Subject: 29.0.50; XRender image transforms many magnitudes faster than Cairo image transforms
From: Po Lu <[email-masked]>
To: [email-masked]
Date: Fri, 26 Nov 2021 16:06:46 +0800
List-Id: <debbugs-submit.debbugs.gnu.org>

Try to pixel-scroll (using better-pixel-scroll-mode', soon to be
pixel-scroll-precise-mode) through an image with transforms (you can see
such an example by using eww to browse to gnu.org).

On a build without Cairo, which uses the X rendering extension to
achieve hardware acceleration of image scaling, everything is perfectly
smooth.  On a build with Cairo, it takes around second to scroll through
one or two pixels of such an image.

If nobody else knows why this is the case, I plan to look into it in a
few days, as this is within my field of expertise.  Thanks.

In GNU Emacs 29.0.50 (build 318, x86_64-pc-linux-gnu, GTK+ Version 3.24.30)
 of 2021-11-26 built on trinity
Repository revision: 4e9f78d2a4c5608eeb056b5ecd159b6ea251372e
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.12101003
System Description: Fedora Linux 35 (Workstation Edition)

Configured using:
 'configure --without-cairo --with-xinput2 --cache-file=/tmp/ccache'

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