Eli Zaretskii <eliz <at> gnu.org>,
 77104 <at> debbugs.gnu.org
Subject: Re: bug#77104: 31.0.50; Transparent png image background is ignored
 in frames with alpha-background
Date: Wed, 19 Mar 2025 16:57:01 +0000

Alan Third <alan <at> idiocy.org> writes:

> I think the problem is that Emacs has never handled image
> transparency, so it sets the transparent pixels of the image to the
> frame's background colour. There's quite a bit of fiddling to manage
> this with changing backgrounds and so on.
>
> I'm not sure what would be required for drawing transparent images to
> the glass. On NS I think it would be near trivial as the toolkit will
> handle it for us, but I don't know about others. For example I have a
> memory of being told that some terms don't clear under images so
> drawing a transparent image might show the previous pixels.

Then, how does :mask 'heuristic works? It somehow manages to set certain
pixels to be truly transparent. If so, why is it a problem setting
pixels that have transparent color in the image truly transparent in
exactly the same way?

-- 
Ihor Radchenko // yantar92,
Org mode maintainer,
Learn more about Org mode at <https://orgmode.org/>.
Support Org development at <https://liberapay.com/org-mode>,
or support my work at <https://liberapay.com/yantar92>

Message #20 received at 77104 <at> debbugs.gnu.org (full text, mbox):

From: Eli Zaretskii <eliz <at> gnu.org>
To: Alan Third <alan <at> idiocy.org>
Cc: luangruo <at> yahoo.com, dybfysiat <at> 163.com, alan <at> idiocy.org,
 77104 <at> debbugs.gnu.org, yantar92 <at> posteo.net
Subject: Re: bug#77104: 31.0.50; Transparent png image background is ignored
 in frames with alpha-background
Date: Wed, 19 Mar 2025 19:02:44 +0200

> Date: Wed, 19 Mar 2025 16:23:44 +0000
> From: Alan Third <alan <at> idiocy.org>
> Cc: Eli Zaretskii <eliz <at> gnu.org>, Ihor Radchenko <yantar92 <at> posteo.net>,
> 	77104 <at> debbugs.gnu.org, dybfysiat <at> 163.com
> 
> On Wed, Mar 19, 2025 at 09:06:07PM +0800, Po Lu wrote:
> > Eli Zaretskii <eliz <at> gnu.org> writes:
> > 
> > > Po Lu and Alan, is this a bug or a missing feature?
> > 
> > I'm tending towards "bug", but only Alan can be sure.
> 
> I'd lean more towards missing feature, but it's a toss up really.
> 
> I think the problem is that Emacs has never handled image
> transparency, so it sets the transparent pixels of the image to the
> frame's background colour.

That's what I thought, indeed.

> There's quite a bit of fiddling to manage this with changing
> backgrounds and so on.
> 
> I'm not sure what would be required for drawing transparent images to
> the glass. On NS I think it would be near trivial as the toolkit will
> handle it for us, but I don't know about others. For example I have a
> memory of being told that some terms don't clear under images so
> drawing a transparent image might show the previous pixels.

Patches welcome.

Message #23 received at 77104 <at> debbugs.gnu.org (full text, mbox):

From: Alan Third <alan <at> idiocy.org>
To: Ihor Radchenko <yantar92 <at> posteo.net>
Cc: Po Lu <luangruo <at> yahoo.com>, dybfysiat <at> 163.com, Eli Zaretskii <eliz <at> gnu.org>,
 77104 <at> debbugs.gnu.org
Subject: Re: bug#77104: 31.0.50; Transparent png image background is ignored
 in frames with alpha-background
Date: Wed, 19 Mar 2025 20:45:37 +0000

On Wed, Mar 19, 2025 at 04:57:01PM +0000, Ihor Radchenko wrote:
> Alan Third <alan <at> idiocy.org> writes:
> 
> > I think the problem is that Emacs has never handled image
> > transparency, so it sets the transparent pixels of the image to the
> > frame's background colour. There's quite a bit of fiddling to manage
> > this with changing backgrounds and so on.
> >
> > I'm not sure what would be required for drawing transparent images to
> > the glass. On NS I think it would be near trivial as the toolkit will
> > handle it for us, but I don't know about others. For example I have a
> > memory of being told that some terms don't clear under images so
> > drawing a transparent image might show the previous pixels.
> 
> Then, how does :mask 'heuristic works? It somehow manages to set certain
> pixels to be truly transparent. If so, why is it a problem setting
> pixels that have transparent color in the image truly transparent in
> exactly the same way?

As I recall on X it uses a mask, so there's a second image that has
the transparent pixels marked (so probably a one bit bitmap) and the
image drawing function only draws the corresponding non-transparent
pixels from the main image.

Other terms may work differently. I don't think NS has native support
for masking bitmaps, for example. I can't remember how we work around
that.
-- 
Alan Third

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