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To help newly registered users get more familiar with the wiki (and maybe older users too) there is now a {{Welcome to the wiki}} template. Have a look at it and feel free to add it to new users discussion pages (and perhaps your own).

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Difference between revisions of "Compositor"

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{{Template:Non-stable|version=2019.2|progress=80}}
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{{Template:Non-stable|version=2019.2|build_flags=-DENABLE_COMPOSITOR|progress=100}}
  
  
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{{infobox subsystem
 
{{infobox subsystem
|image      = Clouds experiment.jpg
+
|image      = ALS Compositor pipeline.jpg
 
|name        = Compositor Subsystem
 
|name        = Compositor Subsystem
|started    = 01/2018
+
|started    = 01/2018 (Available since FlightGear 2019.2)
 
|description = Dynamic rendering pipeline configured via the property tree and XML
 
|description = Dynamic rendering pipeline configured via the property tree and XML
|status      = * experimental as of 02/2018
+
|status      = Stable
* pending review (06/2018) <ref>https://sourceforge.net/p/flightgear/mailman/message/36355833/</ref>
+
|developers  = Fernando García Liñán
* put up for review/discussion as of 08/2018
+
|folders =  
* pending merge-request (02/2019) <ref>https://sourceforge.net/p/flightgear/mailman/message/36578956/</ref>
+
* {{flightgear file|src/Viewer}}
|maintainers = none
+
* {{simgear file|simgear/scene/viewer}}
|developers  = Icecode
+
* {{fgdata file|Compositor}}
|topic-sg = https://sourceforge.net/u/fgarlin/simgear/ci/new-compositor/tree/
+
|topic-fg = https://sourceforge.net/u/fgarlin/flightgear-src/ci/new-compositor/tree/
+
|topic-fgdata = https://sourceforge.net/u/fgarlin/flightgear/ci/new-compositor/tree/
+
|merge-requests =
+
* https://sourceforge.net/p/flightgear/simgear/merge-requests/47/
+
* https://sourceforge.net/p/flightgear/flightgear/merge-requests/149/
+
* https://sourceforge.net/p/flightgear/fgdata/merge-requests/131/
+
 
}}
 
}}
  
The '''Compositor''' aims to bring multi-pass rendering to FlightGear. It encapsulates a rendering pipeline and exposes its parameters to the [[Property Tree]].
+
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 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 (using the [[PropertyList XML File]] format).
+
  
The Compositor introduces a new dedicated fgdata directory for new/custom rendering pipelines: $FG_ROOT/Compositor.
+
The Compositor introduces a new dedicated fgdata directory for new/custom rendering pipelines: {{Fgdata file|Compositor}}.
 
+
This is also where the default pipeline can be found, namely: $FG_ROOT/Compositor/default.xml
+
  
 
== Background ==
 
== Background ==
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   |script_version = 0.36  
 
   |script_version = 0.36  
 
   }}</ref>
 
   }}</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:
 
Zan's original patches can still be found in his newcameras branches which allow the user to define the rendering pipeline in preferences.xml:
Line 76: Line 65:
 
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>https://forum.flightgear.org/search.php?st=0&sk=t&sd=d&sr=posts&keywords=zan++rembrandt+canvas</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>https://forum.flightgear.org/search.php?st=0&sk=t&sd=d&sr=posts&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 (i.e. property-driven subsystems).
+
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.
  
 
== Status ==
 
== Status ==
'''02/2019'''
+
'''06/2019'''
  
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.
+
The Compositor core is complete and stable. Effects and shaders still need to be adapted to the Compositor.
 
+
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. <ref>https://sourceforge.net/p/flightgear/mailman/message/36355833/</ref>
+
 
+
As of 02/2019, a set of merge requests has been created for the Compositor framework for SimGear, FlightGear and FGData respectively.
+
 
+
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.
+
 
+
(There is a known issue where FG will crash with a segfault if a valid
+
Compositor XML isn't found.)
+
 
+
The FGData merge request only includes the Low-spec pipeline, which
+
imitates the current near/far camera forward pipeline.<ref>https://sourceforge.net/p/flightgear/mailman/message/36578956/</ref>
+
 
+
James and Richard agreed that śince compilation is disabled by default, it might be easier to merge more or less as-is (unless there’s something really awkward), and then make any adjustments on top?
+
 
+
That also means we can enable this option on Jenkins to get platform coverage.<ref>https://sourceforge.net/p/flightgear/mailman/message/36579111/</ref> <ref>https://sourceforge.net/p/flightgear/mailman/message/36583393/</ref>
+
 
+
And this would make things easier for other people willing to collaborate.
+
 
+
Everything is structured so nothing is affected when the compositor is
+
disabled at compile time, so there shouldn't be any breakage from the
+
commit.<ref>https://sourceforge.net/p/flightgear/mailman/message/36583493/</ref>
+
 
+
James and Stuart are hoping to review the merge request in time for 2019.1 <ref>https://sourceforge.net/p/flightgear/mailman/message/36587786/</ref> <ref>https://sourceforge.net/p/flightgear/mailman/message/36587830/</ref>
+
 
+
Richard offered to assist making changes that are required to the animation parsing/handling; including putting stuff into the property tree. <ref>https://forum.flightgear.org/viewtopic.php?f=47&t=35095&p=339699&#p339788</ref>
+
  
 
== Features ==
 
== Features ==
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* 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 <condition> block.
  
 
== Elements ==
 
== Elements ==
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; wrap-s, wrap-t and wrap-r (Optional)
 
; wrap-s, wrap-t and wrap-r (Optional)
 
: 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.
 
: 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.
 +
; condition
 +
: A valid boolean condition to enable the buffer at startup (doesn't work at runtime).
  
 
A typical [[PropertyList XML File|property tree structure]] describing a buffer may be as follows:
 
A typical [[PropertyList XML File|property tree structure]] describing a buffer may be as follows:
Line 171: Line 135:
 
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:
 
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:
  
; condition
 
: A valid XML expression that enables the pass when met.
 
 
; clear-color, clear-accum, clear-depth and clear-stencil
 
; clear-color, clear-accum, clear-depth and clear-stencil
 
: Default values are black, black, 1.0 and 0 respectively.
 
: Default values are black, black, 1.0 and 0 respectively.
 
; clear-color-bit, clear-accum-bit, clear-depth-bit and clear-stencil-bit
 
; clear-color-bit, clear-accum-bit, clear-depth-bit and clear-stencil-bit
 
: Default values are true, false, true, false, respectively.
 
: Default values are true, false, true, false, respectively.
; effect-override
+
; effect-scheme
: Use the specified effect to draw every object. This is useful for special passes like G-buffer and shadow map optimization.
+
: The pass will try to use the specified effect scheme to draw every object.
 +
; condition
 +
: A valid boolean condition to enable the pass at startup (doesn't work at runtime).
 +
 
 +
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:
 +
 
 +
; buffer
 +
: The name of the buffer to output to.
 +
; component
 +
: FBO attachment point. Possible values are color0 to color15, depth, stencil and depth-stencil.
 +
; level (Optional)
 +
: Controls the mip map level of the texture that is attached. Default value is 0.
 +
; face (Optional)
 +
: Controls the face of texture cube map or z level of 3d texture. Default value is 0.
 +
; mipmap-generation (Optional)
 +
: Controls whether mipmap generation should be done for texture. Default value is false.
 +
; multisample-samples (Optional)
 +
: MSAA samples. Default value is 0.
 +
; multisample-color-samples (Optional)
 +
: MSAA color samples. Default value is 0.
 +
; condition
 +
: A valid boolean condition to enable the attachment at startup (doesn't work at runtime).
 +
 
 +
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:
 +
 
 +
; buffer
 +
: The name of the buffer to bind.
 +
; unit
 +
: The texture unit to place the texture on. Effects will be able to access the buffer on this texture unit.
 +
; condition
 +
: A valid boolean condition to enable the binding at startup (doesn't work at runtime).
  
 
There are specific pass types, each with their own set of custom parameters:
 
There are specific pass types, each with their own set of custom parameters:
  
* '''scene'''
+
==== scene ====
 
Renders the scene from the point of view given by the CameraGroup.
 
Renders the scene from the point of view given by the CameraGroup.
 
; cull-mask
 
; cull-mask
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: 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.
 
: 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'''
+
==== 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.
 
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.
 
; geometry
 
; geometry
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: The quad will use this effect.
 
: The quad will use this effect.
  
* '''shadow-map'''
+
==== shadow-map ====
 
Renders the scene from a light's point of view.
 
Renders the scene from a light's point of view.
 
; light-num
 
; light-num
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; near-m and far-m
 
; near-m and far-m
 
: They specify the range of the shadow map.
 
: They specify the range of the shadow map.
 
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:
 
 
; buffer
 
: The name of the buffer to output to.
 
; component
 
: FBO attachment point. Possible values are color0 to color15, depth, stencil and depth-stencil.
 
; level (Optional)
 
: Controls the mip map level of the texture that is attached. Default value is 0.
 
; face (Optional)
 
: Controls the face of texture cube map or z level of 3d texture. Default value is 0.
 
; mipmap-generation (Optional)
 
: Controls whether mipmap generation should be done for texture. Default value is false.
 
; multisample-samples (Optional)
 
: MSAA samples. Default value is 0.
 
; multisample-color-samples (Optional)
 
: MSAA color samples. Default value is 0.
 
 
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:
 
 
; buffer
 
: The name of the buffer to bind.
 
; unit
 
: The texture unit to place the texture on. Effects will be able to access the buffer on this texture unit.
 
  
 
Example XML for a ''scene'' type pass:
 
Example XML for a ''scene'' type pass:
Line 266: Line 234:
 
[[File:Low-spec Compositor pipeline.png|thumb|Screenshot showing OSG stats of the Compositor-based low-spec rendering pipeline.]]
 
[[File:Low-spec Compositor pipeline.png|thumb|Screenshot showing OSG stats of the Compositor-based low-spec rendering pipeline.]]
  
== Canvas integration ==
+
=== ALS ===
  
[[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).]]
+
The ALS pipeline tries to bring multipass rendering to the current ALS framework, effectively combining the best from ALS and Project Rembrandt.
  
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.
+
==== Cascaded shadow mapping ====
  
<syntaxhighlight lang="nasal">
+
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:
var (width,height) = (612,612);
+
* 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.
var title = 'Compositor&Canvas test';
+
* 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?
var window = canvas.Window.new([width,height],"dialog")
+
* 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.
.set('title',title);
+
* 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).
var myCanvas = window.createCanvas().set("background", canvas.style.getColor("bg_color"));
+
 
var root = myCanvas.createGroup();
+
==== Post-processing ====
var path = "buffer://test-compositor/test-buffer";
+
 
var child = root.createChild("image")
+
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.
    .setTranslation(50, 50)
+
 
    .setSize(512, 512)
+
==== Real-time dynamic reflections ====
    .setFile(path);
+
 
</syntaxhighlight>
+
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?
  
 
== TODO ==
 
== TODO ==
  
 
* Bring back distortion correction.
 
* Bring back distortion correction.
* Finish Clustered Forward Rendering.
+
* Add spotlight support to clustered shading.
 
* 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.
 
* 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.
* Coherently support static conditional branching (<condition> tags available for both passes and buffers), as well as using property tree parameters (via a <property> tag ala Effects). For simplicity and to avoid major architectural changes, these conditional checks should only be performed on startup (no dynamic branching). For example, the user should be able to select if they want shadows and their size in the launcher, not on the in-sim Rendering dialog.
+
* Bring back Canvas integration so aircraft devs have access to the rendering pipeline. This allows to render exterior views in cockpit displays etc.
  
 
== 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.
 
* computeIntersections() always uses pass 0, which might fail. The scene pass should be used (which one if there are several?).
 
  
 
== Related ==
 
== Related ==
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{{Appendix}}
 
{{Appendix}}
 +
 +
[[Category:Core development projects]]

Latest revision as of 16:19, 13 June 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.

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 Subsystem
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
Status Stable
Folders

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 [1] 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 [2]

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

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. [3]

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. [4]

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.

Status

06/2019

The Compositor core is complete and stable. Effects and shaders still need to be adapted to the Compositor.

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.

Elements

Buffers

A buffer represents a texture or, more generically, a region of GPU memory. It can have the following parameters:

name
Passes will be able to address the buffer by this name.
type
Any texture type allowed by OpenGL: 1d, 2d, 2d-array, 2d-multisample, 3d, rect or cubemap.
width
Texture width. It's possible to write 'screen' to use the physical viewport width.
screen-width-scale
If 'screen' was used, this controls the width scaling factor.
height
Texture height. It's possible to write 'screen' to use the physical viewport height.
screen-height-scale
If 'screen' was used, this controls the height scaling factor.
depth
Texture depth.
internal-format, source-format and source-type
They specify the texture format. They correspond to the arguments internalformat, format and type respectively of the OpenGL function glTexImage2D.
min-filter and mag-filter (Optional)
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.
wrap-s, wrap-t and wrap-r (Optional)
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.
condition
A valid boolean condition to enable the buffer at startup (doesn't work at runtime).

A typical property tree structure describing a buffer may be as follows:

<buffer>
  <name>color</name>
  <type>2d</type>
  <width>screen</width>
  <height>screen</height>
  <screen-width-scale>1.5</screen-width-scale>
  <screen-height-scale>1.5</screen-height-scale>
  <internal-format>rgba8</internal-format>
  <source-format>rgba</source-format>
  <source-type>ubyte</source-type>
  <min-filter>nearest-mipmap-nearest</min-filter>
  <mag-filter>nearest-mipmap-nearest</mag-filter> 
  <wrap-s>repeat</wrap-s>
  <wrap-t>repeat</wrap-t>
</buffer>

Passes

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

clear-color, clear-accum, clear-depth and clear-stencil
Default values are black, black, 1.0 and 0 respectively.
clear-color-bit, clear-accum-bit, clear-depth-bit and clear-stencil-bit
Default values are true, false, true, false, respectively.
effect-scheme
The pass will try to use the specified effect scheme to draw every object.
condition
A valid boolean condition to enable the pass at startup (doesn't work at runtime).

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:

buffer
The name of the buffer to output to.
component
FBO attachment point. Possible values are color0 to color15, depth, stencil and depth-stencil.
level (Optional)
Controls the mip map level of the texture that is attached. Default value is 0.
face (Optional)
Controls the face of texture cube map or z level of 3d texture. Default value is 0.
mipmap-generation (Optional)
Controls whether mipmap generation should be done for texture. Default value is false.
multisample-samples (Optional)
MSAA samples. Default value is 0.
multisample-color-samples (Optional)
MSAA color samples. Default value is 0.
condition
A valid boolean condition to enable the attachment at startup (doesn't work at runtime).

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:

buffer
The name of the buffer to bind.
unit
The texture unit to place the texture on. Effects will be able to access the buffer on this texture unit.
condition
A valid boolean condition to enable the binding at startup (doesn't work at runtime).

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.

cull-mask
A 32 bit number that specifies the cull mask to be used. See simgear/scene/util/RenderConstants.hxx to know which bits enable what.
z-near and z-far
They change the depth range to be used. If both of them are zero, the default Z range in the CameraGroup is used.
clustered-forward
Enables the use of clustered forward rendering for this pass.
cubemap-face
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.

geometry
Specifies the x, y, width and height of the fullscreen quad inside the viewport using normalized coordinates.
effect
The quad will use this effect.

shadow-map

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

light-num
The OpenGL light number to use for this shadow map.
near-m and far-m
They specify the range of the shadow map.

Example XML for a scene type pass:

<pass>
  <name>forward-lighting</name>
  <type>scene</type>
  <clear-color type="vec4d">0 0 0 0</clear-color>

  <clustered-forward/>

  <binding>
    <buffer>shadowmap</buffer>
    <unit>10</unit>
  </binding>
  <attachment>
    <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>

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 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?

TODO

  • Bring back distortion correction.
  • Add spotlight support to clustered shading.
  • 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.

Known Issues

  • Setting a buffer scale factor different from 1.0 and rendering to it might not scale the splash screen correctly.

Related

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References