View source for Compositor

{{Template:Non-stable|version=2019.2|build_flags=-DENABLE_COMPOSITOR=ON|progress=100}}



{{forum|47|Effects & Shaders}}

{{infobox subsystem
|image       = ALS Compositor pipeline.jpg
|name        = Compositor Framework
|started     = 01/2018 (Available since FlightGear 2019.2)
|description = Dynamic rendering pipeline configured via the property tree and XML
|status      = Stable
|developers  = Fernando García Liñán
|folders = 
* {{flightgear file|src/Viewer}}
* {{simgear file|simgear/scene/viewer}}
* {{fgdata file|Compositor}}
}}

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 ==
{{See also|Supporting multiple renderers|Howto:Canvas View Camera Element}}

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
  |url    =  https://sourceforge.net/p/flightgear/mailman/message/28946515/ 
  |title  =  <nowiki> Re: [Flightgear-devel] [Rembrandt] the plan </nowiki> 
  |author =  <nowiki> Mathias Fröhlich </nowiki> 
  |date   =  Mar 7th, 2012 
  |added  =  Mar 7th, 2012 
  |script_version = 0.36 
  }}</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 and at the time plans were discussed to unify this with the ongoing Rembrandt implementation (no longer maintained).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]] (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
  |url    =  https://sourceforge.net/p/flightgear/mailman/message/28944773/ 
  |title  =  <nowiki> Re: [Flightgear-devel] [Rembrandt] the plan </nowiki> 
  |author =  <nowiki> Frederic Bouvier </nowiki> 
  |date   =  Mar 7th, 2012 
  |added  =  Mar 7th, 2012 
  |script_version = 0.36 
  }}</ref>

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}}.

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
  |url    =  https://sourceforge.net/p/flightgear/mailman/message/28944733/ 
  |title  =  <nowiki> [Flightgear-devel] [Rembrandt] the plan </nowiki> 
  |author =  <nowiki> Lauri Peltonen </nowiki> 
  |date   =  Mar 7th, 2012 
  |added  =  Mar 7th, 2012 
  |script_version = 0.36 
  }}</ref>

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.

== News / Changelog ==

== Developer Discussion ==

The Compositor is disabled by default at compilation time in
FlightGear (not SimGear). Both Rembrandt and the forward pipeline are
still available if this flag is disabled.<ref>https://sourceforge.net/p/flightgear/mailman/message/36578956/</ref>

it is worth remembering that at the moment compositor is 
identical to the default pipeline and thus there will be no advantage 
for flying - it is currently there so we can develop it further<ref>https://sourceforge.net/p/flightgear/mailman/message/36605836/</ref>

Would it be practical to get to a point where both the legacy and
compositor CameraGroup classes can co-exist at runtime? I recently made a
fix (pull request coming shortly) that involved a lot of duplicate code in
the legacy and compositor code, and then I had to change compile options,
recompile, etc. to test it; yuck. I'm very enthusiastic about the
compositor, but it will get much more of a workout if it  is compiled and
available to everyone.<ref>https://sourceforge.net/p/flightgear/mailman/message/36605878/</ref>

The vision is we would replace the legacy rendering with 
compositor; this will then allow us to clean up a lot of the hardcoded 
stuff that sets up the pipeline, remove the Rembrandt code and generally 
have a good tidy up.

There is a new scheme that Fernando is currently working on whereby we 
have a definition of the effects/shaders within the pipeline keep them 
within the pipeline folder structure; together with defining what the 
shader does (if applicable) so it can be controlled by a future UI. 
There is a very rough draft design doc for this available here 
http://www.zaretto.com/sites/zaretto.com/files/compositor-effects-registry.pdf <ref>https://sourceforge.net/p/flightgear/mailman/message/36606242/</ref>

Tim Moore suggested to dump the legacy renderer and going with
the compositor. He did some work to eliminate the
need for the far camera, and decided to gamble on doing the work with the
compositor renderer instead of the legacy renderer. He had to add render
buffers to the compositor pass syntax, but otherwise it went well. Going
forward, other work he is doing will use the compositor.

This work uses a floating point depth buffer, the OpenGL clip control
extension to restrict Z values from 0 to 1, and the scheme of reversing the
Z values so that 0 is at the far plane. Tim will submit a merge request for
it once he determines if something similar will work for systems that don't
support clip control.<ref>https://sourceforge.net/p/flightgear/mailman/message/36659646/</ref>

== Gallery ==

<gallery mode=packed widths=230px heights=230px>
Aircrane-compositor-11-2019.jpeg|Aircrane helicopter in [[Compositor]] mode, screen shot by wlbragg <ref>https://forum.flightgear.org/viewtopic.php?f=47&t=36269&start=45#p357570</ref>
</gallery>

{{#ev:youtube|5KYZylR3Wao}}

== 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, [[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.
* Fully configurable via an XML interface without compromising performance (ala Effects, using [[PropertyList XML File|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 [[Conditions|<condition> block]].

== How to enable the Compositor ==

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 pipeline, start FlightGear with the command line argument: <code>--compositor=Compositor/ALS/als</code>


== Good to know ==
* still untested, but it's supposed to work with multiple monitors <ref>https://forum.flightgear.org/viewtopic.php?f=47&t=36269#p354669</ref>
* Spot lights aren't supported yet for clustered shading, i.e. they are registered and parsed but aren't processed as lights. Point lights are, but I don't remember what's their current status regarding shaders. <ref>https://forum.flightgear.org/viewtopic.php?f=47&t=36269#p353179</ref>
* shadows are only supported on the ALS pipeline <ref>https://forum.flightgear.org/viewtopic.php?f=47&t=36269&start=60#p357586</ref>
* Transparency on trees require MSAA (anti-aliasing) <ref>https://forum.flightgear.org/viewtopic.php?f=47&t=36269&start=45#p357580</ref>

== 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 <tt>/sim/debug/show-light-volumes</tt> to true.

{|cellpadding=10|
|valign=top style="width: 20%;"|
<syntaxhighlight lang="xml">
<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>
</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>.
* <tt>'''position'''</tt>. The position of the light source in model space and in meters.
* <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
! scope="col" style="width:33%;" |Look-at point
! scope="col" style="width:33%;" |Rotation angles
|-
| 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>
<y>0</y>
<z>-0.013</z>
</syntaxhighlight>
| 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">
<lookat-x-m>-8.031</lookat-x-m>
<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">
<pitch-deg>90</pitch-deg>
<roll-deg>0</roll-deg>
<heading-deg>0</heading-deg>
</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.
* <tt>'''debug-color'''</tt> ('''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 <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.
* Try to mark as many cockpit objects as possible as <tt>interior</tt>.
<syntaxhighlight lang="xml">
<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>
</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.

== 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.

[[File:Low-spec Compositor pipeline.png|thumb|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 aircrafts with complex cockpit geometry).

==== 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 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 ==

* Bring back distortion correction.
* $FG_ROOT/Compositor could use a per-pipeline structure, so that effects/shaders and pipelines are neatly separated and organized, this would also make it easier for maintainers to track a certain pipeline instead of having to track all changes under $FG_ROOT/Compositor (git log)
* 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 [[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.
* Automatically calculate light source attenuation based on radius and radius based on attenuation.

== 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).

== References ==
{{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]]