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Compositor (view source)

Revision as of 00:13, 2 January 2019

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{{Template:Non-stable|version=2019.3|progress=80}}
{{Template:Non-stable|version=2019.3|progress=60}}
 


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{{infobox subsystem
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A '''Compositor''' is a rendering pipeline configured by the [[Property Tree]]. Every configurable element of the pipeline wraps an OpenGL/OSG object and exposes its parameters to the property tree. This way, the rendering pipeline becomes dynamically-reconfigurable at runtime.
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 the <compositor/> tag containing a valid path to the Compositor XML file.
 
Related efforts: [[Howto:Canvas View Camera Element]]


== Background ==
== Background ==
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   |script_version = 0.36  
   |script_version = 0.36  
   }}</ref>
   }}</ref>
In 02/2018, Icecode GL re-implemented a more generic version of Zan's idea using Canvas concepts, which means the rendering pipeline is not just xml-configurable, but using listeners to be dynamically-reconfigurable at runtime via properties.


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.
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.


== Status ==
== Features ==
'''08/2018'''
 
Fernando has been working on and off on multi-pass rendering support for
FlightGear since late 2017. It went through several iterations and
design changes, but he think it's finally on the right track. It's
heavily based on the Ogre3D Compositor and inspired by many
data-driven rendering pipelines. Its features include:


* 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.
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* 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 a XML interface without compromising performance (ala Effects, using PropertyList files).
* Optional at compile time to aid merge request efforts.
* 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.


Unlike Rembrandt, the Compositor makes use of scene graph cameras
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.
instead of viewer level cameras.
 
This allows CameraGroup to manage windows, near/far cameras and other
slaves without interfering with what is being rendered
(post-processing, shadows...).
 
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. Still, I think it's time to
announce it so people much more knowledgeable than me can point me in
the right direction to get this upstream and warn me about possible
issues and worries. :)
 
[[File:Compositor-08-2018.png|thumb|A screenshot showing post-processed gaussian blur and a blue filter:[https://i.imgur.com/Zzxre8a.png]]]
 
The next major (and hopefully final) iteration of the Compositor is
mostly done now, just some more cleanup is needed. I've tried to keep
the changes contained in CameraGroup.cxx/hxx and in new files in
SimGear to ease the merging process.
 
== Use Cases ==
* tail cameras
* mirrors
* [[Procedural Texturing]]
* [[Shuttle ADI ball]]
 
== FAQs ==
{{See also|Unifying the 2D rendering backend via canvas}}
 
I started the Compositor to hopefully unify rendering in FG in the most
non-radical way. Instead of changing a large part of the codebase like
Rembrandt, the Compositor aims to bring the same funcionality and slowly
deprecate parts of the FG Viewer code.
 
Indeed, this should help bring more interesting effects to the table.
I'm specially looking forward to porting ALS to a multi-pass
environment through PBR.
 
=== Shadows ===
It includes shadows indeed. The Compositor stacks several passes of
different types on top of each other, and exchanges buffers between
them. Nothing prevents someone from creating a new pass type that
implements a shadow map pre-pass. I've already tried myself
successfully, but the problem is telling the pass which light should be
used, which brings me to spotlights.
 
=== Light Sources ===
Allowing aircraft/scenery developers to include "real" lights is a whole
different aspect of the scene graph that shouldn't be managed by the
Compositor. Some kind of system that exposes OpenGL lights should be
implemented and linked to the Compositor. Some decisions have to be made
though. In a forward renderer lights aren't as cheap as in a deferred
renderer, so if the aircraft developer says there has to be a light
there, someone using a forward renderer might see a performance drop or
not see them at all.
 
 
=== Show me the code ===
I've submitted all I have to my FG fork:
 
 
https://sourceforge.net/u/fgarlin/simgear/ci/new-compositor/tree/
 
https://sourceforge.net/u/fgarlin/flightgear-src/ci/new-compositor/tree/
 
https://sourceforge.net/u/fgarlin/flightgear/ci/new-compositor/tree/
 
I haven't had the time to update the repos to the latest changes in the  official repos, but it shouldn't matter for now.
 
=== Merging ===
I'd love to have this merged as soon as possible, but maybe next month is a
bit too rushed. I don't even know if the approach I'm using will be the
definitive one or the small implications the Compositor might have on the
rest of the viewer related code. I'm not in a rush though, I can wait for
the next release if I can't make a decent enough merge request in June/July.


== Elements ==
== Elements ==
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=== Buffers ===
=== Buffers ===


A buffer represents a texture or, more generically, a region of GPU memory. Textures can be of any type allowed by OpenGL: 1D, 2D, rectangle, 2Darray, 3D or cubemap.
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.


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:


<syntaxhighlight lang="xml">
<syntaxhighlight lang="xml">
      <buffer>
<buffer>
            <name>buffer-name</name>
  <name>color</name>
    <type>2d</type>
  <type>2d</type>
    <width>512</width>
  <width>screen</width>
    <height>512</height>
  <height>screen</height>
    <scale-factor>1.0</scale-factor>
  <screen-width-scale>1.5</screen-width-scale>
    <internal-format>rgba8</internal-format>
  <screen-height-scale>1.5</screen-height-scale>
    <source-format>rgba</source-format>
  <internal-format>rgba8</internal-format>
            <source-type>ubyte</source-type>
  <source-format>rgba</source-format>
      </buffer>
  <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>
</syntaxhighlight>
</syntaxhighlight>


=== Passes ===
=== Passes ===


A pass wraps around an [http://public.vrac.iastate.edu/vancegroup/docs/OpenSceneGraphReferenceDocs-3.0/a00089.html osg::Camera]. As of February 2018, there are two types of passes supported:
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:


* '''scene'''. Renders a scene. The scene can be an already loaded scene graph node (terrain, aircraft etc.) or a path to a custom 3D model.
; '''shadow-map'''
* '''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 the scene from a light's point of view.
; '''scene'''
: Renders the scene from the point of view given by the CameraGroup.
; '''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.


Passes can render to a buffer (Render to Texture), to several buffers (Multiple Render Targets) or directly to the OSG context. This allows chaining of multiple passes, sharing buffers between them. They can also receive other buffers as input, so effects can access them as normal textures.
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.


Example XML for a ''quad'' type pass:
Example XML for a ''scene'' type pass:


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


          <effect>Effects/test</effect>
  <clustered-forward/>
   
          <input-buffer>
              <buffer>some-buffer</buffer>
              <unit>0</unit>
          </input-buffer>


          <output-buffer>
  <attachment>
              <buffer>color-buffer</buffer>
    <buffer>color</buffer>
              <component>color</component>
    <component>color0</component>
          </output-buffer>
    <multisample-samples>4</multisample-samples>
          <output-buffer>
    <multisample-color-samples>4</multisample-color-samples>
              <buffer>depth-buffer</buffer>
  </attachment>
              <component>depth</component>
  <attachment>
          </output-buffer>
    <buffer>depth</buffer>
      </pass-quad>
    <component>depth</component>
  </attachment>
</pass>
</syntaxhighlight>
</syntaxhighlight>


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== Known Issues ==
== Known Issues ==


* Canvas (sc::Image) doesn't update its texture unless .update() is invoked in Nasal. Is this a feature?
* Setting a buffer scale factor different from 1.0 and rendering to it might not scale the splash screen correctly.
* The scene graph and the viewer are still managed by FGRenderer. Maybe hijacking the init sequence is a good idea?
* No error handling when the Compositor wasn't found. FlightGear should quit gracefully but right now it segfaults.
* light sources ?
* Hardcoded Z handling. Better handling of depth is required to allow, for example, near and far cameras to prevent Z-fighting.


== Related ==
== Related ==
{{Appendix}}
{{Appendix}}
Icecode
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