LAB6: Scene Graph

This lab is based on “Chapter 6: Scene Managers” in the book “Ogre 3d 1.7 Beginner's Guide” by Felix Kerger.

Discussion thread for this lab is here: Lab 6 Discussion Thread

The primary goal of this lab is to see how the representation of geometry in a scene can affect rendering performance. In particular, a scene where everything is represented as a movable object can get super heavy. Instead, you can “bake” meshes that won't move into static batches, which get optimized for rendering performance.

Continue with your project or create a new project from the template, you need to be able to move the camera.

Follow these steps to complete the lab project:

1. Create a New Project Create a new empty project called “Lab6” in the same way you have created new projects for other lab projects.
2. Create a Manual Grass Object You will use the createManualObject() method in Ogre to specify new geometry inside your code representing a single bundle of grass straws. This method is handy for simple geometries or when you want to procedurally generate them. The grass bundle you create is essentially a set of three quads, each at a 60 degree angle to the next, and that all crossing in the middle - producing a star with 6 spikes if viewed from the top. If viewed from the side, you always see the front of one or two quads, regardless of where you stand. You can therefore make this object hold a texture that is visible from all directions, a property that is called billboarding. To get started, create a new method in your MyApplication class which you should call createGrass(). Inside this method construct the quads and assign grass tecture to them. You construct them by specifying all the vertices first (in model space) and then the indices of each triangle that make up the three quads. Here is the code with the initialization of the manual object and the first quad specified:

   		Ogre::ManualObject* manual = _sceneManager->createManualObject("grass");
   		manual->begin("Examples/GrassBlades", Ogre::RenderOperation::OT_TRIANGLE_LIST);
   		// Vertices for quad 1
                   manual->position(5,0,0);
   		manual->textureCoord(1,1);
   		manual->position(-5,10,0);
   		manual->textureCoord(0,0);
   		manual->position(-5,0,0);
   		manual->textureCoord(0,1);
   		manual->position(5,10,0);
   		manual->textureCoord(1,0);
                   // Put vertices for other 2 quads here
                   // ...
                   // Triangles for quad 1
   		manual->index(0);
   		manual->index(1);
   		manual->index(2);
    
   		manual->index(0);
   		manual->index(3);
   		manual->index(1);
                   // Put triangles for other 2 quads here                
                   // ...
                   // And now wrap up the object and add to scene graph for display
                   manual->end();
   		Ogre::SceneNode* grassNode = _sceneManager->getRootSceneNode()->createChildSceneNode("GrassNode2");
   		grassNode->attachObject(manual);

   To add the two missing quads, use the following corner vertex positions: Quad 2 = (2.5,0,4.3),(-2.5,10,-4.3), (-2.5,0,-4.3), (2.5,10,4.3), Quad 3 = (2.5,0,-4.3),(-2.5,10,4.3),(-2.5,0,4.3),(2.5,10,-4.3). Don't forget to give the vertices texture coordinates, which are the same for all quads. Now call createGrass() from your createScene() method and you should see a single bundle of grass right in the middle of the scene.

3. Instance Grass Object To create multiple instances of the object you created, you need to first convert it into a proper Ogre Mesh. In the createGrass() method, replace your last two lines with the following line:

   manual->convertToMesh("BladesOfGrass");

   This essentially creates a new model with the name “BladesOfGrass”, which you can now refer to in subsequent createEntity(<name>) calls. So do that next: Let's create 2500 instances of this model, spread across the ground in a 50×50 square patch! You can do this using a nested for-loop (over i=0-49 and j=0-49) and inside the loop do the following:

   Ogre::Entity* ent = _sceneManager->createEntity("BladesOfGrass");
   Ogre::SceneNode* node = _sceneManager->getRootSceneNode()->createChildSceneNode(Ogre::Vector3(i*3,-10,j*3));
   node->attachObject(ent);

   Notice that you are not naming each entity or node when you create them! But Ogre requires unique names for each of its nodes and entities. When a name is not given, Ogre actually assigns unique names. you can take a look at these names by printing them out inside the loop:

   std::cout << node->getName() << "::"<< ent->getName() << std::endl;

   (Just remember to remove the print-outs for later steps). Do you notice that with 2500 grass models the rendering has slowed dramatically?

4. Optimize using Static Geometry If we assume that the each grass model will stay in the same location in world space throughout your application life, we can optimize rendering by bundling them up into a packet of pre-transformed static geometry. First create a single new StaticGeometry instance. Add this line after the convertToMesh call:

   Ogre::StaticGeometry* field = _sceneManager->createStaticGeometry("FieldOfGrass");

   And now, instead of instancing the grass entities into the scene graph directly, we add them into the static geometry object in the body of our nested for-loop. So, the body of the loop becomes:

   Ogre::Entity* ent = _sceneManager->createEntity("BladesOfGrass");
   field->addEntity(ent,Ogre::Vector3(i*3,-10,j*3));

   Finally, after we have added all the entities to our field, we need to ask the static geometry to “bake”/optimize with the following call:

   field->build();

   Now you should try running the scene again and see if there is any improved performance!

Upload your source files into Lab6 in MySchool. The lab projects will not be graded, but their completion counts towards your participation grade.