H331: Computer Graphics

1. H331: Computer Graphics http://www.cs.kuleuven.ac.be/~graphics/H331/Philip Dutré Department of Computer Science Wednesday May 8, 2003

2. Announcements • Practicum 3: Ray Tracing • Showcase online • Exam info: website + this lecture

3. Topics • Surfel Rendering • Morphable faces • Environment Matting • Interactive shadows for Cinematic Design

4. Surfels : Surface Elements as Rendering Primitives • What’s wrong with polygons? • “Reality starts at 80 million polygons” • Organic shapes are difficult to represent with polygons (Pfister et al. SIGGRAPH 2000)

5. Example (movie clip)

6. Surfels: Concepts • Object is represented as oriented surface points, sampled from polygons • Hierarchical resolutions • Each point has texture information

7. Surfels: Sampling • Generate Layered Depth Image for each dimension: “Layered Depth Cube” LDI LDI LDI

8. Surfels: Sampling • Analog in 2D • Sampling density not determined by curvature of geometry

9. Surfels: Sampling • Adequate density? • Choose density such that each pixel is covered by at least one surfel • Distance between sample rays: h • then: imaginary triangle mesh on surface has lengths of at most h*sqrt(3)

10. Surfels: Texture pre-filtering • Texture color per surfel: filter in texture space using filter of size h.sqrt(3) • Also use larger filters: typically 3 or 4 colors per surfel

11. Surfels: Data Structure • Octree • Each cell in tree is LDI at half the resolution • 2 levels • Construction: bottom-up

12. Surfels: Rendering • Choose blocks in octree at correct level • each pixel must be covered • Cull blocks that are not visible • all surfels in block point away from camera • Project surfels on image plane • What about holes?

13. Surfels: Hole pixels? • During reprojection, reproject ‘disks’ and record depth in each pixel, NOT color • Each pixel has pointer to closest surfel

14. Surfels: Hole Pixels? • For each hole pixel, we have a reference to the nearest surfel • Color for hole pixels • interpolate between different levels of the texture colors of the nearest surfel

15. Surfels: Examples (2.8 M surfels) (2 M surfels) (video clips)

17. A Morphable Model For the Synthesis of 3D Faces • Basic idea: find a set of basis functions for modeling human faces • Max-Planck Institut fur Biologische Kybernetik (SIGGRAPH 99)

18. General Principle

19. Building database • Scan 200 heads (100 male, 100 female) • geometric data • texture data • Conditions: • no facial hair (facial + head) • no make-up • Rotate each face in standard orientation • each face: 70,000 vertices

20. Building database

21. Morphable model • Face has n vertices • shape of face is point in R3n space • texture of face is point in R3n space • Pick set of m ‘basis’ faces • each face is linear combination of m basis faces

22. Morphable model • We can make arbitrary faces by making linear combinations of the m basis faces(m-1 degrees of freedom)“Face Space” • For more variety:different models for ear, nose, eyes, etc.

23. Morphable model

24. Morphable model • Label initial set of 200 faces with attributes: • male/female • smiling/frowning • ... • Find ‘tendencies’ for each of these attributes in Face Space

25. Morphable model

26. Morphable model

27. Match to image of a new face • Place morphable model over image • Iterative procedure to adapt morphable model until ‘closest fit’ is found • Correct texture due to illumination effects

28. Match to image of a new face

29. Morphable model

30. Morphable model (clip)

32. Environment Matting • Classic matting and compositing • place object in front of controlled backdrop • Extract object • Replace background • Transparancy: alpha-matting (Zongker 99)

33. Environment Matting • What if object affects the background?(e.g. transparent objects?)

34. Basic Idea • Capture the way light is reflected through the object • Apply this reflection to any new background

35. Basic Idea • Use textures of different frequencies to capture the way light passes through the object backdrops sidedrops

36. Basic idea • Textures are projected on a CRT

37. Given all deformations of all textures, find what portion of the background ends up in what pixel • multi-dimensional optimization problem • Assumptions • axis-aligned region on texture maps

38. Results Photograph Alpha-matte Environment matte

39. Results Photograph Alpha-matte Environment matte

40. Extensions • Reflections • (clips)

41. Interactive shadow editing • Designing a scene: • Where to place light sources? • Usually shadows are an important aspect!!! • How to specify where shadows are to be located?

44. Examen

45. Hoe verloopt het? • Mondeling + schriftelijke voorbereiding • Gesloten boek • ~ 5 vragen • Zowel kennis als inzicht

46. Welke stof moet gekend zijn? • “Alles wat in de les gezien is” • Relevante secties in boek staan op website • Indien niet in boek: slides • Zowel boek-versie als slide-versie zijn goed als antwoord, zolang het maar juist is!

47. Welke stof moet NIET gekend zijn? • Dingen in boek niet behandeld in les ;-) • OpenGL • Geen OpenGL code op examen

48. Type Vragen: Kennis • “Leg uit: Phong-shading” • “Wat is het XYZ kleurendiagramma?” • “Bespreek het klassieke ray tracing algoritme” • “Wat is een viewport? Welke plaats neemt het in de grafische pipeline?”

49. Type vragen: inzicht • We willen de volgende animatie maken: • Werk de transformatiematrix uit in functie van t … t = 10 t = 5 t = 0

50. Type Vragen: Inzicht • In stochastische ray tracing delen we een lichtbron op in 4 aparte lichtbronnen. Hoeveel schaduwstralen per lichtbron moeten we in beide gevallen gebruiken om dezelfde kwaliteit van beeld te bekomen?