Introduction to Modeling

1. Introduction to Modeling

2. What is CG Modeling? • Combination of Sculpting, Architecture, Drafting, and Painting. • The core component of computer animation.

3. What are the challenges: – Time! • Time to create the models. • Time to render the models. • Time to animate the models. What are our options? • To improve rendering time reduce polygon count. • To improve realism increase polygon count. • To create models faster reduce polygon count. • To reduce animation time reduce number of manipulators. • To improve animation realism increase the number of manipulators.

4. Low polygon model

5. High polygon model

6. Few manipulators

7. Many manipulators

8. How to create models? • Hand code the model – no one does this anymore. • Use a modeling tool – eg Maya, Blender. • Scan physical models using a 3D scanner.

9. Generate by Hand Pros/Cons Very very exact. You know exactly what you get. Very very time consuming. Not possible to build large or Complex models.

10. Use a Modeling Tool – Maya… Maya Pros/Cons Quick to build models. Can build very complex models Lots of controls and manipulators Easy to build overly complex models Tools can be difficult t learn. Can be difficult to control output. Can be hard to port to other tools Blender Pros:A lot of features in a software considering it's free - modeling , texturing, painting, sculpting, fluids, rigging, tracking... The list goes on. Very fast poly modeling tools once you grasp the way of work. Incredibly customizable input. Cons:Slow viewport performance with high-poly scenes. Unintuitive layers system.

11. Use a Modeling Tool – or Blender • Blender • Pros: • A lot of features in a software considering it's free - modeling , texturing, painting, sculpting, fluids, rigging, tracking... The list goes on. • Very fast poly modeling tools once you grasp the way of work. • Incredibly customizable input. • Cons: • Slow viewport performance with high-poly scenes. • Unintuitive layers system.

12. Use a 3D Scanner Pros/Cons Very quick to build models. Can be done automatically. Can hire students to do it. Few controls and manipulators. Requires extensive re-editing. Generates models with too many polygons. Limits animators flexibility.

13. What are the Parts of Modeling? • Scene – when modeling work on a scene at a time. • Spaces – Where your models sit in space. • Objects – the primitives used to build your models. • Structures – combine objects to make structures. • Operators – Tools to build structures from objects.

14. Space Structure Object

15. Spaces • Coordinate Spaces • X Y Z • Cartesian Coordinates • Rectangular Coordinates • Global Coordinates • Local Coordinates

16. Projections • How we go from 3D to 2D • Two types: Perspective and Orthographic

17. 3D Space +Y +Y +Z +Z +X +X Right-handed system Left-handed system

18. Objects • Any 3D structure in modeling. • There are two types of models: • Solid – more realistic but difficult to build • Shell/boundary – represents a surface of an object, but not its volume.

19. +Y Polygons +Z +X Objects • Made up of sets of polygons • Which are made up of lines • Which are made of points • No curved surfaces • Just a “shell” • Not a solid object • Everything is a set of points • In local coordinate system

20. Three Coordinate Systems • World-centered: Where objects are in the world • Object-centered: Relative to position of object • View-centered: Relative to the position of viewer • Simplest case is viewing down z-axis +Y +Z +X

21. Projections • Mapping a 3D object onto a 2D viewing surface Perspective projection View Plane Parallel projection

22. Projections • Parallel • If viewing down z-axis, just discard z component • Perspective • If viewing down z-axis, scale points based on distance. • x_screen = x / z • y_screen = y / z

23. Projections • Usually not viewing down center of z axis. • Usually x = 0 and y = 0 at bottom left • Correct by adding 1/2 screen size • x_screen = x/z + 1/2 screen width • y_screen = y/z + 1/2 screen height • To get perspective right, need to now field of view, distance to screen, aspect ratio. • Often add scaling factor to get it to look right • x_screen = x*scale /z + 1/2 screen width

24. Field of View • To simulate human vision: • 110-120 degrees horizontally • < 90 vertically • Think of the viewing pyramid or frustum

25. Clipping Yon clip plane Hither clip plane Viewing frustum View plane

26. How can you model objects? • Polygonal Models. • NURBS • Subdivision Surfaces • Parametric Models • Implicit Surfaces

27. Polygonal Models Pros/Cons Simple to understand. Simple to render. Simple to combine to make shapes. Hardware supported rendering Need lots of polygons for complex shapes. Difficult to animate lots of polygons. Lots of controls needed. Advanced modeling operations are difficult. Continuity problems. • Polygonal Models consist of: • Vertices • Edges • Faces Polygon Vertex Face Edge

28. NURB Models Non-Uniform Rational B-Splines • NURBs are made of: • Patches • Curves • Control Points Pros/Cons Need fewer patches for complex objects. Easier to animate patches. Fewer controls needed. No Continuity problems. Advanced modeling operations are easier. Hard to create corners! Difficult to understand. Difficult to render. Simple to combine to make shapes. Render by converting to polygons

29. Subdivision Surfaces • Provides `Localized Refinement • Allows you to mix sharp and smooth corners. • Continuity controlled locally. • Increased rendering time • Requires clear understanding to generate good models. • Can be difficult to animate.

30. Parametric Models and Implicit Surfaces Mathematical representations of complex surfaces. X^2*Y^3-Z^4- 12 X=r(t), Y=r(t)+2*r^2(t), Z=t Not in production use at this time. Have very beneficial mathematical properties. Are extremely difficult to render, modify, and animate.

31. Solid Modeling • Which surfaces should be drawn? • Object space methods • Hidden Surface Removal • Painters Algorithm • BSP Trees • Image space methods • Z-Buffering • Ray Casting

32. Z-buffer • Z-buffer holds the z-coordinate of ever pixel • Usually 16 or 32-bits/pixel • Initialize all values to maximum depth • Compute the z value of every point of every non-back facing polygon • Not too hard if all polygons are triangles or rectangles • Do this during the filling of the triangles • If z of point < z in Z-buffer, save the color of the current point and update Z-buffer • otherwise throw away point and move on • Expect z-buffers on all PC in a few years

33. Ray Tracing • Image space technique that mimics physical processes of light • Extremely computationally intensive, but beautiful • Hidden surface removal • Transparency • Reflections • Refraction • Ambient lighting • Point source lighting • Shadows

34. What Kinds of Modeling Operations Exist? • Insert and Delete point, edge, face. • Add Polygon/NURB • Boolean Operations: Union, Intersect, Difference. • Advanced Operations: • Extrude along path. • Loft surface • Revolve • Bend • Split • Very Advanced Operations • Smooth Model

35. Shading • Compute lighting based on angle of light on polygon surface. Surface normal

36. Gouraud Shading • Compute shading for each pixel by averaging shading based on distance and shading of vertices.

37. Transparency • Use an extra set of bits to determine transparency • Alpha • Blend present value of the color buffer with new values.

38. Texture Mapping • Apply stored bit map to a surface • Average textels covered by pixel image Texture map (Texels) Surface Pixel

39. 3D Collision Detection • Can’t be done in image space • Usually use hierarchical approach • First find objects in same 3D cells • Second test for overlaps in bounding sphere or box • Third • Good enough! • Check for polygon collisions • Accurate 3D collision detection is very expensive

40. What types of animation controls are available? • Simple – directly keying specific components. • Moderate – An I/K skeleton, Solvers. • Complex – Programmed Mel Scripts, advanced solvers. Linked Solvers.

41. How do you control color and appearance. • Assign Materials. • Assign Shaders. • Assign Textures. • Advanced Shaders – eg Renderman.

42. What about rendering? • Everything is translated to Triangles.

43. How do you Triangulate? • Polygons • NURBS • Subdivision Surfaces • Parametric Surfaces • Implicit Surfaces

44. How is everything rendered. • First everything is converted to polygons • Second everything is converted to triangles. • Then everything is rendered thru the graphics pipeline discussed in the prior lecture.

45. What kinds of modeling jobs are there? • General Modelers • Photo-realistic modelers • High poly-count modelers • Low poly-count modelers • Character Animators • Prop Modelers