SceneGraph

Note: A SceneGraph is currently used in the C# SuperTux editor, but

not in the game itself. At this point the effort to switch the game over to a SceneGraph doesn't seem worth it, as the DrawingRequests that are used in the game already offer enough flexibility and speed.

A scenegraph is a graph of nodes which change the OpenGL state (color, texture, drawing mode, ...) or draw some OpenGL primitives (rectangles, triangles, ...).

Let's look at a simple example, to get an impression on why a scene graph is useful. Say we have a node CUBE that can draw a cube. Drawing in OpenGL can be done in several different modes: You can change the textures, color, drawing mode and many other things. To keep the code flexible the CUBE node will simply draw the cube, but not care about the OpenGL environment at all. We will introduce some nodes that are only there to change the environment for example a node COLOR which will change the drawing color. A red and a blue cube could be constructed like this:

             ROOT
       /              \
      /                \
    COLOR (red)    COLOR (blue)
      |                 |
     CUBE             CUBE

The Draw() function of the rootnode would simply call the draw functions of it's child nodes. So the Draw() functions of the 2 COLOR nodes would be called. The COLOR nodes, would change the OpenGL drawing color, then call the Draw() function of all child nodes and change the color back to what it was before after that. In effect a red and a blue cube would be drawn.

Commonly used node types are: State changing nodes like DrawingColor, Transformations (rotate,zoom,translate), blend mode, active texture. Or nodes that help organisation like simply nodes that do nothing and contain a list of child nodes, nodes that sort their child nodes in some form.

Links

• Wikiepdia about SceneGraphs