Difference between revisions of "Compositor"

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{{Template:Non-stable|version=2019.3|progress=80}}
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{{Template:Non-stable|version=2019.2|build_flags=-DENABLE_COMPOSITOR=ON|progress=100}}
  
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{{forum|47|Effects & Shaders}}
  
 
{{infobox subsystem
 
{{infobox subsystem
|image      = Clouds experiment.jpg
+
|image      = ALS Compositor pipeline.jpg
|name        = Compositor Subsystem
+
|name        = Compositor Framework
|started    = 01/2018
+
|started    = 01/2018 (Available since FlightGear 2019.2)
|description = Dynamic rendering pipeline configured via the property tree
+
|description = Dynamic rendering pipeline configured via the property tree and XML
|status      = * experimental as of 02/2018
+
|status      = Stable
* put up for review/discussion as of 08/2018
+
|developers  = Fernando García Liñán <ref>https://sourceforge.net/u/fgarlin/profile/</ref>
|maintainers = none
+
|changelog = https://sourceforge.net/u/fgarlin/profile/feed.rss
|developers  = Icecode
+
|folders =  
|topic-sg = https://sourceforge.net/u/fgarlin/simgear/ci/new-compositor/tree/
+
* {{flightgear file|src/Viewer}}
|topic-fg = https://sourceforge.net/u/fgarlin/flightgear-src/ci/new-compositor/tree/
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* {{simgear file|simgear/scene/viewer}}
|topic-fgdata = https://sourceforge.net/u/fgarlin/flightgear/ci/new-compositor/tree/
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* {{fgdata file|Compositor}}
 
}}
 
}}
  
The '''Compositor''' aims to bring multi-pass rendering to FlightGear. It encapsulates a rendering pipeline and exposes its parameters to the [[Property Tree]]. The pipeline used on each physical viewport defined on the [[Howto:Configure camera view windows|CameraGroup settings]] can be changed via a <code><compositor/></code> tag containing a valid path to the Compositor XML file.
+
The '''Compositor''' aims to bring multi-pass rendering to FlightGear. It encapsulates a rendering pipeline and exposes its parameters to a [[Property Tree]] interface. At startup, FlightGear reads the pipeline definition file for each physical viewport defined on the [[Howto:Configure camera view windows|CameraGroup settings]]. If no Compositor file is specified for a physical camera, the one given by the <code>--compositor=</code> startup command will be used. If such startup option is not used either, FlightGear will look for a valid Compositor file in $FG_ROOT/Compositor/default.xml
 +
 
 +
The Compositor introduces a new dedicated fgdata directory for new/custom rendering pipelines: {{Fgdata file|Compositor}}.
  
 
== Background ==
 
== Background ==
{{See also|Supporting multiple renderers}}
+
{{See also|Supporting multiple renderers|Howto:Canvas View Camera Element}}
  
First discussed in 03/2012 during the early [[Rembrandt]] days, Zan came up with patches demonstrating how to create an XML-configurable rendering pipeline.
+
First discussed in 03/2012 during the early [[Rembrandt]] days, Zan (Lauri Peltonen) came up with a set of patches demonstrating how to create an XML-configurable rendering pipeline.
  
 
Back then, this work was considered to look pretty promising <ref>{{cite web
 
Back then, this work was considered to look pretty promising <ref>{{cite web
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   }}</ref> and at the time plans were discussed to unify this with the ongoing Rembrandt implementation (no longer maintained).
 
   }}</ref> and at the time plans were discussed to unify this with the ongoing Rembrandt implementation (no longer maintained).
  
Adopting Zan's approach would have meant that efforts like Rembrandt could have been implemented without requiring C++ space modifications.
+
Adopting Zan's approach would have meant that efforts like [[Rembrandt]] (deferred rendering) could have been implemented without requiring C++ space modifications, i.e. purely in [[Base package]] space.
  
 
Rembrandt's developer (FredB) suggested to extend the format to avoid duplicating the stages when you have more than one viewport, i.e.  specifying a pipeline as a template, with conditions like in effects, and have the current camera layout refer the pipeline that would be duplicated, resized and positioned for each declared viewport <ref>{{cite web
 
Rembrandt's developer (FredB) suggested to extend the format to avoid duplicating the stages when you have more than one viewport, i.e.  specifying a pipeline as a template, with conditions like in effects, and have the current camera layout refer the pipeline that would be duplicated, resized and positioned for each declared viewport <ref>{{cite web
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   }}</ref>
 
   }}</ref>
  
 
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Zan's original patches can still be found in his newcameras branches which allow the user to define the rendering pipeline in preferences.xml: {{gitorious source|proj=fg|repo=zans-flightgear|branch=newcameras|text=FlightGear}}, {{gitorious source|proj=fg|repo=zans-simgear|branch=newcameras|text=SimGear}}.
Zan's original patches can still be found in his newcameras branches which allow the user to define the rendering pipeline in preferences.xml:
+
 
+
* [https://gitorious.org/fg/zans-flightgear?p=fg:zans-flightgear.git;a=shortlog;h=refs/heads/newcameras FlightGear]
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* [https://gitorious.org/fg/zans-simgear?p=fg:zans-simgear.git;a=log;h=refs/heads/newcameras SimGear]
+
  
 
At that point, it didn't have everything Rembrandt's pipeline needs, but most likely could be easily enhanced to support those things.  
 
At that point, it didn't have everything Rembrandt's pipeline needs, but most likely could be easily enhanced to support those things.  
  
Basically the original version added support for multiple camera passes, texture targets, texture formats, passing textures from one pass to another etc, while preserving the standard rendering line if user wants that. <ref>{{cite web
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Basically, the original version added support for multiple camera passes, texture targets, texture formats, passing textures from one pass to another etc, while preserving the standard rendering line if user wants that. <ref>{{cite web
 
   |url    =  https://sourceforge.net/p/flightgear/mailman/message/28944733/  
 
   |url    =  https://sourceforge.net/p/flightgear/mailman/message/28944733/  
 
   |title  =  <nowiki> [Flightgear-devel] [Rembrandt] the plan </nowiki>  
 
   |title  =  <nowiki> [Flightgear-devel] [Rembrandt] the plan </nowiki>  
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   }}</ref>
 
   }}</ref>
  
The corresponding set of patches (topic branches) were put up for review in 08/2018 to discuss the underlying approach and hopefully get this merged in 2019.
+
Since the early days of Zan's groundwork, providing the (hooks) infrastructure to enable base package developers to prototype, test and develop distinct rendering pipelines without requiring C++ space modifications has been a long-standing idea, especially after the [[Canvas]] system became available in early 2012, which demonstrated how RTT-rendering buffers (FBOs) could be set up, created and manipulated procedurally (i.e. at run-time) using XML, the property tree and [[Nasal]] scripting. <ref>{{forum link|type=search|title=Zan's Rembrandt and Canvas work|keywords=zan+rembrandt+canvas}}</ref>
 +
 
 +
The new '''Compositor''' is an improved re-implementation of Zan's original work using not just XML, but also [[Property Tree|properties]] and a handful of [[Canvas]] concepts.
  
 
== Features ==
 
== Features ==
  
* Completely independent of other parts of the simulator, i.e. it's part of SimGear and can be used in a standalone fashion if needed, ala Canvas.
+
* Completely independent of other parts of the simulator, i.e. it's part of [[SimGear]] and can be used in a standalone fashion if needed, ala Canvas.
* Although independent, its aim is to be fully compatible with the current rendering framework in FG. This includes the Effects system, CameraGroup, Rembrandt and ALS (and obviously the Canvas).
+
* Although independent, its aim is to be fully compatible with the current rendering framework in FG. This includes the [[Effects]] system, [[Howto:Configure camera view windows|CameraGroup]], [[Rembrandt]] and [[ALS]] (and obviously the [[Canvas]]).
 
* Its functionality overlaps Rembrandt: what can be done with Rembrandt can be done with the Compositor, but not vice versa.
 
* Its functionality overlaps Rembrandt: what can be done with Rembrandt can be done with the Compositor, but not vice versa.
* Fully configurable via a XML interface without compromising performance (ala Effects, using PropertyList files).
+
* Fully configurable via an XML interface without compromising performance (ala Effects, using [[PropertyList XML File|PropertyList files]]).
 
* Flexible, expandable and compatible with modern graphics.
 
* Flexible, expandable and compatible with modern graphics.
 
* It doesn't increase the hardware requirements, it expands the hardware range FG can run on. People with integrated GPUs (Intel HD etc) can run a Compositor with a single pass that renders directly to the screen like before, while people with more powerful cards can run a Compositor that implements deferred rendering, for example.
 
* It doesn't increase the hardware requirements, it expands the hardware range FG can run on. People with integrated GPUs (Intel HD etc) can run a Compositor with a single pass that renders directly to the screen like before, while people with more powerful cards can run a Compositor that implements deferred rendering, for example.
 +
* Static branching support. Every pipeline element can be enabled/disabled at startup via a [[Conditions|<condition> block]].
  
The Compositor is in an usable state right now: it works but there are no effects or pipelines developed for it. There are also some bugs and features that don't work as expected because of some hardcoded assumptions in the FlightGear Viewer code.
+
== How to enable the Compositor ==
  
== Elements ==
+
Currently the Compositor can only be enabled at compile time via the <code>-DENABLE_COMPOSITOR=ON</code> CMake flag in FlightGear. SimGear doesn't require any extra parameters. Once you have a binary with the Compositor enabled and you run it, you will be presented with the default rendering pipeline. At the time of writing, this is the low spec rendering pipeline. If you want to try the [[#ALS|ALS pipeline]], start FlightGear with the command line argument: <code>--compositor=Compositor/ALS/als</code>
  
=== Buffers ===
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If you want to enable shadows on all objects in the ALS pipeline use these options as a startup parameters (in QT GUI or in the commandline) <code>--prop:bool:/sim/rendering/als/shadows/enabled=true</code> and <code>--prop:int:/sim/rendering/als/shadows/sun-atlas-size=2048</code>. If you feel like the shadows are too low-quality (specially in the cockpit), increase the shadow resolution to 4096 or 8192 instead of 2048.
  
A buffer represents a texture or, more generically, a region of GPU memory. It can have the following parameters:
+
If you are having trouble with lights using an integrated GPU and the Mesa drivers under Linux, try starting FlightGear with the environment variable <tt>MESA_GL_VERSION_OVERRIDE="3.1COMPAT"</tt>
  
; name
+
== Notes for aircraft developers ==
: Passes will be able to address the buffer by this name.
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; type
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: Any texture type allowed by OpenGL: 1d, 2d, 2d-array, 2d-multisample, 3d, rect or cubemap.
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; width
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: Texture width. It's possible to write 'screen' to use the physical viewport width.
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; screen-width-scale
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: If 'screen' was used, this controls the width scaling factor.
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; height
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: Texture height. It's possible to write 'screen' to use the physical viewport height.
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; screen-height-scale
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: If 'screen' was used, this controls the height scaling factor.
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; depth
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: Texture depth.
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; internal-format, source-format and source-type
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: They specify the texture format. They correspond to the arguments ''internalformat'', ''format'' and ''type'' respectively of the OpenGL function ''glTexImage2D''.
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; min-filter and mag-filter (Optional)
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: They change the minification and magnification filtering respectively. Possible values are: linear, linear-mipmap-linear, linear-mipmap-nearest, nearest, nearest-mipmap-linear and nearest-mipmap-nearest. The default value for both filters is linear.
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; wrap-s, wrap-t and wrap-r (Optional)
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: They change the wrap mode for each coordinate. Possible values are: clamp, clamp-to-edge, clamp-to-border, repeat and mirror. The default value for every coordinate is clamp-to-border.
+
  
A typical [[PropertyList XML File|property tree structure]] describing a buffer may be as follows:
+
=== Lights ===
  
 +
The Compositor introduces a new way of defining lights that is renderer agnostic, so every rendering pipeline will be able to access the lights that have been implemented like this. As of 2019/11, the only pipeline that supports dynamic lights is the ALS pipeline. The resulting light volumes can be visualized for debugging purposes by setting the property <tt>/sim/debug/show-light-volumes</tt> to true.
 +
 +
{|cellpadding=10|
 +
|valign=top style="width: 20%;"|
 
<syntaxhighlight lang="xml">
 
<syntaxhighlight lang="xml">
<buffer>
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<light>
   <name>color</name>
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   <name>my-spotlight</name>
   <type>2d</type>
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   <type>spot</type>
   <width>screen</width>
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   <position>
  <height>screen</height>
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    <x-m>-7.7476</x-m>
  <screen-width-scale>1.5</screen-width-scale>
+
    <y-m>0</y-m>
   <screen-height-scale>1.5</screen-height-scale>
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    <z-m>-1.7990</z-m>
   <internal-format>rgba8</internal-format>
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   </position>
   <source-format>rgba</source-format>
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  <direction>
   <source-type>ubyte</source-type>
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    <x>-1.0</x>
   <min-filter>nearest-mipmap-nearest</min-filter>
+
    <y>0</y>
   <mag-filter>nearest-mipmap-nearest</mag-filter>  
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    <z>-0.013</z>
   <wrap-s>repeat</wrap-s>
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   </direction>
   <wrap-t>repeat</wrap-t>
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  <ambient>
</buffer>
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    <r>0.03</r>
 +
    <g>0.03</g>
 +
    <b>0.03</b>
 +
    <a>1</a>
 +
   </ambient>
 +
  <diffuse>
 +
    <r>0.95</r>
 +
    <g>0.9</g>
 +
    <b>0.9</b>
 +
    <a>1</a>
 +
   </diffuse>
 +
  <specular>
 +
    <r>0.95</r>
 +
    <g>0.9</g>
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    <b>0.9</b>
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    <a>1</a>
 +
   </specular>
 +
  <attenuation>
 +
    <c>1.0</c>
 +
    <l>0.09</l>
 +
    <q>0.032</q>
 +
   </attenuation>
 +
  <spot-exponent>5</spot-exponent>
 +
   <spot-cutoff>40</spot-cutoff>
 +
   <range-m>50</range-m>
 +
</light>
 
</syntaxhighlight>
 
</syntaxhighlight>
 +
|valign=top style="width: 80%;"|
 +
* <tt>'''name'''</tt>. An {{tag|animation}} will be able to reference the light by this name. Most animations will work as expected (rotate, translate, spin etc).
 +
* <tt>'''type'''</tt>. <tt>spot</tt> or <tt>point</tt>.
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* <tt>'''position'''</tt>. The position of the light source in model space and in meters.
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* <tt>'''direction'''</tt>. Only available in <tt>spot</tt> lights. It indicates the direction of the spotlight. This parameter can be specified in three different ways:
 +
{| class="wikitable" style="border: 1px solid darkgray;"
 +
! scope="col" style="width:33%;" |Direction vector
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! scope="col" style="width:33%;" |Look-at point
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! scope="col" style="width:33%;" |Rotation angles
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|-
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| style="padding: 10px" | A vector in model space that specifies the direction. Doesn't have to be normalized.
 +
<syntaxhighlight lang="xml">
 +
<x>-1.0</x>
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<y>0</y>
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<z>-0.013</z>
 +
</syntaxhighlight>
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| style="padding: 10px" | The spotlight will calculate its direction by looking at this position from the light position. The point is in model space and in meters.
 +
<syntaxhighlight lang="xml">
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<lookat-x-m>-8.031</lookat-x-m>
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<lookat-y-m>0</lookat-y-m>
 +
<lookat-z-m>-2</lookat-z-m>
 +
</syntaxhighlight>
 +
| style="padding: 10px" | A three angle rotation in degrees that rotates the spotlight around the three axes. A 0 degree angle in all axes makes the spotlight point downwards (negative Z).
 +
<syntaxhighlight lang="xml">
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<pitch-deg>90</pitch-deg>
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<roll-deg>0</roll-deg>
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<heading-deg>0</heading-deg>
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</syntaxhighlight>
 +
|}
 +
* <tt>'''ambient'''</tt>, <tt>'''diffuse'''</tt> and <tt>'''specular'''</tt>. Four-component vectors that specify the light color.
 +
* <tt>'''attenuation'''</tt>. Three-component vector where <code><c></code> specifies the constant factor, <code><l></code> specifies the linear factor and <code><nowiki><q></nowiki></code> specifies the quadratic factor. These factors are plugged into the OpenGL light attenuation formula [[File:Spotlight_attenuation.png]] where d is the distance of the fragment to the light source. See this [http://wiki.ogre3d.org/tiki-index.php?page=-Point+Light+Attenuation table] for a list of attenuation values based on the range of the light.
 +
* <tt>'''range-m'''</tt>. Maximum range from the light source position in meters. This value will be used by the renderers to determine if a fragment is illuminated by this source. Every fragment outside this range isn't guaranteed to be affected by the light, even if the attenuation factor isn't 0 in that particular fragment.
 +
* <tt>'''cutoff'''</tt>. Only available in <tt>spot</tt> lights. It specifies the maximum spread angle of a light source. Only values in the range 0 90 are accepted. If the angle between the direction of the light and the direction from the light to the fragment being lighted is greater than the spot cutoff angle, it won't be lit.
 +
* <tt>'''exponent'''</tt>. Only available in <tt>spot</tt> lights. Higher spot exponents result in a more focused light source, regardless of the spot cutoff angle.
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* <tt>'''debug-color'''</tt> ('''Optional'''). Sets the color of the debug light volume. By default it's red.
 +
|}
  
=== Passes ===
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=== Shadows ===
  
A pass wraps around an [http://public.vrac.iastate.edu/vancegroup/docs/OpenSceneGraphReferenceDocs-3.0/a00089.html osg::Camera]. The following types of passes are supported:
+
The shadow mapping algorithm can be customized entirely by the rendering pipeline. This means that each one will have its own requirements when it comes to shadows. Here are some general recommendations:
  
; '''shadow-map'''
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* Use the <code><noshadow></code> animation to disable shadows on objects that don't need them. An example would be billboarded lights or really small cockpit elements that don't need shadows and would cause degraded performance.
: Renders the scene from a light's point of view.
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* Try to mark as many cockpit objects as possible as <tt>interior</tt>.
; '''scene'''
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<syntaxhighlight lang="xml">
: Renders the scene from the point of view given by the CameraGroup.
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<model>
; '''quad'''
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  <name>interior</name>
: Renders a fullscreen quad with an optional [[Effects|effect]] applied. Useful for screen space shaders (like SSAO, Screen Space Reflections or bloom) and deferred rendering.
+
  <usage>interior</usage>
 +
  <path>Aircraft/JA37/Models/ja37-interior.xml</path> <!-- All the objects that should only be seen when inside the cockpit are in this file -->
 +
</model>
 +
</syntaxhighlight>
 +
* Unlike in Rembrandt, polygons facing the Sun are the ones used to generate the shadow map, so single sided surfaces and non-closed objects should be rendered correctly.
  
Passes can render to a buffer (Render to Texture), to several buffers (Multiple Render Targets) or directly to the framebuffer. This allows chaining of multiple passes, sharing buffers between them. They can also receive other buffers as input, so effects can access them as textures.
+
== Pipelines ==
  
Example XML for a ''scene'' type pass:
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=== Low-Spec pipeline ===
  
<syntaxhighlight lang="xml">
+
A fixed function forward rendering pipeline mainly targeted to low spec systems. It imitates the classic forward pipeline used before multi-pass rendering was introduced by using two near/far cameras rendering directly to the screen.
<pass>
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  <name>forward-lighting</name>
+
  <type>scene</type>
+
  <clear-color type="vec4d">0 0 0 0</clear-color>
+
  
  <clustered-forward/>
+
[[File:Low-spec Compositor pipeline.png|thumb|Screenshot showing OSG stats of the Compositor-based low-spec rendering pipeline.]]
  
  <attachment>
+
=== ALS ===
    <buffer>color</buffer>
+
    <component>color0</component>
+
    <multisample-samples>4</multisample-samples>
+
    <multisample-color-samples>4</multisample-color-samples>
+
  </attachment>
+
  <attachment>
+
    <buffer>depth</buffer>
+
    <component>depth</component>
+
  </attachment>
+
</pass>
+
</syntaxhighlight>
+
  
== Canvas integration ==
+
The ALS pipeline tries to bring multipass rendering to the current ALS framework, effectively combining the best from ALS and Project Rembrandt.
  
[[File:Blur_and_blue_filter_applied_to_a_buffer.png|thumb|Post processed image (right) after applying gaussian blur and a blue filter to a scene pass (left).]]
+
==== Cascaded shadow mapping ====
  
Apart from serving as a debugging tool for visualizing the contents of a buffer, integrating the Compositor with [[Canvas]] allows aircraft developers to access RTT capabilities. Compositor buffers can be accessed within Canvas via a new custom Canvas Image protocol '''buffer://'''. For example, the path <code>buffer://test-compositor/test-buffer</code> displays the buffer test-buffer declared in test-compositor.
+
The main issue with shadow mapping in FlightGear is the complexity of the scene graph. Culling times can become huge if we don't carefully select which parts of the scene graph we want to render in the shadow maps. Some possible optimizations:
 +
* Study the minimum shadow map distance we can get without noticeable light leaking. Select an appropiate amount of cascades (more cascades = more passes over all geometry, and in general we want to keep the amount of forward passes to a minimum). We should have at least three cascades: the first just for cockpit/internal shadows, the second for the whole aircraft and the third for the rest of the scenery geometry. A fourth can be added if the transition between the second and the third is too harsh.
 +
* Improve the culling masks (simgear/scene/util/RenderConstants.hxx). The CASTSHADOW_BIT flag is present in almost every object in the scene graph. Turning this flag off for trees, random buildings and other geometry intensive objects improves framerates by a very considerable amount. Should the user be able to select which objects cast shadows?
 +
* Should the terrain cast shadows? The terrain is rarely steep enough to cast shadows. Apart from that, the terrain in FlightGear messes with automatic near/far computations for the shadow passes since the geometry is not tessellated enough. Also, the terrain LOD is not good enough to have decent cull times at far cascades.
 +
* Adding a "internal only" shadow flag for aircraft developers. This allows farther shadow cascades to cull complex objects that are only visible in the nearest cascades. (Very important optimization for aircraft with complex cockpit geometry).
 +
* Vegetation shadows will be done by the "legacy" method currently in use. Shadow mapping on vegetation is much more expensive in terms of performance and the current algorithm does the job well enough. <ref>https://forum.flightgear.org/viewtopic.php?f=47&p=357606</ref>
  
<syntaxhighlight lang="nasal">
+
==== Post-processing ====
var (width,height) = (612,612);
+
 
var title = 'Compositor&Canvas test';
+
Gamma correction, night vision and other ALS filters should happen in a quad pass. The current filter_combined() should be left for post-processing that requires as much precision as possible - e.g. dithering to prevent banding). HDR is not a planned feature for now so ALS will be using rgba8 buffers for most of its features.
var window = canvas.Window.new([width,height],"dialog")
+
 
.set('title',title);
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==== Real-time dynamic reflections ====
var myCanvas = window.createCanvas().set("background", canvas.style.getColor("bg_color"));
+
 
var root = myCanvas.createGroup();
+
Rendering dynamically to a cubemap is possible. As with shadow mapping, minimizing the object count and number of forward passes is vital to get good performance in FlightGear. Rendering to six cubemap faces requires six forward passes, but we can render to a dual paraboloid map instead, reducing this number to two.
var path = "buffer://test-compositor/test-buffer";
+
 
var child = root.createChild("image")
+
==== Transparency ====
    .setTranslation(50, 50)
+
 
    .setSize(512, 512)
+
When shadows (and multipass rendering in general) come into play, transparent objects have to be treated differently, even when we are dealing with a forward renderer. In OSG there are two ways to separate transparent surfaces:
    .setFile(path);
+
 
</syntaxhighlight>
+
* Using RenderBins. After a single scene cull traversal, surfaces which belong to a special RenderBin type (DepthSortedBin) are removed or moved to another camera. This is how Rembrandt does it and it is the most backwards compatible approach since RenderBins can be changed directly inside Effects.
 +
* Using cull masks. Two separate traversals are done: one for opaque objects and another for translucent objects. This requires offering aircraft developers another way of tagging a surface as transparent. A trivial approach would be to add a new <animation> type called 'transparent', but that wouldn't be backwards compatible. Maybe we can add some kind of system where we can change cull masks inside Effects? Would that be too hacky or out of place?
 +
 
 +
== Creating a custom rendering pipeline ==
 +
 
 +
Since the Compositor is completely data-driven, new rendering pipelines can be created by writing a custom XML pipeline definition. This section tries to document most of the available parameters, but the best and most up-to-date resource is the Compositor parsing code in SimGear ({{simgear file|simgear/scene/viewer}}). See existing pipelines in {{fgdata file|Compositor}} for practical examples on how to use these parameters.
 +
 
 +
=== Buffers ===
 +
 
 +
A buffer represents a texture or, more generically, a region of GPU memory.
 +
 
 +
{| class="wikitable" style="text-align: center; font-size: 85%; width: auto; table-layout: fixed;
 +
! scope="col" | Parameter Name
 +
! scope="col" | Optional
 +
! scope="col" | Value
 +
! scope="col" | Default Value
 +
! scope="col" | Description
 +
|-
 +
! scope="row"| <tt>name</tt>
 +
| {{No}}
 +
| string
 +
|
 +
| Passes will be able to address the buffer by this name
 +
|-
 +
! scope="row"| <tt>type</tt>
 +
| {{No}}
 +
| <tt>1d, 2d, 2d-array, 2d-multisample, 3d, rect, cubemap</tt>
 +
|
 +
| Any texture type allowed by OpenGL
 +
|-
 +
! scope="row"| <tt>width</tt>
 +
| {{No}}
 +
| Any unsigned integer or <tt>screen</tt> to use the physical viewport width. The <code><property></code> tag can also be used to use a property value
 +
|
 +
| Texture width
 +
|-
 +
! scope="row"| <tt>screen-width-scale</tt>
 +
| {{Yes}}
 +
| float
 +
| <tt>1.0</tt>
 +
| If <tt>screen</tt> was used, this controls the width scaling factor
 +
|-
 +
! scope="row"| <tt>height</tt>
 +
| {{No}}
 +
| Any unsigned integer or <tt>screen</tt> to use the physical viewport height. The <code><property></code> tag can also be used to use a property value
 +
|
 +
| Texture height
 +
|-
 +
! scope="row"| <tt>screen-height-scale</tt>
 +
| {{Yes}}
 +
| float
 +
| <tt>1.0</tt>
 +
| If <tt>screen</tt> was used, this controls the height scaling factor
 +
|-
 +
! scope="row"| <tt>depth</tt>
 +
| {{No}}
 +
| Any unsigned integer. The <code><property></code> tag can also be used to use a property value
 +
|
 +
| Texture depth
 +
|-
 +
! scope="row"| <tt>format</tt>
 +
| {{Yes}}
 +
| See {{simgear file|simgear/scene/viewer/CompositorBuffer.cxx}} for the latest available values
 +
| <tt>rgba8</tt>
 +
| Specifies the texture format. It corresponds to the ''internalformat'', ''format'' and ''type'' arguments of the OpenGL function ''glTexImage2D''
 +
|-
 +
! scope="row"| <tt>min-filter, mag-filter</tt>
 +
| {{Yes}}
 +
| <tt>linear, linear-mipmap-linear, linear-mipmap-nearest, nearest, nearest-mipmap-linear, nearest-mipmap-nearest</tt>
 +
| <tt>linear</tt>
 +
| Change the minification and magnification filtering respectively
 +
|-
 +
! scope="row"| <tt>wrap-s, wrap-t, wrap-r</tt>
 +
| {{Yes}}
 +
| <tt>clamp, clamp-to-edge, clamp-to-border, repeat, mirror</tt>
 +
| <tt>clamp-to-border</tt>
 +
| They change the wrap mode for each coordinate
 +
|-
 +
! scope="row"| <tt>anisotropy</tt>
 +
| {{Yes}}
 +
| float
 +
| <tt>1.0</tt>
 +
|
 +
|-
 +
! scope="row"| <tt>border-color</tt>
 +
| {{Yes}}
 +
| vec4
 +
| <tt>(0.0f, 0.0f, 0.0f, 0.0f)</tt>
 +
|
 +
|-
 +
! scope="row"| <tt>shadow-comparison</tt>
 +
| {{Yes}}
 +
| bool
 +
| <tt>true</tt>
 +
|
 +
|-
 +
! scope="row"| <tt>shadow-texture-mode</tt>
 +
| {{Yes}}
 +
| <tt>luminance, intensity, alpha</tt>
 +
| <tt>luminance</tt>
 +
|
 +
|-
 +
! scope="row"| <tt>shadow-compare-func</tt>
 +
| {{Yes}}
 +
| <tt>never, less, equal, lequal, greater, notequal, gequal, always</tt>
 +
| <tt>lequal</tt>
 +
|
 +
|}
 +
 
 +
=== Passes ===
 +
 
 +
A pass wraps around an [http://public.vrac.iastate.edu/vancegroup/docs/OpenSceneGraphReferenceDocs-3.0/a00089.html osg::Camera]. Passes all have some common parameters:
 +
 
 +
{| class="wikitable" style="text-align: center; font-size: 85%; width: auto; table-layout: fixed;
 +
! scope="col" | Parameter Name
 +
! scope="col" | Optional
 +
! scope="col" | Value
 +
! scope="col" | Default Value
 +
! scope="col" | Description
 +
|-
 +
! scope="row"| <tt>clear-color, clear-accum, clear-depth and clear-stencil</tt>
 +
| {{Yes}}
 +
| vec4
 +
| black, black, <tt>1.0</tt>, <tt>0</tt> respectively
 +
| Pass clear colors
 +
|-
 +
! scope="row"| <tt>clear-mask</tt>
 +
| {{Yes}}
 +
| <tt>color, stencil, depth, accum</tt>
 +
| <tt>color depth</tt>
 +
| Pass clear mask
 +
|-
 +
! scope="row"| <tt>effect-scheme</tt>
 +
| {{Yes}}
 +
| Valid effect scheme name
 +
| None
 +
| The pass will try to use the specified effect scheme to draw every object.
 +
|}
 +
 
 +
Passes can render to a buffer (Render to Texture), to several buffers (Multiple Render Targets) or directly to the framebuffer. This is accomplished by the <code><attachment></code> tag. Possible parameters of an attachment are:
 +
 
 +
{| class="wikitable" style="text-align: center; font-size: 85%; width: auto; table-layout: fixed;
 +
! scope="col" | Parameter Name
 +
! scope="col" | Optional
 +
! scope="col" | Value
 +
! scope="col" | Default Value
 +
! scope="col" | Description
 +
|-
 +
! scope="row"| <tt>buffer</tt>
 +
| {{No}}
 +
| Valid buffer name
 +
|
 +
| The name of the buffer to output to
 +
|-
 +
! scope="row"| <tt>component</tt>
 +
| {{No}}
 +
| <tt>color, color0</tt> to <tt>color15, depth, stencil, depth-stencil</tt>
 +
|
 +
| FBO attachment point
 +
|-
 +
! scope="row"| <tt>level</tt>
 +
| {{Yes}}
 +
| int
 +
| <tt>0</tt>
 +
| Mipmap level of the texture that is attached
 +
|-
 +
! scope="row"| <tt>face</tt>
 +
| {{Yes}}
 +
| int
 +
| <tt>0</tt>
 +
| Face of cube map texture or z-level of 3d texture
 +
|-
 +
! scope="row"| <tt>mipmap-generation</tt>
 +
| {{Yes}}
 +
| bool
 +
| <tt>false</tt>
 +
| Whether mipmap generation should be done for texture
 +
|-
 +
! scope="row"| <tt>multisample-samples</tt>
 +
| {{Yes}}
 +
| int
 +
| <tt>0</tt>
 +
| Multisample anti-aliasing (MSAA) samples
 +
|-
 +
! scope="row"| <tt>multisample-color-samples</tt>
 +
| {{Yes}}
 +
| int
 +
| <tt>0</tt>
 +
| Multisample anti-aliasing (MSAA) color samples
 +
|}
 +
 
 +
Passes can also receive buffers as input and use them in their shaders. This is accomplished by the <code><binding></code> tag, which has the following parameters:
 +
 
 +
{| class="wikitable" style="text-align: center; font-size: 85%; width: auto; table-layout: fixed;
 +
! scope="col" | Parameter Name
 +
! scope="col" | Optional
 +
! scope="col" | Value
 +
! scope="col" | Default Value
 +
! scope="col" | Description
 +
|-
 +
! scope="row"| <tt>buffer</tt>
 +
| {{No}}
 +
| Valid buffer name
 +
|
 +
| The name of the buffer to bind
 +
|-
 +
! scope="row"| <tt>unit</tt>
 +
| {{No}}
 +
| int
 +
|
 +
| The texture unit to place the texture on. Effects will be able to access the buffer on this texture unit
 +
|}
 +
 
 +
There are specific pass types, each with their own set of custom parameters.
 +
 
 +
==== scene ====
 +
Renders the scene from the point of view given by the CameraGroup.
 +
{| class="wikitable" style="text-align: center; font-size: 85%; width: auto; table-layout: fixed;
 +
! scope="col" | Parameter Name
 +
! scope="col" | Optional
 +
! scope="col" | Value
 +
! scope="col" | Default Value
 +
! scope="col" | Description
 +
|-
 +
! scope="row"| <tt>cull-mask</tt>
 +
| {{Yes}}
 +
| A 32 bit number. See {{simgear file|simgear/scene/util/RenderConstants.hxx}} to know which bits enable what
 +
| <tt>0xffffffff</tt>
 +
| Specifies the cull mask to be used in the underlying <tt>osg::Camera</tt>
 +
|-
 +
! scope="row"| <tt>z-near, z-far</tt>
 +
| {{Yes}}
 +
| int
 +
| Default Z range in the CameraGroup
 +
| They change the depth range to be used
 +
|-
 +
! scope="row"| <tt>cubemap-face</tt>
 +
| {{Yes}}
 +
| int
 +
| <tt>-1</tt> (don't use cubemap)
 +
| Ignores the given view and projection matrices and uses a custom one that renders the scene as if it was seen from inside a cubemap looking towards the specified face
 +
|}
 +
 
 +
==== quad ====
 +
Renders a fullscreen quad with an optional [[Effects|effect]] applied. Useful for screen space shaders (like SSAO, Screen Space Reflections or bloom) and deferred rendering.
 +
{| class="wikitable" style="text-align: center; font-size: 85%; width: auto; table-layout: fixed;
 +
! scope="col" | Parameter Name
 +
! scope="col" | Optional
 +
! scope="col" | Value
 +
! scope="col" | Default Value
 +
! scope="col" | Description
 +
|-
 +
! scope="row"| <tt>geometry</tt>
 +
| {{Yes}}
 +
| float values for <code><x>, <y>, <width>, <height></code>
 +
| <tt>0.0, 0.0, 1.0, 1.0</tt> respectively
 +
| Size of the fullscreen quad inside the viewport using normalized coordinates.
 +
|-
 +
! scope="row"| <tt>effect</tt>
 +
| {{Yes}}
 +
| Valid Effect file
 +
| None
 +
| This Effect will be applied to the quad geometry
 +
|}
 +
 
 +
==== shadow-map ====
 +
Renders the scene from a light's point of view.
 +
{| class="wikitable" style="text-align: center; font-size: 85%; width: auto; table-layout: fixed;
 +
! scope="col" | Parameter Name
 +
! scope="col" | Optional
 +
! scope="col" | Value
 +
! scope="col" | Default Value
 +
! scope="col" | Description
 +
|-
 +
! scope="row"| <tt>light-name</tt>
 +
| {{No}}
 +
| Valid light name that exists in the scene graph
 +
|
 +
| The name of the <tt>osg::LightSource</tt> to use for this shadow map
 +
|-
 +
! scope="row"| <tt>near-m, far-m</tt>
 +
| {{No}}
 +
| Valid Effect file
 +
|
 +
| They specify the depth range of the shadow map
 +
|}
  
 
== TODO ==
 
== TODO ==
  
 
* Bring back distortion correction.
 
* Bring back distortion correction.
* Finish Clustered Forward Rendering.
+
* Some kind of versioning system to be able to make breaking changes in the future if/when the compositor is updated in any significant way, without people having to manually update their configs.
* Rewrite the uniform system (maybe integrating with Effects?).
+
* Bring back [[Howto:Canvas View Camera Element|Canvas integration]] so aircraft devs have access to the rendering pipeline. This allows to render exterior views in cockpit displays etc.
* Find a way to override an entire pass stateset with an Effect.
+
* Automatically calculate light source attenuation based on radius and radius based on attenuation.
 +
* Add 1-bit transparency to shadow mapping (maybe even full blown transparency with multiple depth buffers?).
  
 
== Known Issues ==
 
== Known Issues ==
  
 
* Setting a buffer scale factor different from 1.0 and rendering to it might not scale the splash screen correctly.
 
* Setting a buffer scale factor different from 1.0 and rendering to it might not scale the splash screen correctly.
* No error handling when the Compositor wasn't found. FlightGear should quit gracefully but right now it segfaults.
+
* Clustered shading crashes FG if compiled under OSG 3.6. This is related to osg::TextureBuffer changing definition from OSG 3.4 to OSG 3.6 (Images vs BufferData).
* Hardcoded Z handling. Better handling of depth is required to allow, for example, near and far cameras to prevent Z-fighting.
+
* Relative path Effects in aircrafts don't work. This is '''not''' permanent, they will work again once the Compositor replaces the legacy renderer.
* computeIntersections() always uses pass 0, which might fail. The scene pass should be used (which one if there are several?).
+
* [[Earthview]] doesn't work.
 +
* There is some kind of moiré pattern at certain sunlight angles (specially at dusk/dawn).
 +
* hud.eff doesn't work under the ALS pipeline.
 +
* Spotlights sometimes disappear at certain view angles.
 +
* Vegetation appears to flicker at long distances.
  
== Related ==
+
== References ==
 
{{Appendix}}
 
{{Appendix}}
 +
 +
== Related content ==
 +
=== Wiki articles ===
 +
* [[Uniform Buffer Objects]]
 +
* [[CompositeViewer Support]]
 +
* [[FlightGear CIGI Support (Common Image Generator Interface)]]
 +
 +
=== Forum topics ===
 +
* {{forum link|t=36269|text=The Compositor}}
 +
* {{forum link|t=35095|text=Clustered Forward Rendering}} (12/2018)
 +
* {{forum link|t=33045|text=Getting started with RTT}}
 +
* {{forum link|t=32846|text=Canvas::View Development}}
 +
* {{forum link|t=23929|text=Gear View in Cockpit}}
 +
* {{forum link|t=20057|text=Rear-View mirror}}
 +
* {{forum link|t=18905|text=Progress on synthetic terrain}}
 +
* {{forum link|t=17184|text=Instruments with heightmaps}}
 +
 +
[[Category:Core development projects]]

Latest revision as of 14:11, 7 December 2019

This article describes content/features that may not yet be available in the latest stable version of FlightGear (2018.3).
You may need to install some extra components, use the latest development (Git) version or even rebuild FlightGear from source, possibly from a custom topic branch using special build settings: -DENABLE_COMPOSITOR=ON.

This feature is scheduled for FlightGear 2019.2. 100}% completed

If you'd like to learn more about getting your own ideas into FlightGear, check out Implementing new features for FlightGear.


Compositor Framework
ALS Compositor pipeline.jpg
Started in 01/2018 (Available since FlightGear 2019.2)
Description Dynamic rendering pipeline configured via the property tree and XML
Contributor(s) Fernando García Liñán [1]
Status Stable
Folders
Changelog https://sourceforge.net/u/fgarlin/profile/feed.rss

The Compositor aims to bring multi-pass rendering to FlightGear. It encapsulates a rendering pipeline and exposes its parameters to a Property Tree interface. At startup, FlightGear reads the pipeline definition file for each physical viewport defined on the CameraGroup settings. If no Compositor file is specified for a physical camera, the one given by the --compositor= startup command will be used. If such startup option is not used either, FlightGear will look for a valid Compositor file in $FG_ROOT/Compositor/default.xml

The Compositor introduces a new dedicated fgdata directory for new/custom rendering pipelines: fgdata/Compositor.

Background

First discussed in 03/2012 during the early Rembrandt days, Zan (Lauri Peltonen) came up with a set of patches demonstrating how to create an XML-configurable rendering pipeline.

Back then, this work was considered to look pretty promising [2] and at the time plans were discussed to unify this with the ongoing Rembrandt implementation (no longer maintained).

Adopting Zan's approach would have meant that efforts like Rembrandt (deferred rendering) could have been implemented without requiring C++ space modifications, i.e. purely in Base package space.

Rembrandt's developer (FredB) suggested to extend the format to avoid duplicating the stages when you have more than one viewport, i.e. specifying a pipeline as a template, with conditions like in effects, and have the current camera layout refer the pipeline that would be duplicated, resized and positioned for each declared viewport [3]

Zan's original patches can still be found in his newcameras branches which allow the user to define the rendering pipeline in preferences.xml: FlightGear, SimGear.

At that point, it didn't have everything Rembrandt's pipeline needs, but most likely could be easily enhanced to support those things.

Basically, the original version added support for multiple camera passes, texture targets, texture formats, passing textures from one pass to another etc, while preserving the standard rendering line if user wants that. [4]

Since the early days of Zan's groundwork, providing the (hooks) infrastructure to enable base package developers to prototype, test and develop distinct rendering pipelines without requiring C++ space modifications has been a long-standing idea, especially after the Canvas system became available in early 2012, which demonstrated how RTT-rendering buffers (FBOs) could be set up, created and manipulated procedurally (i.e. at run-time) using XML, the property tree and Nasal scripting. [5]

The new Compositor is an improved re-implementation of Zan's original work using not just XML, but also properties and a handful of Canvas concepts.

Features

  • Completely independent of other parts of the simulator, i.e. it's part of SimGear and can be used in a standalone fashion if needed, ala Canvas.
  • Although independent, its aim is to be fully compatible with the current rendering framework in FG. This includes the Effects system, CameraGroup, Rembrandt and ALS (and obviously the Canvas).
  • Its functionality overlaps Rembrandt: what can be done with Rembrandt can be done with the Compositor, but not vice versa.
  • Fully configurable via an XML interface without compromising performance (ala Effects, using PropertyList files).
  • Flexible, expandable and compatible with modern graphics.
  • It doesn't increase the hardware requirements, it expands the hardware range FG can run on. People with integrated GPUs (Intel HD etc) can run a Compositor with a single pass that renders directly to the screen like before, while people with more powerful cards can run a Compositor that implements deferred rendering, for example.
  • Static branching support. Every pipeline element can be enabled/disabled at startup via a <condition> block.

How to enable the Compositor

Currently the Compositor can only be enabled at compile time via the -DENABLE_COMPOSITOR=ON CMake flag in FlightGear. SimGear doesn't require any extra parameters. Once you have a binary with the Compositor enabled and you run it, you will be presented with the default rendering pipeline. At the time of writing, this is the low spec rendering pipeline. If you want to try the ALS pipeline, start FlightGear with the command line argument: --compositor=Compositor/ALS/als

If you want to enable shadows on all objects in the ALS pipeline use these options as a startup parameters (in QT GUI or in the commandline) --prop:bool:/sim/rendering/als/shadows/enabled=true and --prop:int:/sim/rendering/als/shadows/sun-atlas-size=2048. If you feel like the shadows are too low-quality (specially in the cockpit), increase the shadow resolution to 4096 or 8192 instead of 2048.

If you are having trouble with lights using an integrated GPU and the Mesa drivers under Linux, try starting FlightGear with the environment variable MESA_GL_VERSION_OVERRIDE="3.1COMPAT"

Notes for aircraft developers

Lights

The Compositor introduces a new way of defining lights that is renderer agnostic, so every rendering pipeline will be able to access the lights that have been implemented like this. As of 2019/11, the only pipeline that supports dynamic lights is the ALS pipeline. The resulting light volumes can be visualized for debugging purposes by setting the property /sim/debug/show-light-volumes to true.

<light>
  <name>my-spotlight</name>
  <type>spot</type>
  <position>
    <x-m>-7.7476</x-m>
    <y-m>0</y-m>
    <z-m>-1.7990</z-m>
  </position>
  <direction>
    <x>-1.0</x>
    <y>0</y>
    <z>-0.013</z>
  </direction>
  <ambient>
    <r>0.03</r>
    <g>0.03</g>
    <b>0.03</b>
    <a>1</a>
  </ambient>
  <diffuse>
    <r>0.95</r>
    <g>0.9</g>
    <b>0.9</b>
    <a>1</a>
  </diffuse>
  <specular>
    <r>0.95</r>
    <g>0.9</g>
    <b>0.9</b>
    <a>1</a>
  </specular>
  <attenuation>
    <c>1.0</c>
    <l>0.09</l>
    <q>0.032</q>
  </attenuation>
  <spot-exponent>5</spot-exponent>
  <spot-cutoff>40</spot-cutoff>
  <range-m>50</range-m>
</light>
  • name. An <animation> will be able to reference the light by this name. Most animations will work as expected (rotate, translate, spin etc).
  • type. spot or point.
  • position. The position of the light source in model space and in meters.
  • direction. Only available in spot lights. It indicates the direction of the spotlight. This parameter can be specified in three different ways:
Direction vector Look-at point Rotation angles
A vector in model space that specifies the direction. Doesn't have to be normalized.
<x>-1.0</x>
<y>0</y>
<z>-0.013</z>
The spotlight will calculate its direction by looking at this position from the light position. The point is in model space and in meters.
<lookat-x-m>-8.031</lookat-x-m>
<lookat-y-m>0</lookat-y-m>
<lookat-z-m>-2</lookat-z-m>
A three angle rotation in degrees that rotates the spotlight around the three axes. A 0 degree angle in all axes makes the spotlight point downwards (negative Z).
<pitch-deg>90</pitch-deg>
<roll-deg>0</roll-deg>
<heading-deg>0</heading-deg>
  • ambient, diffuse and specular. Four-component vectors that specify the light color.
  • attenuation. Three-component vector where <c> specifies the constant factor, <l> specifies the linear factor and <q> specifies the quadratic factor. These factors are plugged into the OpenGL light attenuation formula Spotlight attenuation.png where d is the distance of the fragment to the light source. See this table for a list of attenuation values based on the range of the light.
  • range-m. Maximum range from the light source position in meters. This value will be used by the renderers to determine if a fragment is illuminated by this source. Every fragment outside this range isn't guaranteed to be affected by the light, even if the attenuation factor isn't 0 in that particular fragment.
  • cutoff. Only available in spot lights. It specifies the maximum spread angle of a light source. Only values in the range 0 90 are accepted. If the angle between the direction of the light and the direction from the light to the fragment being lighted is greater than the spot cutoff angle, it won't be lit.
  • exponent. Only available in spot lights. Higher spot exponents result in a more focused light source, regardless of the spot cutoff angle.
  • debug-color (Optional). Sets the color of the debug light volume. By default it's red.

Shadows

The shadow mapping algorithm can be customized entirely by the rendering pipeline. This means that each one will have its own requirements when it comes to shadows. Here are some general recommendations:

  • Use the <noshadow> animation to disable shadows on objects that don't need them. An example would be billboarded lights or really small cockpit elements that don't need shadows and would cause degraded performance.
  • Try to mark as many cockpit objects as possible as interior.
<model>
  <name>interior</name>
  <usage>interior</usage>
  <path>Aircraft/JA37/Models/ja37-interior.xml</path> <!-- All the objects that should only be seen when inside the cockpit are in this file -->
</model>
  • Unlike in Rembrandt, polygons facing the Sun are the ones used to generate the shadow map, so single sided surfaces and non-closed objects should be rendered correctly.

Pipelines

Low-Spec pipeline

A fixed function forward rendering pipeline mainly targeted to low spec systems. It imitates the classic forward pipeline used before multi-pass rendering was introduced by using two near/far cameras rendering directly to the screen.

Screenshot showing OSG stats of the Compositor-based low-spec rendering pipeline.

ALS

The ALS pipeline tries to bring multipass rendering to the current ALS framework, effectively combining the best from ALS and Project Rembrandt.

Cascaded shadow mapping

The main issue with shadow mapping in FlightGear is the complexity of the scene graph. Culling times can become huge if we don't carefully select which parts of the scene graph we want to render in the shadow maps. Some possible optimizations:

  • Study the minimum shadow map distance we can get without noticeable light leaking. Select an appropiate amount of cascades (more cascades = more passes over all geometry, and in general we want to keep the amount of forward passes to a minimum). We should have at least three cascades: the first just for cockpit/internal shadows, the second for the whole aircraft and the third for the rest of the scenery geometry. A fourth can be added if the transition between the second and the third is too harsh.
  • Improve the culling masks (simgear/scene/util/RenderConstants.hxx). The CASTSHADOW_BIT flag is present in almost every object in the scene graph. Turning this flag off for trees, random buildings and other geometry intensive objects improves framerates by a very considerable amount. Should the user be able to select which objects cast shadows?
  • Should the terrain cast shadows? The terrain is rarely steep enough to cast shadows. Apart from that, the terrain in FlightGear messes with automatic near/far computations for the shadow passes since the geometry is not tessellated enough. Also, the terrain LOD is not good enough to have decent cull times at far cascades.
  • Adding a "internal only" shadow flag for aircraft developers. This allows farther shadow cascades to cull complex objects that are only visible in the nearest cascades. (Very important optimization for aircraft with complex cockpit geometry).
  • Vegetation shadows will be done by the "legacy" method currently in use. Shadow mapping on vegetation is much more expensive in terms of performance and the current algorithm does the job well enough. [6]

Post-processing

Gamma correction, night vision and other ALS filters should happen in a quad pass. The current filter_combined() should be left for post-processing that requires as much precision as possible - e.g. dithering to prevent banding). HDR is not a planned feature for now so ALS will be using rgba8 buffers for most of its features.

Real-time dynamic reflections

Rendering dynamically to a cubemap is possible. As with shadow mapping, minimizing the object count and number of forward passes is vital to get good performance in FlightGear. Rendering to six cubemap faces requires six forward passes, but we can render to a dual paraboloid map instead, reducing this number to two.

Transparency

When shadows (and multipass rendering in general) come into play, transparent objects have to be treated differently, even when we are dealing with a forward renderer. In OSG there are two ways to separate transparent surfaces:

  • Using RenderBins. After a single scene cull traversal, surfaces which belong to a special RenderBin type (DepthSortedBin) are removed or moved to another camera. This is how Rembrandt does it and it is the most backwards compatible approach since RenderBins can be changed directly inside Effects.
  • Using cull masks. Two separate traversals are done: one for opaque objects and another for translucent objects. This requires offering aircraft developers another way of tagging a surface as transparent. A trivial approach would be to add a new <animation> type called 'transparent', but that wouldn't be backwards compatible. Maybe we can add some kind of system where we can change cull masks inside Effects? Would that be too hacky or out of place?

Creating a custom rendering pipeline

Since the Compositor is completely data-driven, new rendering pipelines can be created by writing a custom XML pipeline definition. This section tries to document most of the available parameters, but the best and most up-to-date resource is the Compositor parsing code in SimGear (simgear/simgear/scene/viewer). See existing pipelines in fgdata/Compositor for practical examples on how to use these parameters.

Buffers

A buffer represents a texture or, more generically, a region of GPU memory.

Parameter Name Optional Value Default Value Description
name No string Passes will be able to address the buffer by this name
type No 1d, 2d, 2d-array, 2d-multisample, 3d, rect, cubemap Any texture type allowed by OpenGL
width No Any unsigned integer or screen to use the physical viewport width. The <property> tag can also be used to use a property value Texture width
screen-width-scale Yes float 1.0 If screen was used, this controls the width scaling factor
height No Any unsigned integer or screen to use the physical viewport height. The <property> tag can also be used to use a property value Texture height
screen-height-scale Yes float 1.0 If screen was used, this controls the height scaling factor
depth No Any unsigned integer. The <property> tag can also be used to use a property value Texture depth
format Yes See simgear/simgear/scene/viewer/CompositorBuffer.cxx for the latest available values rgba8 Specifies the texture format. It corresponds to the internalformat, format and type arguments of the OpenGL function glTexImage2D
min-filter, mag-filter Yes linear, linear-mipmap-linear, linear-mipmap-nearest, nearest, nearest-mipmap-linear, nearest-mipmap-nearest linear Change the minification and magnification filtering respectively
wrap-s, wrap-t, wrap-r Yes clamp, clamp-to-edge, clamp-to-border, repeat, mirror clamp-to-border They change the wrap mode for each coordinate
anisotropy Yes float 1.0
border-color Yes vec4 (0.0f, 0.0f, 0.0f, 0.0f)
shadow-comparison Yes bool true
shadow-texture-mode Yes luminance, intensity, alpha luminance
shadow-compare-func Yes never, less, equal, lequal, greater, notequal, gequal, always lequal

Passes

A pass wraps around an osg::Camera. Passes all have some common parameters:

Parameter Name Optional Value Default Value Description
clear-color, clear-accum, clear-depth and clear-stencil Yes vec4 black, black, 1.0, 0 respectively Pass clear colors
clear-mask Yes color, stencil, depth, accum color depth Pass clear mask
effect-scheme Yes Valid effect scheme name None The pass will try to use the specified effect scheme to draw every object.

Passes can render to a buffer (Render to Texture), to several buffers (Multiple Render Targets) or directly to the framebuffer. This is accomplished by the <attachment> tag. Possible parameters of an attachment are:

Parameter Name Optional Value Default Value Description
buffer No Valid buffer name The name of the buffer to output to
component No color, color0 to color15, depth, stencil, depth-stencil FBO attachment point
level Yes int 0 Mipmap level of the texture that is attached
face Yes int 0 Face of cube map texture or z-level of 3d texture
mipmap-generation Yes bool false Whether mipmap generation should be done for texture
multisample-samples Yes int 0 Multisample anti-aliasing (MSAA) samples
multisample-color-samples Yes int 0 Multisample anti-aliasing (MSAA) color samples

Passes can also receive buffers as input and use them in their shaders. This is accomplished by the <binding> tag, which has the following parameters:

Parameter Name Optional Value Default Value Description
buffer No Valid buffer name The name of the buffer to bind
unit No int The texture unit to place the texture on. Effects will be able to access the buffer on this texture unit

There are specific pass types, each with their own set of custom parameters.

scene

Renders the scene from the point of view given by the CameraGroup.

Parameter Name Optional Value Default Value Description
cull-mask Yes A 32 bit number. See simgear/simgear/scene/util/RenderConstants.hxx to know which bits enable what 0xffffffff Specifies the cull mask to be used in the underlying osg::Camera
z-near, z-far Yes int Default Z range in the CameraGroup They change the depth range to be used
cubemap-face Yes int -1 (don't use cubemap) Ignores the given view and projection matrices and uses a custom one that renders the scene as if it was seen from inside a cubemap looking towards the specified face

quad

Renders a fullscreen quad with an optional effect applied. Useful for screen space shaders (like SSAO, Screen Space Reflections or bloom) and deferred rendering.

Parameter Name Optional Value Default Value Description
geometry Yes float values for <x>, <y>, <width>, <height> 0.0, 0.0, 1.0, 1.0 respectively Size of the fullscreen quad inside the viewport using normalized coordinates.
effect Yes Valid Effect file None This Effect will be applied to the quad geometry

shadow-map

Renders the scene from a light's point of view.

Parameter Name Optional Value Default Value Description
light-name No Valid light name that exists in the scene graph The name of the osg::LightSource to use for this shadow map
near-m, far-m No Valid Effect file They specify the depth range of the shadow map

TODO

  • Bring back distortion correction.
  • Some kind of versioning system to be able to make breaking changes in the future if/when the compositor is updated in any significant way, without people having to manually update their configs.
  • Bring back Canvas integration so aircraft devs have access to the rendering pipeline. This allows to render exterior views in cockpit displays etc.
  • Automatically calculate light source attenuation based on radius and radius based on attenuation.
  • Add 1-bit transparency to shadow mapping (maybe even full blown transparency with multiple depth buffers?).

Known Issues

  • Setting a buffer scale factor different from 1.0 and rendering to it might not scale the splash screen correctly.
  • Clustered shading crashes FG if compiled under OSG 3.6. This is related to osg::TextureBuffer changing definition from OSG 3.4 to OSG 3.6 (Images vs BufferData).
  • Relative path Effects in aircrafts don't work. This is not permanent, they will work again once the Compositor replaces the legacy renderer.
  • Earthview doesn't work.
  • There is some kind of moiré pattern at certain sunlight angles (specially at dusk/dawn).
  • hud.eff doesn't work under the ALS pipeline.
  • Spotlights sometimes disappear at certain view angles.
  • Vegetation appears to flicker at long distances.

References

References

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