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1 H331: Computer Graphics Philip Dutré Department of Computer Science Wednesday May 8, 2003.

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Presentatie over: "1 H331: Computer Graphics Philip Dutré Department of Computer Science Wednesday May 8, 2003."— Transcript van de presentatie:

1 1 H331: Computer Graphics Philip Dutré Department of Computer Science Wednesday May 8, 2003

2 2 Announcements u Practicum 3: Ray Tracing –Showcase online u Exam info: website + this lecture

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

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

5 5 Example (movie clip)

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

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

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

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

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

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

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

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

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

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

16 16

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

18 18 General Principle

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

20 20 Building database

21 21 Morphable model u Face has n vertices –shape of face is point in R 3n space –texture of face is point in R 3n space u Pick set of m ‘basis’ faces –each face is linear combination of m basis faces

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

23 23 Morphable model

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

25 25 Morphable model

26 26 Morphable model

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

28 28 Match to image of a new face

29 29 Morphable model

30 30 Morphable model (clip)

31 31

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

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

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

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

36 36 Basic idea u Textures are projected on a CRT

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

38 38 Results Alpha-matte Environment matte Photograph

39 39 Results Alpha-matte Environment matte Photograph

40 40 Extensions u Reflections u (clips)

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

42 42

43 43

44 44 Examen

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

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

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

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

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

50 50 Type Vragen: Inzicht u 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?

51 51 Practica? u Tellen mee voor 7 punten op 20 u Verdeling: 3 /4 of 4/ 3 (afhankelijk van beste score)


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