Intro To Computer Graphics

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Intro To Computer Graphics

• Geb Thomas

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Learning Objectives

• Learn the differences between image-order,
  object-order and volume rendering.
• Learn how the eye perceives color and how
  monitors present color.
• Learn how light and objects interact and how to
  mathematically define ambient, diffuse and
  specular reflections.
• Learn the difference between flat and Gouraud
  shading.

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How Does a VR System Use Graphics?

• Processor talks to graphics card about geometry
  and lighting
• Graphics card crunches geometry and lighting
  calculations
• Stores this in a buffer
• Another circuit converts the buffer to the video
  signal

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Illustrated Graphics Card
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The Graphics Pipeline
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How Do We See Things?
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Ray Tracing
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Object Order
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Volume Rendering

• Similar to ray-tracing, but instead of being
  obstructed by intervening objects, the ray may
  just be attenuated, or color-filtered.

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Color

• Light hits the eye in a continuous spectrum of
  color from different frequencies.
• Our eyes have three different types of cones to
  receive data. Each cone is sensitive to
  different frequencies.
• The signals from our color-sensitive cones
  provide the perception of color.
• By providing emitters designed to stimulate each
  type of cone, the monitor can fool our eye into
  believing it is perceiving natural color.

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Eye Details
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Light Is Reflected/Absorbed in All Spectra
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The Absorption Characteristic of the 3 Types of
Cones
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Simulating Colors
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For Example
Natural Reflected Light
Cone response Blue -gt .8 Red -gt .7
Green -gt .5
Intensity
Wavelength
Simulated with a Monitor
Cone response Blue -gt .8 Red -gt .7
Green -gt .5
Blue -gt .8 Red -gt .7 Green -gt .5
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Lights

• Infinitely distant point light creates parallel
  rays
• Constant direction across field of view
• No radiant energy drop-off
• Local light sources
• 1/R2 energy drop-off
• Radial directions from source
• Even more complex if the source is distributed
  rather than point-like

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Ambient Light

• A general surround light that represents the
  random light rays resulting from multiple
  reflections.
• Generally provides the colors of objects in
  shadows.
• Rc Lc Oc
• Where Rc is resultant color, Lc is the light
  intensity curve, and Oc is the object intensity
  curve.
• Also works Rc ambientRGBo where ambient ranges
  from 0-1 and RGBo is the RGB values of the object
  color.

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Diffuse Light(Lambertian reflection)

• This is light from the light source, determined
  by the angle of incidence.
• Objects are brighter when they directly face the
  light
• Rc Lc Oc(-Ln.On)
• Where Ln On are the light normal and object
  normal, respectively (normalized length).
• Gouraud shading interpolates the object normal
  across adjacent faces to make the object look
  smooth.
• http//www.wiley.com/legacy/compbooks/vrml2sbk/toc
  /ch20.htm

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Specular Reflection

• Add hot spots characteristic of shiny objects.

Ln
Light
On
S
-Ln
Cn
-Cn
Object
Camera
RcLcOcS . (-Cn)Osp
S 2On.(-Ln) On Ln
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Exercise 1

• Given an object with RGB values of .5 .2 .1 and
  an ambient light of .5, what is the color of a
  pixel containing the object (assuming no other
  light sources).

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Exercise 2

• Try to calculate color in 2D. Assume that a
  bright white light (intensity 1) reflects off a
  plane oriented at 35 degrees with a color of RGB
  values 1 0 0. What is the color of a pixel
  showing the plane, if the camera is off to the
  right?

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Exercise 3

• Same situation as Exercise 2, but calculate the
  specular reflection, if the specular power is .08

35o
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Learning Objectives

• Learn the differences between image-order,
  object-order and volume rendering.
• Learn how the eye perceives color and how
  monitors present color.
• Learn how light and objects interact and how to
  mathematically define ambient, diffuse and
  specular reflections.
• Learn the difference between flat and Gouraud
  shading.

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