bug#65432: 30.0.50; jerky image movements when zoomed

[Eli Zaretskii]

Ping!  Should we mention this issue in PROBLEMS?

> Cc: 65432@debbugs.gnu.org, manuel@ledu-giraud.fr
> Date: Sat, 26 Aug 2023 10:58:56 +0300
> From: Eli Zaretskii <eliz@gnu.org>
> 
> > From: Po Lu <luangruo@yahoo.com>
> > Cc: Manuel Giraud <manuel@ledu-giraud.fr>,  65432@debbugs.gnu.org
> > Date: Tue, 22 Aug 2023 10:59:22 +0800
> > 
> > Eli Zaretskii <eliz@gnu.org> writes:
> > 
> > >> Date: Mon, 21 Aug 2023 16:45:31 +0200
> > >> From:  Manuel Giraud via "Bug reports for GNU Emacs,
> > >>  the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>
> > >> 
> > >> When opening a rather big image into a quite big Emacs window, movements
> > >> (C-n, C-p, C-f, C-b) are smooth at original image size but are jerky
> > >> whenever there is some zooming (in or out) involved.
> > >> 
> > >> Recipe:
> > >>         - emacs -Q
> > >>         - 'C-x C-f' "a big image.jpg"
> > >>         - 's o' (image-transform-reset-to-original)
> > >>         - keep 'C-n' pressed, keep 'C-f' pressed -> movements are smooth
> > >>         - 'i -' (image-decrease-size)
> > >>         - keep 'C-n' pressed, keep 'C-f' pressed -> movements are jerky
> > >
> > > I cannot reproduce this, on MS-Windows.  I see no difference between
> > > "jerkyness" of motion commands before and after 'i -'.
> > 
> > This is a known inefficiency with Cairo's implementation of affine
> > transforms.  AFAICT, Cairo performs the transform on the CPU, which is
> > aggravated by uploading the complete bitmap to the X server every time
> > the image is displayed.
> > 
> > Building without Cairo should induce Emacs to utilize XRender transforms
> > instead, ameliorating this problem.
> 
> Should this be in PROBLEMS?
> 
> 
> 
>