Rendering%20Pipeline%20and%20Graphics%20Hardware

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Rendering Pipeline and Graphics Hardware

• Aaron Bloomfield
• CS 445 Introduction to Graphics
• Fall 2006

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Overview

• Framebuffers
• Rendering Pipeline
• Transformations
• Lighting
• Clipping
• Modeling
• Camera
• Visible Surface Determination
• History

How is the rasterized scene kept in memory?
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Framebuffers

• So far weve talked about the physical display
  device
• How does the interface between the device and the
  computers 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

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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 toEndian-ness
• How can 24-bit and 32-bit mean the same thing
  here?

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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

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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

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Overview

• Framebuffers
• Rendering Pipeline
• Transformations
• Lighting
• Clipping
• Modeling
• Camera
• Visible Surface Determination
• History

How does the graphics hardware process the
graphical display?
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The Rendering Pipeline A Tour
Model CameraParameters
Rendering Pipeline
Framebuffer
Display
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The Parts You Know
Model CameraParameters
Rendering Pipeline
Framebuffer
Display
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The Rendering Pipeline
Model CameraParameters
Rendering Pipeline
Framebuffer
Display
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2-D Rendering Rasterization
Model CameraParameters
Rendering Pipeline
Framebuffer
Display

• Well talk about this soon

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The Rendering Pipeline 3-D
Model CameraParameters
Rendering Pipeline
Framebuffer
Display
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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
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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
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Overview

• Framebuffers
• Rendering Pipeline
• Transformations
• Lighting
• Clipping
• Modeling
• Camera
• Visible Surface Determination
• History

How do you transform the objects so they can be
displayed?
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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

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Rendering Transformations

• Modeling transforms
• Size, place, scale, and rotate objects parts of
  the model w.r.t. each other
• Object coordinates ? world coordinates
• The scene now has its origin at (0,0,0)

Y
Z
X
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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
• The scene now has its origin at the camera

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Rendering Transformations

• Projection transform
• Apply perspective foreshortening
• Distant small the pinhole camera model
• View coordinates ? screen coordinates
• The scene is now in 2 dimensions

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Rendering Transformations

• All these transformations involve shifting
  coordinate systems (i.e., basis sets)
• Matrices do that
• Represent coordinates as vectors, transforms as
  matrices
• Multiply matrices concatenate transforms!

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Rendering Transformations

• Homogeneous coordinates represent coordinates in
  3 dimensions with a 4-vector
• Denoted x, y, z, wT
• Note that w 1 in model coordinates
• To get 3-D coordinates, divide by wx, y,
  zT x/w, y/w, z/wT
• Transformations are 4x4 matrices
• Why? To handle translation and projection
• Well see this a bit more later in the semester

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Overview

• Framebuffers
• Rendering Pipeline
• Transformations
• Lighting
• Clipping
• Modeling
• Camera
• Visible Surface Determination
• History

How do we compute the radiance for each sample
ray?
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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
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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

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Lighting Simulation

• Lighting parameters
• Light source emission
• Surface reflectance
• Atmospheric attenuation
• Camera response

Light Source
Surface
Camera
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Lighting Simulation

• Local illumination
• Ray casting
• Polygon shading
• Global illumination
• Ray tracing
• Monte Carlo methods
• Radiosity methods

Light Source
Surface
N
More on these methods later!
Camera
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Overview

• Framebuffers
• Rendering Pipeline
• Transformations
• Lighting
• Clipping
• Modeling
• Camera
• Visible Surface Determination
• History

How do you only display those parts of the scene
that are visible?
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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
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Rendering Clipping

• Clipping a 3-D primitive returns its intersection
  with the view frustum

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Rendering Clipping

• Clipping is tricky!
• We will a lot more on clipping

In 3 vertices Out 6 vertices
Clip
In 1 polygon Out 2 polygons
Clip
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Overview

• Framebuffers
• Rendering Pipeline
• Transformations
• Lighting
• Clipping
• Modeling
• Camera
• Visible Surface Determination
• History

How is the 3D scene described in a computer?
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The Rendering Pipeline 3-D
Model CameraParameters
Rendering Pipeline
Framebuffer
Display
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Modeling The Basics

• Common interactive 3-D primitives points, lines,
  polygons (i.e., triangles)
• Organized into objects
• Not necessarily in the OOP sense
• 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

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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
Body
Head
Arm
Trunk
Leg
Eye
Mouth
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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
Arm
Trunk
Eye
Mouth
MatrixStack
Active
Foot
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Overview

• Framebuffers
• Rendering Pipeline
• Transformations
• Lighting
• Clipping
• Modeling
• Camera
• Visible Surface Determination
• History

How is the viewing device described in a computer?
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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

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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 pre-multiplies the viewing
  matrix, which pre-multiplies the modeling
  matrices
• Actually, OpenGL lumps viewing and modeling
  transforms into modelview matrix

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Camera Models

• The most common model is pin-hole camera
• All captured light rays arrive along paths toward
  focal point without lens distortion (everything
  is in focus)
• Sensor response proportional to radiance

Other models consider ... Depth of field Motion
blur Lens distortion
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Camera Parameters

• Position
• Eye position (px, py, pz)
• Orientation
• View direction (dx, dy, dz)
• Up direction (ux, uy, uz)
• Aperture
• Field of view (xfov, yfov)
• Film plane
• Look at point
• View plane normal

View Plane
Up direction
Look at Point
back
View direction
right
Eye Position
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Overview

• Framebuffers
• Rendering Pipeline
• Transformations
• Lighting
• Clipping
• Modeling
• Camera
• Visible Surface Determination
• History

How can the front-most surface be found with an
algorithm?
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Visible Surface Determination

• The color of each pixel on the view planedepends
  on the radiance emanating from visible surfaces

View plane
Eye position
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Ray Casting

• For each sample
• Construct ray from eye position through view
  plane
• Find first surface intersected by ray through
  pixel
• Compute color of sample based on surface radiance

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Ray Casting

• For each sample
• Construct ray from eye position through view
  plane
• Find first surface intersected by ray through
  pixel
• Compute color of sample based on surface radiance

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Visible Surface Determination

• For each sample
• Construct ray from eye position through view
  plane
• Find first surface intersected by ray through
  pixel
• Compute color of sample based on surface radiance

More efficient algorithms utilize spatial
coherence!
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Rendering Algorithms

• Rendering is a problem in sampling and
  reconstruction!

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Overview

• Framebuffers
• Rendering Pipeline
• Transformations
• Lighting
• Clipping
• Modeling
• Camera
• Visible Surface Determination
• History

Whats the history of computer graphics hardware?
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Graphical Hardware Companies

• In the beginning there was SGI
• and they remained the king for 15 years
• Are now in bankruptcy protection
• Why buy a really expensive server when you can
  get a PC that is almost as fast, but 1/10th the
  cost?
• NVidia and ATI provide high-end graphics cards
  for PCs
• ATI tens to focus more on increasing the number
  of triangles rendered per frame
• NVidia tends to focus more on adding new
  graphical capabilities
• So researchers use it more

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A much older graphics pipeline

• SGI Onyx2
• From 1997 or so
• A fully configured system could easily run 100k
• A 200 graphics card today can perform 2-3 times
  as much
• In all fairness, the Onyx2 had a lot of
  advantages

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Summary

• Major issues in 3D rendering
• 3D scene representation
• 3D viewer representation
• Visible surface determination
• Lighting simulation
• Concluding note
• Accurate physical simulation is complex and
  intractable
• Rendering algorithms apply many approximations
  to simplify representations and computations