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Project 3: Scene Graphs and Culling

In this project you will need to implement a scene graph to render a group of animated robots.

The total score for this project is 100 points. Additionally, you can obtain up to 10 points of extra credit.

We recommend the following approach for the timing of your work:

  1. Parts 1-4 require knowledge of scene graphs. They will be covered in the lecture on Oct 22nd.
  2. Part 5 (culling) will be covered in lecture on Oct 29th and also in discussion on Oct 28th.

1. Scene Graph Engine (25 Points)

To create a robot with multiple moving body parts (head, torso, limbs, eyes, antennae), we need to first implement a simple scene graph structure for our rendering engine. This scene graph should consist of at least three nodes: Node (5 points), Transform (10 points) and Geometry (10 points). You are free to add more scene graph node types as you see fit.

  • Class Node should be abstract and serve as the common base class. It should implement the following class methods:
    • an abstract draw method: virtual void draw(Matrix4 C)=0
    • an abstract virtual void update()=0 method to separate bounding sphere updates from rendering
  • Transform should be derived from Node and have the following features:
    • store a 4x4 transformation matrix M
    • store a list of pointers to child nodes (std::list<Node*>)
    • provide a class methods to add a child node (addChild()) to the list
    • its draw method needs to traverse the list of children and call each child node's draw function
    • when draw(C) is called, multiply matrix M with matrix C.
  • Geometry should be derived from Node and have the following features:
    • set the modelview matrix to the current C matrix
    • an initialization method to load a 3D model (OBJ file) whose filename is passed to it (init(string filename). Your OBJ loader from project 2 should work.
    • have a class method which draws the 3D model associated with this node.

2. Robot (20 Points)

Now that we have the scene graph classes, it is time to put them to work. Build your own robot using the addChild methods. Use at least 3 different types of parts for your robot (e.g., body, head and limb). In total, your robot needs to consist of at least 4 parts, 3 of which need to be moving independently from one another and they need to be connected to the 4th part.

Start with code that uses your trackball code, and modify it so that trackball rotations control the camera instead, so that trackball rotations control the camera's direction vector - the camera stays in place. (If you don't have trackball rotation code, keyboard controls will suffice, but the camera still needs to be stationary and pivot about its location.) Also allow zooming in and out (i.e., changing the field of view).

Thanks to our tutors Weichen and former tutor Yining, you have the following robot components to choose from: head, body (torso), limb, eye, antenna. You will find the OBJ files in this ZIP file.

Here is an example of a robot with two antennas, two eyeballs, one head, one torso and 4 limbs (2 legs and 2 arms):


Use your creativity to build the most creative robot in class! The 5 most creative robots in class are going to get extra credit, after a vote on Canvas.

Once you've created your scene graph, you need to get your rendering engine ready to recursively traverse the scene graph for rendering by creating a root node of type Transform and calling its draw() function with the identity matrix as its parameter.

3. Animated Robot (15 Points)

Animate the robot to make it look like it is walking, by changing the matrices in the Transform nodes. Walking in place is fine, the robot does not need to actually move forward.

4. Lots of Robots (15 Points)

  • Construct a scene which consists of a large amount of robots, at least 100. The robots can all be identical clones. (5 points)
  • Distribute the robots on a 2D grid (i.e., place them on a plane with uniform spacing). For 100 robots, use a 10x10 grid. (5 points)
  • Enable the animation for all the robots so that they look like they are walking. (5 points)
  • Make sure your camera can be rotated and zoomed so that you can change between having all robots on screen all the way to none and back.

This image illustrates the grid layout of the robots (background image not required):



  • -5 if camera does not allow gradual changes between showing all robots and none.

5. Culling (25 Points)

Implement object level culling, to allow the existence of thousands of instances of your robot, without having to render them all at once.

  • Determine the parameters for a bounding sphere (Vector3 for its center point, and a radius) for each of your robots, which contains all parts of the robot. You do not need to find the tightest possible bounding spheres. Just make them as tight as you reasonably can. Estimating the bounding sphere size is fine. You don't need to have a separate bounding sphere for each animation step - one that encloses all steps is fine.
  • Add the optional ability to render the bounding spheres. Support a keyboard key to toggle the rendering of the bounding spheres on or off. You can render the spheres by using the sphere OBJ file from project 2. Other ways of rendering spheres are also acceptable. We recommend rendering a wireframe sphere by rendering OpenGL lines instead of triangles. (5 points)
  • Add view frustum culling, and support a keyboard key to turn it on or off. (10 points)
  • Display the number of robots that are visible (=not culled) on standard out or in the window's title bar. Make sure that by turning culling on this number goes down when part of the robot army is off screen. (5 points)
  • Debug Mode: Implement a demo mode in which you zoom the camera out (increase the FOV by a few degrees) but do the culling with the original FOV, so that one can see when the robots get culled. Allow toggling between demo mode and normal mode with a keyboard key. (5 points)


  • Lighthouse3D has an excellent description of how to calculate the view frustum plane equations.

6. Extra Credit (Up to 10 Points)

  • Hierarchical Culling: Create a hierarchical culling algorithm by storing bounding sphere information at every level of the scene graph, so that you can cull an entire branch of the scene graph at once. Structure your scene graph so that you have multiple levels (for instance, by subdividing your army into four quarters, and create a node above each quarter army. Show how many times you were able to cull based on the higher level bounding spheres by displaying the number in the text window. Use differently colored spheres for the different levels of your culling hierarchy. (5 points)
  • Creativity Contest (up to 5 Points): We're going to have a contest for the top 5 most creative robot creations. Submit a JPEG image or GIF animation of your robot to Canvas by 11:59pm on Friday 11/1 by adding an entry to the respective discussion thread on Piazza. To create a GIF animation you can use this converter. To convert any image file format to a GIF image, we recommend IrfanView. The top five most voted for robots are going to get extra credit. Voting starts once submissions are closed and ends on the following Tuesday at 9am. The winner of the contest will get 5 points of extra credit. The second place will get 4 points, third gets 3, fourth 2, fifth 1 point.