Why render terrains?

1. Real-time Terrain Rendering and ProcessingShibenBhattacharjee200607022shiben@research.iiit.ac.inAdvisor: Prof. P. J. Narayanan

2. Why render terrains? • Terrains are a basic need in most graphics application • Geographical information systems • E.g. Google Earth • Entertainment: • Computer Games • Animation Movies • Study of terrains

3. Chapters • What is terrain rendering? • Real-time Terrain Rendering • Spherical Terrain Rendering • Painterly Rendering of Terrains

4. What is terrain rendering?

5. Terrain Data

8. Terrain Rendering • Do not render geometry which is not visible • View Frustum Culling • Control the detail of geometry rendered according to amount of visibility • Level of Detail scheme • Keep a representation (data structure) • Memory management • Fast processing • Accessibility

9. Theoretically… • Divide the terrain in tiles • Discard tiles not in view • Reduce the detail of tiles farther from camera

12. Real-time Terrain Rendering

13. Representation Array of tilelets (Rendering by GPU) Array of tiles (Rendering by CPU) Blocks (Used for caching)

14. Global LoD System • Merge level • LoD based on elevation • Whole GPU cache resolution is changed • Lowered, if camera is going higher • Increased, if camera is coming to the ground • Distance level • Tiles’ detail is reduced based on camera distance

15. Terrain Rendering • Divided in two stages • Stage 1: CPU • View frustum culling (level1) • Render tiles

16. Terrain Rendering • Stage 2: GPU • Tessellate region between vertices • Access height data for elevation of vertices • 2nd Level of culling • Only for tiles intersecting with view frustum

17. 2nd Level of Culling • Test tilelet bounds against screen bounds • If outside, GS doesn’t generate tilelet • CPU load reduced Distance based lod Culled tilelet Tilelets which undergo 2nd level of culling Tilelets assumed to be inside

18. Constant Memory Usage Camera moving up Wide view range required Detail is not necessary

19. Caching, types of job Merge 4 blocks into 1 Bring new block from main memory

20. Terrain Deformation • Blocks reside in memory as textures • Edited with fragment shader (like GPGPU) • Procedural deformation or mouse based interactions • Cache is edited directly, thus changes are reflected instantly in the rendering • Deforming a block takes 0.25ms

21. Interactions: Simple Physics • Textures used to keep position and velocities of point objects • A fragment shader pass updates the values according to collision with terrain • Physics uses terrain in cache, behaves according to its deformation

22. Spherical Terrain Rendering

23. Cartography • Latitude-Longitude system • 2D coordinates to refer any point on planet • Map of planet: a 2D image

24. Terrain Rendering and Cartography • Terrain data available in 2D • Convert 2D samples of data to Polar Coordinates • Render Spherical Structure • Though convenient, has serious problems

25. Samples on planet – Not uniform • More samples near polar regions • Fewer samples near equator • Uneven distribution of detail • Poles have singularity • Too many vertices coincide

26. Redundant Information • Data contains a lot of redundant information Compare the size of marked regions

27. Hierarchical Triangular Mesh (HTM) • Start with an octahedron – 8 equilateral base triangles • Recursive subdivision of the triangles • Stop recursion when appropriate detail reached A. SZALAY, J. GRAY, et al., In MSR-TR-2005-123

28. HTM: Regular Sampling of Sphere • Uniform sampling of sphere • No pole singularity Poles not cluttered All regions equally sampled

29. HTM and Terrain Rendering • Transform cartographic data to form 4 images / diamonds • Redundant information removed (saves 50% memory) Eg. 1024x512 4x (256x256)

30. HTM and Terrain Rendering • Each diamond – data for two base triangles • Each sample maps one-to-one to a vertex in the HTM

31. Geometry Clipmaps (State-of-the-art Terrain Rendering) • Real-time Terrain Rendering • High Rendering Throughput • Constant memory usage • Square geometry clipmaps cannot be used directly • Equilateral triangles needed • Data still needed in 2D – friendly with GPUs • Changes needed F. Losasso and H. Hoppe., SIGGRAPH, 2004 A. Asirvatham and H. Hoppe., GPU Gems 2, 2005

32. Terrains World Space Triangles HTM World Space 2D Data Triangles Data is 2D, World Space is not • HTM contains equilateral triangles between samples • A square region in the 2D data = A rhombus in world space • Known terrain rendering techniques use right triangles • New technique needed

33. Hexagonal Geometry Clipmaps • Clip two opposite corners of a square region at ½ distance • Form a six sided polygon • This polygon takes shape of a hexagon in world space

34. Hexagonal Geometry Clipmaps • Clip all clipmaps: we get a Hexagonal Clipmap Pyramid • Note: Clipmap data remains 2D with 7/16 of it unusable High level Intermediate level Low level Etc.

35. Rendering • Clipmaps of different resolution update with different rate • Hexagonal nest distorted periodically due to this • Hexagonal clipmaps tessellated with Rendering Blocks Each red quadrilateral uses a rendering block

36. View Frustum Culling • Camera yaw required • Determine which three sides to be shown • Max fov ~ 120o (use more sides for greater fov)

37. Painterly Rendering of Terrains

38. What is painterly rendering? • Non-photo realistic rendering (NPR) • Convert a photo-realistic image into a artistic hand painted drawing • Use of brush strokes [Perlin, Hertzmann ‘98]

39. For animation • 3D space information used for rendering brush strokes [B Meier, 96] • Brush strokes alpha blended, sorted back to front

40. For real-time rendering • 3D space information used for rendering brush strokes/sprites [Bhattacharjee, Adabala, ICVGIP ‘06] • Brush strokes alpha blended • Sorting not done • Depth image used to decide visibility

41. Terrains • 3D space information can be used from terrain data as well • Brush strokes alpha blended • Getting a depth image is heavy • Terrain have wide view range • Terrains are heavy object • Level of detail complexities

42. How does an artist does it? • An artist paints on a canvas • 3D information vaguely in the imagination • An artist draws far objects first so that front objects cover them • He/she may be creating 3D environments but strokes individually look flat • Artist may be using different brushes but mostly they remain of the same size • E.g. Close by objects drawn with detail but farther objects abstracted with 2 or 3 strokes

43. Rendering • Stage 2: GPU • Convert the incoming vertex into a sprite • Access height data for position of stroke • Access slope map for orientation of stroke • Access texture for color of stroke • Maintain size, Ignore change due to projection

44. Back to front stroke ordering • A scan order of regular 2D samples looks sorted from some orientation camera camera

45. Back to front stroke ordering • A total of 8 such orientations are enough camera

46. Back to front stroke ordering • Ordering used for rendering tiles • Ordering used for rendering strokes • No sorting needed • 8 Scanning orders stored • Order is chosen depending on camera pan • That’s only one decision! • No performance affect on rendering

47. Stroke Orientation • Slope map is a dudv map • Gives the direction of maximum gradient at every point • Stroke accesses the direction • Transforms it in camera space • Orientation of sprite decided

48. Aesthetics considerations • Using different stroke textures can produce varied painterly styles • Apart from using slope-map for stroke orientation • normal map can be used • A constant can be used • User can create his own texture for stroke’s orientation

49. Aesthetics considerations • Shiny oil painting

50. Conclusions