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Transcript

The Rendering Pipeline CS 445: Introduction to Computer Graphics David Luebke University of Virginia Admin Call roll Forums signup Demo Ogre Recap Display Technology: DMDs Digital Micromirror Devices (projectors) – Microelectromechanical (MEM) devices, fabricated with VLSI techniques 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 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 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 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 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… 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 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 Framebuffers: True-Color A true-color (aka 24-bit or 32-bit) framebuffer stores one byte each for red, green, and blue Each pixel can thus be one of 224 colors Pay attention to Endian-ness How can 24-bit and 32-bit mean the same thing here? Framebuffers: IndexedColor 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 colors can a pixel be? Still common on low-end displays (cell phones, PDAs, GameBoys) Cute trick: color-map animation 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 The Rendering Pipeline: A Whirlwind Tour Transform Illuminate Transform Clip Project Rasterize Model & Camera Parameters Rendering Pipeline Framebuffer Display The Display You Know Transform Illuminate Transform Clip Project Rasterize Model & Camera Parameters Rendering Pipeline Framebuffer Display The Framebuffer You Know Transform Illuminate Transform Clip Project Rasterize Model & Camera Parameters Rendering Pipeline Framebuffer Display The Rendering Pipeline Transform Illuminate Transform Clip Project Rasterize Model & Camera Parameters Rendering Pipeline Framebuffer Display 2-D Rendering: Rasterization (Coming Soon) Transform Illuminate Transform Clip Project Rasterize Model & Camera Parameters Rendering Pipeline Framebuffer Display The Rendering Pipeline: 3-D Transform Illuminate Transform Clip Project Rasterize Model & Camera Parameters Rendering Pipeline Framebuffer Display The Rendering Pipeline: 3-D Scene graph Object geometry Result: Modeling Transforms • All vertices of scene in shared 3-D “world” coordinate system Lighting Calculations • Vertices shaded according to lighting model Viewing Transform • Scene vertices in 3-D “view” or “camera” coordinate system Clipping Projection Transform • Exactly those vertices & portions of polygons in view frustum • 2-D screen coordinates of clipped vertices The Rendering Pipeline: 3-D Scene graph Object geometry Result: Modeling Transforms • All vertices of scene in shared 3-D “world” coordinate system Lighting Calculations • Vertices shaded according to lighting model Viewing Transform • Scene vertices in 3-D “view” or “camera” coordinate system Clipping Projection Transform • Exactly those vertices & portions of polygons in view frustum • 2-D screen coordinates of clipped vertices 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 Rendering: Transformations Modeling transforms – Size, place, scale, and rotate objects parts of the model w.r.t. each other – Object coordinates world coordinates Y Y X Z Z X 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 Rendering: Transformations Projection transform – Apply perspective foreshortening Distant = small: the pinhole camera model – View coordinates screen coordinates 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 X cos q = q Y sin -sin q X q cos Y Multiply matrices = concatenate transforms! 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 The Rendering Pipeline: 3-D Scene graph Object geometry Result: Modeling Transforms • All vertices of scene in shared 3-D “world” coordinate system Lighting Calculations • Vertices shaded according to lighting model Viewing Transform • Scene vertices in 3-D “view” or “camera” coordinate system Clipping Projection Transform • Exactly those vertices & portions of polygons in view frustum • 2-D screen coordinates of clipped vertices 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 The Rendering Pipeline: 3-D Scene graph Object geometry Result: Modeling Transforms • All vertices of scene in shared 3-D “world” coordinate system Lighting Calculations • Vertices shaded according to lighting model Viewing Transform • Scene vertices in 3-D “view” or “camera” coordinate system Clipping Projection Transform • Exactly those vertices & portions of polygons in view frustum • 2-D screen coordinates of clipped vertices Rendering: Clipping Clipping a 3-D primitive returns its intersection with the view frustum: Rendering: Clipping Clipping is tricky! – We will have a whole assignment on clipping Clip In: 3 vertices Out: 6 vertices Clip In: 1 polygon Out: 2 polygons The Rendering Pipeline: 3-D Transform Illuminate Transform Clip Project Rasterize Model & Camera Parameters Rendering Pipeline Framebuffer Display 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 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 Mouth Body Eye Leg Trunk Arm 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 Active Matrix Stack Mouth Eye Leg Foot Trunk Arm Modeling: The Camera Finally: need a model of the virtual camera – Can be very sophisticated 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 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