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CSCE 552 Spring 2009

CSCE 552 Spring 2009. 3D Models. By Jijun Tang. Triangles. Fundamental primitive of pipelines Everything else constructed from them (except lines and point sprites) Three points define a plane Triangle plane is mapped with data Textures Colors “Rasterized” to find pixels to draw. Mesh.

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CSCE 552 Spring 2009

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  1. CSCE 552 Spring 2009 3D Models By Jijun Tang

  2. Triangles • Fundamental primitive of pipelines • Everything else constructed from them • (except lines and point sprites) • Three points define a plane • Triangle plane is mapped with data • Textures • Colors • “Rasterized” to find pixels to draw

  3. Mesh

  4. Vertices • A vertex is a point in space • Plus other attribute data • Colors • Surface normal • Texture coordinates • Whatever data shader programs need • Triangles use three vertices • Vertices shared between adjacent triangles

  5. Textures • Array of texels • Same as pixel, but for a texture • Nominally R,G,B,A but can mean anything • 1D, 2D, 3D and “cube map” arrays • 2D is by far the most common • Basically just a 2D image bitmap • Often square and power-of-2 in size • Cube map - six 2D arrays makes hollow cube • Approximates a hollow sphere of texels • For environmental

  6. Cube Map

  7. Texture Example

  8. High-Level Organization • Gameplay and Rendering • Render Objects • Render Instances • Meshes • Skeletons • Volume Partitioning

  9. Gameplay and Rendering • Rendering speed varies according to scene • Some scenes more complex than others • Typically 15-60 frames per second • Gameplay is constant speed • Camera view should not change game • In multiplayer, each person has a different view, but there is only one shared game • 1 update per second (RTS) to thousands (FPS) • Keep the two as separate as possible!

  10. Render Objects • Description of renderable object type • Mesh data (triangles, vertices) • Material data (shaders, textures, etc) • Skeleton (+rig) for animation • Shared by multiple instances

  11. Render Instances • A single entity in a world • References a render object • Decides what the object looks like • Position and orientation • Lighting state • Animation state

  12. Meshes • Triangles • Vertices • Single material • “Atomic unit of rendering” • Not quite atomic, depending on hardware • Single object may have multiple meshes • Each with different shaders, textures, etc • Level-Of-Distance (LOD)

  13. LOD • Objects have different mesh for different distance from the player • The mesh should be simpler if object is faraway • Many games have LOD, for example, Microsoft Train Simulator

  14. Volume Partitioning • Cannot draw entire world every frame • Lots of objects – far too slow • Need to decide quickly what is visible • Partition world into areas • Decide which areas are visible • Draw things in each visible area • Many ways of partitioning the world

  15. Volume Partitioning - Portals • Nodes joined by portals • Usually a polygon, but can be any shape • See if any portal of node is visible • If so, draw geometry in node • See if portals to other nodes are visible • Check only against visible portal shape • Common to use screen bounding boxes • Recurse to other nodes

  16. Volume Partitioning – Portals Node View frustum Portal Visible Test first two portals Invisible Not tested ? ? Eye

  17. Volume Partitioning – Portals Node Portal Visible Both visible Invisible Not tested Eye

  18. Volume Partitioning – Portals Node Portal Visible Mark node visible, test all portals going from node Invisible Not tested ? ? Eye

  19. Volume Partitioning – Portals Node Portal Visible One portal visible, one invisible Invisible Not tested Eye

  20. Volume Partitioning – Portals Node Portal ? Visible Mark node as visible, other node not visited at all. Check all portals in visible node Invisible ? ? Not tested Eye

  21. Volume Partitioning – Portals Node Portal Visible One visible, two invisible Invisible Not tested Eye

  22. Volume Partitioning – Portals Node Portal ? Visible Mark node as visible, check new node’s portals Invisible Not tested Eye

  23. Volume Partitioning – Portals Node Portal Visible One portal invisible. No more visible nodes or portals to check. Render scene. Invisible Not tested Eye

  24. Real Example

  25. Volume Partitioning – Portals • Portals are simple and fast • Low memory footprint • Automatic generation is difficult, and generally need to be placed by hand • Hard to find which node a point is in, and must constantly track movement of objects through portals • Best at indoor scenes, outside generates too many portals to be efficient

  26. Volume Partitioning – BSP • Binary space partition tree • Tree of nodes • Each node has plane that splits it in two child nodes, one on each side of plane • Some leaves marked as “solid” • Others filled with renderable geometry

  27. BSP

  28. Volume Partitioning – BSP • Finding which node a point is in is fast • Start at top node • Test which side of the plane the point is on • Move to that child node • Stop when leaf node hit • Visibility determination is similar to portals • Portals implied from BSP planes • Automated BSP generation is common • Generates far more nodes than portals • Higher memory requirements

  29. Volume Partitioning: Quadtree • Quadtree (2D) and octree (3D) • Quadtrees described here • Extension to 3D octree is obvious • Each node is square • Usually power-of-two in size • Has four child nodes or leaves • Each is a quarter of size of parent

  30. Quadtree

  31. Octree

  32. Volume Partitioning: Quadtree • Fast to find which node a point is in • Mostly used for simple frustum culling • Not very good at indoor visibility • Quadtree edges usually not aligned with real geometry • Very low memory requirements • Good at dynamic moving objects • Insertion and removal is very fast

  33. Volume Partitioning - PVS • Potentially visible set • Based on any existing node system • For each node, stores list of which nodes are potentially visible • Use list for node that camera is currently in • Ignore any nodes not on that list – not visible • Static lists • Precalculated at level authoring time • Ignores current frustum • Cannot deal with moving occluders

  34. PVS Room A Room C PVS = B, A, D Room D Room B Viewpoint Room E

  35. Volume Partitioning - PVS • Very fast • No recursion, no calculations • Still need frustum culling • Difficult to calculate • Intersection of volumes and portals • Lots of tests – very slow • Most useful when combined with other partitioning schemes

  36. Volume Partitioning • Different methods for different things • Quadtree/octree for outdoor views • Does frustum culling well • Hard to cull much more for outdoor views • Portals or BSP for indoor scenes • BSP or quadtree for collision detection • Portals not suitable

  37. Rendering Primitives • Strips, Lists, Fans • Indexed Primitives • The Vertex Cache • Quads and Point Sprites

  38. Strips, Lists, Fans Triangle strip 4 2 4 1 2 6 8 5 2 1 2 3 6 7 1 5 3 3 9 8 4 3 Triangle list 1 3 5 4 2 7 4 5 6 5 6 Line list 1 Line strip 6 Triangle fan

  39. Vertex Sharing • List has no sharing • Vertex count = triangle count * 3 • Strips and fans share adjacent vertices • Vertex count = triangle count + 2 • Lower memory • Topology restrictions • Have to break into multiple rendering calls

  40. Vertex Counts • Using lists duplicates vertices a lot! • Total of 6x number of rendering vertices • Most meshes: tri count = 2x vert count • Strips or fans still duplicate vertices • Each strip/fan needs its own set of vertices • More than doubles vertex count • Typically 2.5x with good strips • Hard to find optimal strips and fans • Have to submit each as separate rendering call

  41. Strips vs. Lists 32 triangles, 25 vertices 4 strips, 40 vertices 25 to 40 vertices is 60% extra data!

  42. Indexed Primitives • Vertices stored in separate array • No duplication of vertices • Called a “vertex buffer” or “vertex array” • 3 numbers (int/float/double) per vertex • Triangles hold indices, not vertices • Index is just an integer • Typically 16 bits (65,536) • Duplicating indices is cheap • Indexes into vertex array

  43. Vertex Index Array

  44. The Vertex Cache • Vertices processed by vertex shader • Results used by multiple triangles • Avoid re-running shader for each triangle • Storing results in video memory is slow • So store results in small cache • Requires indexed primitives • Cache typically 16-32 vertices in size, resulted in around 95% efficiency

  45. Cache Performance • Size and type of cache usually unknown • LRU (least recently used) or FIFO (first in first out) replacement policy • Also odd variants of FIFO policy • Variable cache size according to vertex type • Reorder triangles to be cache-friendly • Not the same as finding optimal strips! • Render nearby triangles together • “Fairly good” is easy to achieve • Ideal ordering still a subject for research

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