moth-wm-gl

moth-wm-gl is a fork of Xfce xfwm4 that adds OpenGL compositing. XRender, the original way, stays as a fallback.

There is also a new option, on by default, that turns compositing off while a fullscreen application has focus. It is worth about 3% more frames for applications that do not ask for that themselves.

Compared to XRender it uses about a third less power for the same work and provides about 10% more frames per second to applications. The gap grows with load, up to 25% with eight windows drawing flat out.

Driver	Renderer	FPS	CPU (ms/s)	CPU (W)	GPU (W)
AMD (Mesa radeonsi)	no compositor	180.5	-	18.9
	XRender	161.6	11.2	21.0
	OpenGL	174.2	13.2	20.2
NVIDIA proprietary	no compositor	166.3	-	4.0	37.2
	XRender	160.9	17.5	5.8	43.0
	OpenGL	158.6	16.2	4.7	43.2
NVIDIA Mesa (zink)	no compositor	149.6	-	6.8	N/A
	XRender	149.1	23.5	14.8	N/A
	OpenGL	149.1	22.5	14.8	N/A

FPS is measured in a windowed benchmark running as fast as it can; CPU and power are measured while the same benchmark is locked at 60 fps. The AMD machine has the graphics inside the processor, so its one power figure covers both. The numbers come from wmbench, which measures any window manager, so they can be reproduced or compared with another desktop.

Features

OpenGL compositing is on by default; if the driver cannot do it, XRender is used instead.

OpenGL talks to the driver through EGL by default, because it was the cheapest on the processor on every driver tested. MOTH_WM_GL_BACKEND=glx switches to GLX, and if EGL cannot start, GLX takes over on its own.

Only the parts of the screen that changed are painted, then the whole buffer is swapped, which the display hardware does for free. MOTH_WM_GL_PRESENT set to copy or fbo picks other ways of getting the frame on screen; they are for drivers that behave differently, not for everyday use.

Adaptive vsync can be picked now, with either renderer: frames wait for the screen while they can keep up, and stop waiting when they cannot, so a slow moment costs no extra lag. See Settings.

To check which renderer is in use, open Window Manager Tweaks, Compositor tab:

• Use OpenGL for compositing (default on) — enables the OpenGL renderer; XRender is used if it cannot start.
• Suspend compositing for focused fullscreen windows (default on) — turns compositing off while a fullscreen application has focus. Useful for games. While it is off, avoiding tearing is up to the application: fine for OpenGL and Vulkan, others may tear. An application can ask never to be bypassed, which is what mpv --x11-bypass-compositor=never does.
• Display fullscreen overlay windows directly (default on) — same as the original moth-wm; it only gained a tooltip. It covers quite old games and players that bypass the window manager without saying anything.

Usage

The project installs its own moth-wm executables, settings tools, configuration channel, icons, resources, and theme paths. Configure your session to start moth-wm after installation.

To try it without installing, replace the running window manager:

./path_to_new_build/src/moth-wm --replace

Settings

Moth-wm keeps its configuration independent from xfwm4:

Purpose	Path
System defaults	$(sysconfdir)/moth-wm/defaults.conf (normally /usr/local/etc/moth-wm/defaults.conf)
User settings	$XDG_CONFIG_HOME/moth-wm/settings.conf (normally ~/.config/moth-wm/settings.conf)
User themes	$XDG_CONFIG_HOME/moth-wm/themes/<theme>/moth-wm/
System themes	$(datadir)/moth-wm/themes/<theme>/moth-wm/

The settings applications use the independent moth-wm Xfconf channel for live updates and automatically synchronize its scalar options to settings.conf. The user file has final precedence, so it is safe to edit by hand and is never read from xfwm4's configuration directory.

/general/use_gl_compositing turns the OpenGL renderer on and off. It takes effect immediately.

/general/suspend_compositing_fullscreen turns compositing off while a fullscreen window has focus.

/general/vblank_mode, or --vblank on the command line, read at startup only:

value	description
auto	(default) sync every frame to the screen
adaptive	(new) turns vsync off when the frame rate falls below the refresh rate, avoiding stutter and input lag (needs GLX_EXT_swap_control_tear, otherwise same as auto)
off	no sync, fastest, tears

Two more values exist, glx and xpresent, but with the OpenGL renderer both behave like auto. They only differ after falling back to XRender.

When it falls back to XRender

The OpenGL path is skipped, quietly and without breaking the session, if:

• libepoxy was missing at build time
• the driver is a software renderer such as llvmpipe or swrast
• the driver is older than OpenGL 2.0, has no frame buffer objects, or cannot hand windows over as textures
• the graphics context is lost while running, after a driver reset for instance
• a colour depth the driver cannot hand over

Other changes to moth-wm

Everything this fork changes outside the OpenGL renderer, compared with xfwm4 4.20.0.

New features:

• two new boxes in Window Manager Tweaks, under Compositor: one turns the OpenGL renderer on and off, one turns off the fullscreen behaviour above. The dialog also says which renderer is running
• compositing switches itself off while a fullscreen window has focus, so games and video get the screen to themselves
• --vblank parameter now supports the adaptive value
• buttons in the title bar can light up under the pointer on a window that is not focused, from a new inactive-prelight image in the theme
• any tool can read the current renderer and vsync mode by running xprop -root _MOTH_WM_RENDER_BACKEND _MOTH_WM_VSYNC

Fixes:

• a window keeps its border while it is resized, instead of losing an edge for a moment on every step
• a window that is resized and moved at the same time, by dragging a corner, no longer appears to jump and come back
• a shrinking window leaves no stale strip behind along the edge it gave up
• title bar buttons are redrawn only when they really change
• the whole screen is drawn once when compositing starts, so nothing stale is left on it, and switching compositing off no longer risks a crash
• resizing a window no longer stops dead for half a second the first time. moth-wm was asking programs to report their drawing in a way it never answered, so they waited for a reply that never came
• it now waits a tenth of a second, not half a second, for a program to draw while you resize it, and a program that is slow once keeps smooth resizing afterwards instead of losing it for good
• a window and its dialogs are remembered as a family instead of being worked out by going through every window on screen. That happened on every focus, raise, restack, map, unmap and workspace change
• finding the window an event belongs to is a direct lookup now, not a walk through the whole list

Requirements

No new build dependency. moth-wm already builds against libepoxy for vsync and moth-wm-gl uses it for the OpenGL renderer too. It is optional upstream, so check that the configure summary says Epoxy support: yes.

At runtime the driver needs OpenGL 2.0 or newer, frame buffer objects, and the ability to hand windows over as textures. Every driver of the last 15 years or so has all three.

VirtualBox and QEMU can use the OpenGL renderer as long as they have their 3D acceleration on.

License

Same as moth-wm: GNU General Public License, version 2 or later. See COPYING for the full text.