CS 445: Introduction to Computer Graphics David Luebke University of Virginia

1. The Rendering Pipeline CS 445: Introduction to Computer Graphics David Luebke University of Virginia

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3. Demo • Ogre

4. RecapDisplay Technology: DMDs • Digital Micromirror Devices (projectors) • Microelectromechanical (MEM) devices, fabricated with VLSI techniques

5. Recap:Display Technology: DMDs • DMDs are truly digital pixels • Vary grey levels by modulating pulse length • Color: multiple chips, or color-wheel • Great resolution • Very bright • Flicker problems

6. Display Technologies: Organic LED Arrays • Organic Light-Emitting Diode (OLED) Arrays • The display of the future? Many think so. • OLEDs function like regular semiconductor LEDs • But with thin-film polymer construction: • Thin-film deposition of organic, light-emitting molecules through vapor sublimation in a vacuum. • Dope emissive layers with fluorescent molecules to create color. • Not grown like a crystal, no high-temperature doping • Thus, easier to create large-area OLEDs

7. Display Technologies: Organic LED Arrays • OLED pros: • Transparent • Flexible • Light-emitting, and quite bright (daylight visible) • Large viewing angle • Fast (< 1 microsecond off-on-off) • Can be made large or small

8. Display Technologies: Organic LED Arrays • OLED cons: • Not quite there yet (96x64 displays) except niche markets • Cell phones (especially back display) • Car stereos • Not very robust, display lifetime a key issue • Currently only passive matrix displays • Passive matrix: Pixels are illuminated in scanline order (like a raster display), but the lack of phosphorescence causes flicker • Active matrix: A polysilicate layer provides thin film transistors at each pixel, allowing direct pixel access and constant illumination See http://www.howstuffworks.com/lcd4.htm for more info • Hard to compete with LCDs, a moving target

9. Display Technologies:Other • Liquid Crystal On Silicon (LCOS) • “Next big thing” for projectors • Don’t know much about this one • E-Ink • Tiny black-and-white spheres embedded in matrix • Slow refresh, very high resolution • Over 200 dpi eBook devices available now in Japan • Others…

10. Framebuffers • So far we’ve talked about the physical display device • How does the interface between the device and the computer’s notion of an image look? • Framebuffer: A memory array in which the computer stores an image • On most computers, separate memory bank from main memory (why?) • Many different variations, motivated by cost of memory

11. Framebuffers • So far we’ve talked about the physical display device • How does the interface between the device and the computer’s notion of an image look? • Framebuffer: A memory array in which the computer stores an image • On most computers, separate memory bank from main memory (why?) • Many different variations, motivated by cost of memory

12. Framebuffers: True-Color • A true-color(aka 24-bitor 32-bit)framebufferstores one byte each for red, green, and blue • Each pixel can thus be one of 224 colors • Pay attention toEndian-ness • How can 24-bit and 32-bit mean the same thing here?

13. Framebuffers: Indexed-Color • An indexed-color (8-bit or PseudoColor) framebuffer stores one byte per pixel (also: GIF image format) • This byte indexes into a color map: • How many colorscan a pixel be? • Still common on low-end displays (cell phones, PDAs,GameBoys) • Cute trick: color-map animation

14. Framebuffers: Hi-Color • Hi-Color was a popular PC SVGA standard • Packs pixels into 16 bits: • 5 Red, 6 Green, 5 Blue (why would green get more?) • Sometimes just 5,5,5 • Each pixel can be one of 216 colors • Hi-color images can exhibit worse quantization artifacts than a well-mapped 8-bit image

15. Transform Illuminate Transform Clip Project Rasterize The Rendering Pipeline: A Whirlwind Tour Model & CameraParameters Rendering Pipeline Framebuffer Display

16. Transform Illuminate Transform Clip Project Rasterize The Display You Know Model & CameraParameters Rendering Pipeline Framebuffer Display

17. Transform Illuminate Transform Clip Project Rasterize The Framebuffer You Know Model & CameraParameters Rendering Pipeline Framebuffer Display

18. Transform Illuminate Transform Clip Project Rasterize The Rendering Pipeline Model & CameraParameters Rendering Pipeline Framebuffer Display

19. Transform Illuminate Transform Clip Project Rasterize 2-D Rendering: Rasterization(Coming Soon) Model & CameraParameters Rendering Pipeline Framebuffer Display

20. Transform Illuminate Transform Clip Project Rasterize The Rendering Pipeline: 3-D Model & CameraParameters Rendering Pipeline Framebuffer Display

21. The Rendering Pipeline: 3-D Scene graphObject geometry • Result: • All vertices of scene in shared 3-D “world” coordinate system • Vertices shaded according to lighting model • Scene vertices in 3-D “view” or “camera” coordinate system • Exactly those vertices & portions of polygons in view frustum • 2-D screen coordinates of clipped vertices ModelingTransforms LightingCalculations ViewingTransform Clipping ProjectionTransform

22. The Rendering Pipeline: 3-D Scene graphObject geometry • Result: • All vertices of scene in shared 3-D “world” coordinate system • Vertices shaded according to lighting model • Scene vertices in 3-D “view” or “camera” coordinate system • Exactly those vertices & portions of polygons in view frustum • 2-D screen coordinates of clipped vertices ModelingTransforms LightingCalculations ViewingTransform Clipping ProjectionTransform

23. Rendering: Transformations • So far, discussion has been in screen space • But model is stored in model space(a.k.a. object space or world space) • Three sets of geometric transformations: • Modeling transforms • Viewing transforms • Projection transforms

24. Y X Z Rendering: Transformations • Modeling transforms • Size, place, scale, and rotate objects parts of the model w.r.t. each other • Object coordinates  world coordinates Y Z X

25. Rendering: Transformations • Viewing transform • Rotate & translate the world to lie directly in front of the camera • Typically place camera at origin • Typically looking down -Z axis • World coordinates  view coordinates

26. Rendering: Transformations • Projection transform • Apply perspective foreshortening • Distant = small: the pinhole camera model • View coordinates  screen coordinates

27. ¢ q - q é ù é ù é ù X cos sin X = ê ú ê ú ê ú ¢ q q Y sin cos Y ë û ë û ë û Rendering: Transformations • All these transformations involve shifting coordinate systems (i.e., basis sets) • Oh yeah, that’s what matrices do… • Represent coordinates as vectors, transforms as matrices • Multiply matrices = concatenate transforms!

28. Rendering: Transformations • Homogeneous coordinates: represent coordinates in 3 dimensions with a 4-vector • Denoted [x, y, z, w]T • Note that w = 1 in model coordinates • To get 3-D coordinates, divide by w:[x’, y’, z’]T = [x/w, y/w, z/w]T • Transformations are 4x4 matrices • Why? To handle translation and projection

29. The Rendering Pipeline: 3-D Scene graphObject geometry • Result: • All vertices of scene in shared 3-D “world” coordinate system • Vertices shaded according to lighting model • Scene vertices in 3-D “view” or “camera” coordinate system • Exactly those vertices & portions of polygons in view frustum • 2-D screen coordinates of clipped vertices ModelingTransforms LightingCalculations ViewingTransform Clipping ProjectionTransform

30. Rendering: Lighting • Illuminating a scene: coloring pixels according to some approximation of lighting • Global illumination: solves for lighting of the whole scene at once • Local illumination: local approximation, typically lighting each polygon separately • Interactive graphics (e.g., hardware) does only local illumination at run time

31. The Rendering Pipeline: 3-D Scene graphObject geometry • Result: • All vertices of scene in shared 3-D “world” coordinate system • Vertices shaded according to lighting model • Scene vertices in 3-D “view” or “camera” coordinate system • Exactly those vertices & portions of polygons in view frustum • 2-D screen coordinates of clipped vertices ModelingTransforms LightingCalculations ViewingTransform Clipping ProjectionTransform

32. Rendering: Clipping • Clipping a 3-D primitive returns its intersection with the view frustum:

33. Rendering: Clipping • Clipping is tricky! • We will have a whole assignment on clipping In: 3 vertices Out: 6 vertices Clip In: 1 polygon Out: 2 polygons Clip

34. Transform Illuminate Transform Clip Project Rasterize The Rendering Pipeline: 3-D Model & CameraParameters Rendering Pipeline Framebuffer Display

35. Modeling: The Basics • Common interactive 3-D primitives: points, lines, polygons (i.e., triangles) • Organized into objects • Collection of primitives, other objects • Associated matrix for transformations • Instancing: using same geometry for multiple objects • 4 wheels on a car, 2 arms on a robot

36. Modeling: The Scene Graph • The scene graph captures transformations and object-object relationships in a DAG • Nodes are objects; • Arcs indicate instancing • Each has a matrix Robot Head Body Mouth Eye Leg Trunk Arm

37. Modeling: The Scene Graph • Traverse the scene graph in depth-first order, concatenating transformations • Maintain a matrix stack of transformations Robot Visited Head Body Unvisited Leg Mouth Eye Trunk Arm MatrixStack Active Foot

38. Modeling: The Camera • Finally: need a model of the virtual camera • Can be verysophisticated • Field of view, depth of field, distortion, chromatic aberration… • Interactive graphics (OpenGL): • Camera pose:position & orientation • Captured in viewing transform (i.e., modelview matrix) • Pinhole camera model • Field of view • Aspect ratio • Near & far clipping planes

39. Modeling: The Camera • Camera parameters (FOV, etc) are encapsulated in a projection matrix • Homogeneous coordinates 4x4 matrix! • See OpenGL Appendix F for the matrix • The projection matrix premultiplies the viewing matrix, which premultiplies the modeling matrices • Actually, OpenGL lumps viewing and modeling transforms into modelview matrix