Download 3 - University of Virginia, Department of Computer Science

The Rendering Pipeline
CS 445: Introduction to Computer Graphics
David Luebke
University of Virginia
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Recap
Display Technology: DMDs
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Digital Micromirror Devices (projectors)
– Microelectromechanical (MEM) devices, fabricated with VLSI
techniques
Recap:
Display Technology: DMDs
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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
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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:
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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
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OLED pros:
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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
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OLED cons:
– Not quite there yet (96x64 displays) except niche markets
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Cell phones (especially back display)
Car stereos
– Not very robust, display lifetime a key issue
– Currently only passive matrix displays
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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
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Liquid Crystal On Silicon (LCOS)
– “Next big thing” for projectors
– Don’t know much about this one
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E-Ink
– Tiny black-and-white spheres embedded in matrix
– Slow refresh, very high resolution
– Over 200 dpi eBook devices available now in Japan
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Others…
Framebuffers
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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
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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
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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
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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
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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
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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
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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
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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
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Viewing transform
– Rotate & translate the world to lie directly in front of the camera
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Typically place camera at origin
Typically looking down -Z axis
– World coordinates  view coordinates
Rendering:
Transformations
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Projection transform
– Apply perspective foreshortening
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Distant = small: the pinhole camera model
– View coordinates  screen coordinates
Rendering:
Transformations
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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
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-sin q  X 
 
q
cos  Y 
Multiply matrices = concatenate transforms!
Rendering:
Transformations
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Homogeneous coordinates: represent coordinates in 3
dimensions with a 4-vector
– Denoted [x, y, z, w]T
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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
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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
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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
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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
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Clipping a 3-D primitive returns its intersection with the view
frustum:
Rendering: Clipping
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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
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Common interactive 3-D primitives: points, lines, polygons
(i.e., triangles)
Organized into objects
– Collection of primitives, other objects
– Associated matrix for transformations
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Instancing: using same geometry for multiple objects
– 4 wheels on a car, 2 arms on a robot
Modeling: The Scene
Graph
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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
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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
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Finally: need a model of the virtual camera
– Can be very sophisticated
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Field of view, depth of field, distortion, chromatic aberration…
– Interactive graphics (OpenGL):
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Camera pose: position & orientation
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Captured in viewing transform (i.e., modelview matrix)
Pinhole camera model
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Field of view
Aspect ratio
Near & far clipping planes
Modeling: The Camera
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Camera parameters (FOV, etc) are encapsulated in a projection
matrix
– Homogeneous coordinates  4x4 matrix!
– See OpenGL Appendix F for the matrix
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The projection matrix premultiplies the viewing matrix, which
premultiplies the modeling matrices
– Actually, OpenGL lumps viewing and modeling transforms into
modelview matrix