[doc] install: Add Configuration Widget section

Add a section to cover the configuration widget, the
various tabs and settings and the new post-processing
filter stack.

doc/install.rst

@@ -321,6 +321,72 @@ in a special config file $XDG_CONFIG_HOME/looking-glass/imgui.ini.
 Please refrain from editing this file manually as your changes might
 be overwritten by the client.
 
+.. _client_config_widget:
+
+Configuration Widget
+~~~~~~~~~~~~~~~~~~~~
+
+The Configuration Widget is accessible through the Overlay mode. The
+widget has multiple tabs that allow setting a variety of modes and
+parameters for the application.
+
+Settings tab
+^^^^^^^^^^^^
+
+- Performance Metrics: Enabling this will turn on the Metrics Graphs
+  Widget. Multiple graphs can be enabled and they will stack vertically.
+- EGL: Modify EGL features such as the algorithm used for scaling and
+  enabling night vision mode.
+Changes in the Settings tab are not persistent and will change back to
+their default values when the client is restarted.
+
+EGL Filters tab
+^^^^^^^^^^^^^^^
+
+The EGL Filters tab is a GUI for enabling, configuring and ordering
+the post-processing filter stack. Each filter can be expanded to open
+its settings. Filters can also be re-ordered by dragging them up or down.
+Filters application is cumulative, and order is important (e.g. applying CAS
+before FSR might have different results than the reverse). Users are
+encouraged to experiment with the order and parameters to achieve optimal
+results. The currently available filters include:
+
+- Downscaler: Filter for downscaling resolution. Can be used to undo the
+  poor upscaling that some games implement such that it can then
+  be better upscaled using FSR (see below). The filter has a pixel-size setting
+  that is used to set the effective downscaling ratio and can be set to use
+  different downscaling algorithms.
+- AMD FidelityFX Super Resolution (FSR): Filter implementing  a rendering
+  technique for upscaling resolution. FSR works by accepting the original
+  frames (on the guest) at lower resolution, then applying a spatial upscaling
+  algorithm in the client to make the final result look as though it is
+  rendered in high-resolution. The FSR filter interface allows for fine
+  tuning the sharpness factor of the algorithm and shows the equivalent
+  quality mode based on the ratio between the original frame resolution
+  and the client resolution.
+- AMD FidelityFX Contrast Adaptive Sharpening (CAS): Filter that
+  increases visual quality by applying a sharpening algorithm to the
+  frame. CAS can sometimes restore detail lost in a typical upscaling
+  application. The CAS filter interface has an adjustable sharpness
+  control.
+
+The filter stack settings and order can be saved to presets so that it
+can be conveniently recalled at a later time. As filter settings are
+usually application specific, multiple presets can be defined for each
+case scenario. To save a preset, click on "Save preset as..." and enter
+a preset name. Presets are recalled by selecting them in the "Preset name"
+pull down. Presets are persistent and are stored on disk at
+``$XDG_CONFIG_HOME/looking-glass/presets``.
+
+.. warning::
+   Please refrain from modifying any files under the ``presets`` folder.
+   Those files are meant to be modified only by the Looking-Glass client.
+
+.. note::
+   Although presets are persistent, the client will not remember which
+   preset was used last session, so a preset needs to be recalled once
+   the client starts.
+
 .. _client_full_command_options:
 
 Full Command Line Options