Managing Your Objects Paul Taylor 2009 Assignment Due Fri 5/6/2009

What is an Object A Bunch of Polygons (or one), or a Sprite Each Object can have: – A Translation – A Rotation – A Scale – A Shear (not super common)

Our Test Objects A Sweet Car!

The Simplest Representation Car Body Translation, Rotation and Scale Wheel 1 Translation, Rotation and Scale Wheel 2 Translation, Rotation and Scale

Scale Rotate Translate Scale Rotate Translate Scale Rotate Translate

Using Hierarchical Objects

With our Matricies Scale Rotate Translate Scale Rotate Translate Scale Rotate Translate

Directed Acyclic Graphs (DAG) Technically this is a Specialised case of a Directed Graph The rule is there are no paths that start at a vertex and end at the same vertex (No Loops)

What is the benefit? We can Reuse Objects Cars have four wheels A flock of birds may all be the same A turret may have 4 Cannons

Sweet Car utilising a DAG

Connecting All your Objects We are creating a Scene Graph

In effect Object Storage Looks Like Root Child 1 Child 2Sibling 1Sibling 2 Sibling 2 Child 1 Sibling 2 Child 2 Sibling 2 Child 2 Sibling 1 Child 3

Creating Bounding Boxes BBs are typically used For Collision Detection With a Hierarchical Object generation can still be relatively simple. Outlier detection in X Y Z and –X-Y-Z directions A Bounding Sphere simply needs the most distant vertex from the Object Center More Complicated routines can be used, or you can use a Human!

Constructive Geometry Not generally used during real time renders Consist of using Logical Operators on Polygon Objects to combine them – Intersections, Unions and Difference A Typical Use of Constructive Geometry is in 3D Game Level creation and CAD Design

Constructive Solid Geometry CSG

We have our Objects Nicely packed Why? To Create Structures that define the 3D Worlds we are creating Intersecting the Bounding Boxes/Spheres in the game world is much faster. Once we know that 2 objects intersect (via BB collision) we can go deeper into the collision (If needed)

Space Subdivision We have already covered BSP Trees, now we will look at 1 1/2 more Space Partitioning methods Oct-Trees KD-Trees

Quad-Trees Quad Trees are the 2D baby brother of Oct Trees Computationally similar, conceptually easier Space is Divided into 4 areas If more than one object exists in a region it is again subdivided into 4 sub-regions, and so on….

es/spatialDBMS/quadtree.gif es/spatialDBMS/quadtree.gif

Oct-Trees

/Jean-Daniel_Nahmias/Massive_Model_Rendering.htm /Jean-Daniel_Nahmias/Massive_Model_Rendering.htm

KD-Trees A specialisation of Binary Trees (Similar to BSP Trees) In a similar way to Oct Trees a KD Tree splits space in an Axis Aligned Fashion. Different to Oct-Trees a KD Tree does not always split the world into even parts

KD-Trees a.org/wiki/Kd-tree

hpcc.astro.washington.edu/old_content/papers/marios/perform/n ode3.html hpcc.astro.washington.edu/old_content/papers/marios/perform/n ode3.html

What else can you do? Oct-Trees and KD-Trees can be used for: – View Frustum Culling – Occlusion Culling – Level of Detail Detection – Spatial Indexing (Similar to a BSP Tree) – Ray Casting

Bringing it all together You organise all of your objects – Binary Tree, Object Tree, Hash Table etc You have a method of generating Collisions – Firstly by Simplifying objects Bounding Boxes, Spheres – Secondly by finding Collisions between objects Oct-Trees, KD-Trees – Thirdly we can check the collision of the actual objects if required Finally we need to do something appropriate with the collision!

That’s a lot of work!

Open Scene Graph (OSG) Multipass Object Grouping and Ordering Middleware that does Object Handling Things you could do with it: Prototyping Offline Rendering A Sluggish Game