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Compositor

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Revision as of 15:10, 27 February 2018 by Hooray (Talk | contribs) (Use Cases)

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Compositor Subsystem
Canvas-view-element-prototype-by-icecode gl.png
Started in 01/2018
Description Dynamic rendering pipeline configured via the property tree
Maintainer(s) none
Contributor(s) Icecode
Status experimental as of 02/2018

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.

Related efforts: Howto:Canvas_View_Camera_Element

Background

Use Cases

Elements

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

      <buffer>
            <name>buffer-name</name>
	    <type>2d</type>
	    <width>512</width>
	    <height>512</height>
	    <scale-factor>1.0</scale-factor>
	    <internal-format>rgba8</internal-format>
	    <source-format>rgba</source-format>
            <source-type>ubyte</source-type>
      </buffer>

Passes

A pass wraps around an osg::Camera. As of February 2018, there are two types of passes 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.
  • 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.

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.

Example XML for a quad type pass:

      <pass>
          <type>quad</type>
          <name>pass-name</name>

          <effect>Effects/test.eff</effect>
    
          <output-buffer>
              <buffer>buffer-name</buffer>
              <component>color</component>
          </output-buffer>
      </pass>

Canvas integration

A fullscreen quad with a shader that colors everything magenta is rendered to a buffer, which is displayed by a Canvas Image.

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);

Known Issues

  • Canvas doesn't update its texture unless .update() is invoked in Nasal. Is this a feature?
  • The scene graph and the viewer are still managed by FGRenderer. Maybe hijacking the init sequence is a good idea?
  • Effects need that buffers are created before reading the .eff file. Maybe some kind of listener is needed ala Rembrandt?

Related

References