| This article describes content/features that may not yet be available in the latest stable version of FlightGear (2020.1).|
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:
This feature is scheduled for FlightGear 2019.3.
If you'd like to learn more about getting your own ideas into FlightGear, check out Implementing new features for FlightGear.
|Description||Dynamic rendering pipeline configured via the property tree|
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 CameraGroup settings can be changed via the <compositor/> tag containing a valid path to the Compositor XML file.
First discussed in 03/2012 during the early Rembrandt days, Zan came up with patches demonstrating how to create an XML-configurable rendering pipeline.
Back then, this work was considered to look pretty promising  and at the time plans were discussed to unify this with the ongoing Rembrandt implementation (no longer maintained).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.
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 
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. 
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.
- 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 a 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.
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.
A buffer represents a texture or, more generically, a region of GPU memory. It can have the following parameters:
- Passes will be able to address the buffer by this name.
- Any texture type allowed by OpenGL: 1d, 2d, 2d-array, 2d-multisample, 3d, rect or cubemap.
- Texture width. It's possible to write 'screen' to use the physical viewport width.
- If 'screen' was used, this controls the width scaling factor.
- Texture height. It's possible to write 'screen' to use the physical viewport height.
- If 'screen' was used, this controls the height scaling factor.
- 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 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>
A pass wraps around an osg::Camera. The following types of passes are supported:
- Renders the scene from a light's point of view.
- Renders the scene from the point of view given by the CameraGroup.
- Renders a fullscreen quad with an optional 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 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 scene type pass:
<pass> <name>forward-lighting</name> <type>scene</type> <clear-color type="vec4d">0 0 0 0</clear-color> <clustered-forward/> <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>
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
buffer://test-compositor/test-buffer displays the buffer test-buffer declared in test-compositor.
var (width,height) = (612,612); var title = 'Compositor&Canvas test'; var window = canvas.Window.new([width,height],"dialog") .set('title',title); var myCanvas = window.createCanvas().set("background", canvas.style.getColor("bg_color")); var root = myCanvas.createGroup(); var path = "buffer://test-compositor/test-buffer"; var child = root.createChild("image") .setTranslation(50, 50) .setSize(512, 512) .setFile(path);
- 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.
- Hardcoded Z handling. Better handling of depth is required to allow, for example, near and far cameras to prevent Z-fighting.