Improve Fixing jitter and stutter documentation

- Add section on input lag, containing project and hardware/OS
  configuration recommendations.
- Remove outdated advice about graphics drivers on Linux.
- Mention that compositing can sometimes be disabled on Linux for
  better performance.
- Wrap to 80 characters.

tutorials/rendering/jitter_stutter.rst

@@ -1,17 +1,25 @@
 .. _doc_jitter_stutter:
 
-Fixing jitter and stutter
-=========================
+Fixing jitter, stutter and input lag
+====================================
 
-What are jitter and stutter?
-----------------------------
+What is jitter, stutter and input lag?
+--------------------------------------
 
-*Jitter* and *stutter* are two different alterations to visible motion of objects on screen that may affect a game,
-even when running at full speed. These effects are mostly visible in games where the world moves at a constant speed
-in a fixed direction, like runners or platformers.
+*Jitter* and *stutter* are two different alterations to visible motion of
+objects on screen that may affect a game, even when running at full speed. These
+effects are mostly visible in games where the world moves at a constant speed in
+a fixed direction, like runners or platformers.
 
-Distinguishing between them
----------------------------
+*Input lag* is unrelated to jitter and stutter, but is sometimes discussed
+alongside. Input lag refers to visible on-screen delay when performing actions
+with the mouse, keyboard, controller or touchscreen. It can be related to game
+code, engine code or external factors (such as hardware). Input lag is most
+noticeable in games that use the mouse to aim, such as first-person games.
+Input lag can't be completely eliminated, but it can be reduced in several ways.
+
+Distinguishing between jitter and stutter
+-----------------------------------------
 
 A game running at a normal framerate without exhibiting any effect will appear smooth:
 
@@ -21,24 +29,29 @@ A game exhibiting *jitter* will shake constantly in a very subtle way:
 
 .. image:: img/motion_jitter.gif
 
-Finally, a game exhibiting *stutter* will appear smooth, but appear to *stop* or *roll back a frame* every few seconds:
+Finally, a game exhibiting *stutter* will appear smooth, but appear to *stop* or
+*roll back a frame* every few seconds:
 
 .. image:: img/motion_stutter.gif
 
 
 Jitter
 ------
 
-There can be many causes of jitter, the most typical one happens when the game *physics frequency* (usually 60 Hz) runs
-at a different resolution than the monitor refresh rate. Check whether your monitor refresh rate is different from 60 Hz.
+There can be many causes of jitter, the most typical one happens when the game
+*physics frequency* (usually 60 Hz) runs at a different resolution than the
+monitor refresh rate. Check whether your monitor refresh rate is different from
+60 Hz.
 
 This is generally not a problem, given that most monitors are 60 Hz, and
-starting with Godot 3.1, a frame timer was introduced that tries to synchronize with refresh as well as possible.
+starting with Godot 3.1, a frame timer was introduced that tries to synchronize
+with refresh as well as possible.
 
-Sometimes only some objects appear to jitter (character or background). This happens when they are processed in different
-time sources (one is processed in the physics step while another is processed in the idle step). Godot 3.1 does some
-improvements to this, from allowing kinematic bodies to be animated in the regular _process loop, to further fixes in the
-frame timer.
+Sometimes only some objects appear to jitter (character or background). This
+happens when they are processed in different time sources (one is processed in
+the physics step while another is processed in the idle step). Godot 3.1 does
+some improvements to this, from allowing kinematic bodies to be animated in the
+regular ``_process()`` loop, to further fixes in the frame timer.
 
 .. note::
 
@@ -49,60 +62,159 @@ frame timer.
 Stutter
 -------
 
-Stutter may happen due to two different reasons. The first, and most obvious one, is the game not being able to keep full
-framerate performance. Solving this is game specific and will require optimization.
+Stutter may happen due to two different reasons. The first, and most obvious
+one, is the game not being able to keep full framerate performance. Solving this
+is game specific and will require optimization.
+
+The second is more complicated, because it is often not associated to the engine
+or game but the underlying operating system. Here is some information regarding
+stutter on different OSs.
 
-The second is more complicated, because it is often not associated to the engine or game but the underlying operating system.
-Here is some information regarding stutter on different OSs.
+On platforms that support disabling V-Sync, suttering can be made less
+noticeable by disabling V-Sync in the project settings. This will however cause
+tearing to appear, especially on monitors with low refresh rates. If your
+monitor supports it, consider enabling variable refresh rate (G-Sync/FreeSync)
+while leaving V-Sync enabled. This avoids eliminating some forms of stuttering
+without introducing tearing.
 
 Windows
 ^^^^^^^
 
-Windows is known to cause stutter in windowed games. This mostly depends on the hardware installed, drivers version and
-processes running in parallel (e.g. having many browser tabs open may cause stutter in a running game). To avoid this,
-starting with 3.1, Godot raises the game priority to "Above Normal". This helps considerably but may not completely eliminate
-stutter.
+Windows is known to cause stutter in windowed games. This mostly depends on the
+hardware installed, drivers version and processes running in parallel (e.g.
+having many browser tabs open may cause stutter in a running game). To avoid
+this, starting with 3.1, Godot raises the game priority to "Above Normal". This
+helps considerably but may not completely eliminate stutter.
 
-Eliminating this completely requires giving your game full privileges to become "time critical", which is not advised.
-Some games may do it, but it is advised to learn to live with this problem, as it is common for Windows games and most users
-won't play games windowed (games that are played in a window, e.g. puzzle games, will usually not exhibit this problem anyway).
+Eliminating this completely requires giving your game full privileges to become
+"time critical", which is not advised. Some games may do it, but it is advised
+to learn to live with this problem, as it is common for Windows games and most
+users won't play games windowed (games that are played in a window, e.g. puzzle
+games, will usually not exhibit this problem anyway).
 
-For fullscreen, Windows gives special priority to the game so stutter is no longer visible and very rare.
-This is how most games are played.
+For fullscreen, Windows gives special priority to the game so stutter is no
+longer visible and very rare. This is how most games are played.
 
 Linux
 ^^^^^
 
-Stutter may be visible on Desktop Linux, but this is usually associated with different video drivers and compositors.
-Nouveau drivers often exhibit this, while AMD or NVidia proprietary don't. Some compositors may also trigger this problem
-(e.g. KWin), so it is advised to try using a different one to rule it out as the cause.
+Stutter may be visible on desktop Linux, but this is usually associated with
+different video drivers and compositors. Some compositors may also trigger this
+problem (e.g. KWin), so it is advised to try using a different one to rule it
+out as the cause. Some window managers such as KWin and Xfwm allow you to
+manually disable compositing, which can improve performance (at the cost of
+tearing).
 
-There is no workaround for driver or compositor stuttering other than reporting it as an issue to the driver or compositor
-developers.
+There is no workaround for driver or compositor stuttering, other than reporting
+it as an issue to the driver or compositor developers.
 
 macOS
 ^^^^^
 
-Generally, macOS is stutter-free, although recently some bugs were reported when running on fullscreen (this is a macOS bug).
-If you have a machine exhibiting this behavior, please let us know.
+Generally, macOS is stutter-free, although recently some bugs were reported when
+running on fullscreen (this is a macOS bug). If you have a machine exhibiting
+this behavior, please let us know.
 
 Android
 ^^^^^^^
 
-Generally, Android is stutter and jitter-free because the running activity gets all the priority. That said, there may be
-problematic devices (older Kindle Fire is known to be one). If you see this problem on Android, please let us know.
+Generally, Android is stutter and jitter-free because the running activity gets
+all the priority. That said, there may be problematic devices (older Kindle Fire
+is known to be one). If you see this problem on Android, please let us know.
 
 iOS
 ^^^
 
-iOS devices are generally stutter-free, but older devices running newer versions of the operating system may exhibit problems.
-This is generally unavoidable.
-
-Reporting stutter or jitter problems
-------------------------------------
-
-If you are reporting a stutter or jitter problem (opening an issue) not caused by any of the above reasons, please specify very
-clearly all the information possible about device, operating system, driver versions, etc. This may help to better troubleshoot it.
-
-Also make sure to use the correct term (jitter or stutter) based on the exhibited behavior. This will help understand your issue much faster.
-Provide a project that can be used to reproduce the issue, and if possible a screen capture demonstrating the bug.
+iOS devices are generally stutter-free, but older devices running newer versions
+of the operating system may exhibit problems. This is generally unavoidable.
+
+Input lag
+---------
+
+Project configuration
+^^^^^^^^^^^^^^^^^^^^^
+
+On platforms that support disabling V-Sync, input lag can be made less
+noticeable by disabling V-Sync in the project settings. This will however cause
+tearing to appear, especially on monitors with low refresh rates.
+
+Increasing the number of physics iterations per second can also reduce
+physics-induced input latency. This is especially noticeable when using physics
+interpolation (which improves smoothness but increases latency). To do so, set
+**Physics > Common > Physics Ticks Per Second** to a value higher than the
+default ``60``, or set ``Engine.physics_ticks_per_second`` at run-time in a
+script. Values that are a multiple of the monitor refresh rate (typically
+``60``) work best when physics interpolation is disabled, as they will avoid
+jitter. This means values such as ``120``, ``180`` and ``240`` are good starting
+points. As a bonus, higher physics FPSes make tunneling and physics unstability
+issues less likely to occur.
+
+The downside of increasing physics FPS is that CPU usage will increase, which
+can lead to performance bottlenecks in games that have heavy physics simulation
+code. This can be alleviated by increasing physics FPS only in situations where
+low latency is critical, or by letting players adjust physics FPS to match their
+hardware. However, different physics FPS will lead to different outcomes in
+physics simulation, even when ``delta`` is consistently used in your game logic.
+This can give certain players an advantage over others. Therefore, allowing the
+player to change the physics FPS themselves should be avoided for competitive
+multiplayer games.
+
+Lastly, you can disable input buffering on a per-rendered frame basis by calling
+``Input.set_use_accumulated_input(false)`` in a script. This will make it so the
+``_input()`` and ``_unhandled_input()`` functions in your scripts are called on
+every input, rather than accumulating inputs and waiting for a frame to be
+rendered. Disabling input accumulation will increase CPU usage, so it should be
+done with caution.
+
+Hardware/OS-specific
+^^^^^^^^^^^^^^^^^^^^
+
+If your monitor supports it, consider enabling variable refresh rate
+(G-Sync/FreeSync) while leaving V-Sync enabled, then cap the framerate in the
+project settings to a slightly lower value than your monitor's maximum refresh
+rate as per `this page <https://blurbusters.com/howto-low-lag-vsync-on/>`__.
+For example, on a 144 Hz monitor, you can set the project's framerate cap to
+``141``. This may be counterintuitive at first, but capping the FPS below the
+maximum refresh rate range ensures that the OS never has to wait for vertical
+blanking to finish. This leads to *similar* input lag as V-Sync disabled with
+the same framerate cap (usually less than 1 ms greater), but without any
+tearing.
+
+This can be done by changing the **Debug > Settings > FPS > Force FPS** project
+setting or assigning ``Engine.target_fps`` at run-time in a script.
+
+On some platforms, you can also opt into a low-latency mode in the graphics
+driver options (such as the NVIDIA Control Panel on Windows). The **Ultra**
+setting will give you the lowest possible latency, at the cost of slightly lower
+average framerates. Forcing the GPU into a high power state can also further
+lower input lag, at the cost of higher power consumption (and resulting heat/fan
+noise).
+
+Finally, make sure your monitor is running at its highest possible refresh rate
+in the OS' display settings. Also ensure that your mouse is configured to use
+its highest polling rate (typically 1,000 Hz for gaming mice, sometimes more).
+High USB polling rates can however result in high CPU usage, so 500 Hz may be a
+safer bet on low-end CPUs.
+
+On Linux, disabling compsoting in window managers that allow it (such as KWin or
+Xfwm) can reduce input lag significantly.
+
+Reporting jitter, stutter or input lag problems
+-----------------------------------------------
+
+If you are reporting a stutter or jitter problem (opening an issue) not caused
+by any of the above reasons, please specify very clearly all the information
+possible about device, operating system, driver versions, etc. This may help to
+better troubleshoot it.
+
+If you are reporting input lag problems, please include a capture made with a
+high speed camera (such as your phone's slow motion video mode). The capture
+**must** have both the screen and the input device visible so that the number of
+frames between an input and the on-screen result can can be counted. Also, make
+sure to mention your monitor's refresh rate and your input device's polling rate
+(especially for mice).
+
+Also, make sure to use the correct term (jitter, stutter, input lag) based on the
+exhibited behavior. This will help understand your issue much faster. Provide a
+project that can be used to reproduce the issue, and if possible, include a
+screen capture demonstrating the bug.