bug#65432: 30.0.50; jerky image movements when zoomed

[Eli Zaretskii]

> 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?