Overview of graphics systems

Structures in Computer Graphics
Overview of Graphics Systems
Chapter 1
Er. Jay Nagar +91-9601957620

Outline
• Survey of Computer Graphics
• Overview of Graphics Systems
– Image Basics
– Graphics Hardware
• Input
– Describing something to the computer
• Computation
– Computing what we want to draw
• Output
– Final representation
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What is the goal of computer
graphics?
• High level, why computer graphics?
• Lower level, what is the computer doing?
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Business of Generating Images
• Images are made up of pixels
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RGB
RGB Color cube (what we
use in computer graphics)
Other color spaces include HSV,
YUV, YCrCb, and YIQ

The “goal” of computer graphics
• Solve the function
– Red @ a pixel is f(i,j)=…
– Green @ a pixel is f(i,j)=…
– Blue @ a pixel is f(i,j)=…
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Early Applications of Computer
Graphics• Data Visualization
– Charts and Graphs
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Early Applications of Computer
Graphics
• Computer Aided Design (CAD)
– Q: Why wireframe?
• Why these apps?
– A: Better conceptualization,
interaction, transfer of ideas
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Computer Graphics
Applications
• Virtual Reality
– VR: User interacts and
views with a 3D world
using “more natural” means
– Best VR?
• Data Visualization
– Scientific, Engineering,
Medical data
– Visualizing millions to
billions of data points
– See trends
– Different schemes
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Computer Graphics
Applications
• Education and Training
– Models of physical,
financial, social systems
– Comprehension of complex
systems
• Computer Art
– Fine and commercial art
– Performance Art
– Aesthetic Computing
– SIGGRAPH
• Games/Movies
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Computer Graphics
Applications
• Image Processing
– ~Inverse of Graphics
– Start with a picture
– Process picture
information
• Graphical User
Interfaces (GUIs)
– WIMP interface
– HCI
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Overview of Graphics Systems
• Images
• Hardware
– Input Systems
– Output Systems
• Software
– OpenGL
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Two Dimensional Images
• Images (at least
the ones in this
class) are two
dimensional
shapes.
• The two axes we
will label as X
(horizontal), and
Y (vertical).
X Axis
Y
Axis
(0,0) +X
+Y
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Hardware Pipeline
Input OutputComputation
We want to draw a rectangle, how do we describe it to a computer?
Model (n) - object
description that a
computer
understands.
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Partition the space
(7,3)
(7,9)
(14,3)
(14,9)
Vertex (pl. Vertices) - a point in 2 or 3 dimensional space.
1. Define a set of
points (vertices) in
2D space.
2. Given a set of
vertices, draw
lines between
consecutive
vertices.
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Record every position
Bitmap - a rectangular array of bits mapped one-to-one with
pixels.
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Position relative
Vector display system - graphical output system that was
based on strokes (as opposed to pixels). Also known as:
random, calligraphic, or stroke displays.
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Representing Objects
• Most common method is the VERTEX
method. Define the object as a set of points
with connectivity information.
• Why is connectivity important?
Connectivity - information that defines
which vertices are connected to which
other vertices via edges.
Edge - connects two vertices
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Model file for rectangle
• v 4 e 4
• 7 3
• 7 9
• 14 9
• 14 3
• 1 2
• 2 3
• 3 4
• 4 1
(7,3)
(7,9)
(14,3)
(14,9)
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How do we do this?
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Input Devices
• Locator Devices
• Keyboard
• Scanner
– Images
– Laser
• Cameras (research)
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Locator Devices
When queried, locator devices return a
position and/or orientation.
•Mouse (2D and 3D)
•Trackball
•Joystick (2D and 3D)
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Locator Devices
When queried, locator
devices return a
position and/or
orientation.
• Tablet
• Virtual Reality
Trackers
– Data Gloves
– Digitizers
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Keyboard
• Text input
– List boxes, GUI
– CAD/CAM
– Modeling
• Hard coded
– Vertex locations are inserted into code
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Scanners
• Image Scanners - Flatbed,
etc.
– What type of data is
returned? Bitmap
• Laser Scanners -
Deltasphere
– Emits a laser and does time
of flight. Returns 3D point
• Camera based - research
– Examine camera image(s)
and try to figure out
vertices from them.

Many others
• Light Pens
• Voice Systems
• Touch Panels
• Camera/Vision Based
• Which is best?
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Common
Modeling
Approach
• Hybrid
• Animator jobs

Model file for rectangle
• v 4 e 4
• 7 3
• 7 9
• 14 9
• 14 3
• 1 2
• 2 3
• 3 4
• 4 1
(7,3)
(7,9)
(14,3)
(14,9)

Computation Stage
• Now that we have a model of what we want
to draw, what goes on inside the computer
to generate the output?
Computation
Transformations Rasterization

Computation Stage
Computation
Transformations Rasterization
Model
Transformed
Model
Output

How do we store this?
We would like to allocate memory to hold the
results of the computation stage.

Framebuffer
Framebuffer - A block of memory, dedicated
to graphics output, that holds the contents of
what will be displayed.
Pixel - one element of the framebuffer

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 0 0 0 0 0 0
0 0 0 0 0 0 0 1 0 0 0 0 0 0 1 0 0 0 0 0 0
0 0 0 0 0 0 0 1 0 0 0 0 0 0 1 0 0 0 0 0 0
0 0 0 0 0 0 0 1 0 0 0 0 0 0 1 0 0 0 0 0 0
0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
Framebuffer
Questions:
How big is the framebuffer?What is the largest image you can display?How many pixels are there?How much memory do we need to allocate
for the framebuffer?

Framebuffer in Memory
• If we want a framebuffer of 640 pixels by
480 pixles, we should allocate:
framebuffer = 640*480 bits
• How many bit should we allocate?
Q: What do more bits get you?
A: More values to be stored at each pixel.
Why would you want to store something
other than a 1 or 0?

Framebuffer bit depth
• How many colors does 1 bit get you?
• How many colors do 8 bits get you?
– Monochrome systems use this (green/gray
scale)
• What bit depth would you want for your
framebuffer?
bit depth - number of bits allocated per pixel in a buffer

Framebuffer bit depths
• Remember, we are
asking “how much
memory do we
allocate to store the
color at each
pixel?”
• Common answers:
– 16 and 32 bits

Bit depths
• 16 bits per pixel (high color)
– 5 bits for red, 5/6 bits for green, 5 bits for blue
– potential of 32 reds, 32/64 green, 32 blues
– total colors: 65536
• 32 bits per pixel (true color)
– 8 bits for red, green, blue, and alpha
– potential for 256 reds, greens, and blues
– total colors: 16777216 (more than the eye can
distinguish)
• Let’s look at Display Control Panel

Data Type Refresher
• bit - a 0 or 1. Can represent 2 unique values
• byte - 8 bits. 256 values
• word - 32 bits. 4,294,967,296 values
• int - 32 bits.
• float - 32 bits
• double - 64 bits
• unsigned byte - 8 bits

Memory
unsigned byte framebuffer[640*480*3];
framebuffer =
[255 255 255 0 0 255 0 0 255 0 255 0 255 0 0
0 255 0 0 255 0 …]

Graphic Card Memory
• How much memory is on our graphic card?
– 640 * 480 * 32 bits = 1,228,800 bytes
– 1024 * 768 * 32 bits = 3,145,728 bytes
– 1600 * 1200 * 32 bits = 7,680,000 bytes
• How much memory is on your graphics
card?
• As a side note: Playstation 1 has 2 MB
RAM. How do they do it? What is the TV
resolution? 1 bit alpha, no z buffer.

A: Egads! Not enough memory!
Q: What is dithering?
• Trading spatial resolution for intensity and
color depth.
• Sometimes call digital half-toning
• Increases the number of apparent colors
than are actually capable of being displayed

Dithering
• Why does it work? Spatial integration.
Using the fact that our eyes usually blend
areas of high frequency.
• Why do you need it? If you don’t have
enough bits. Eyes can detect 100 shades of
a color. Banding occurs with fewer colors.
• Where do you see this? Printers and
newspaper! Let’s look at the Alligator.

Let’s transition to OpenGL
• Now that we understand the input and
computation stage, let’s hold off on
different output types till after opengl + 2D

Output
We have an image (framebuffer or model),
now we want to show it. Read Ch. 2 in the
Hearn and Baker handout.
• Hardcopy
• Display
– Vector
– Raster Scan

Hardcopy
• Printers (Resolution, color depth)
– Dot Matrix - uses a head with 7 to 24 pins to
strike a ribbon (single or multiple color)
– Ink Jet Printers (fires small balls of colored ink)
– Laser Printers (powder adheres to positive
charged paper)
– Pen Plotters (similar to vector displays).
“infinite” resolution.

Framebuffer -> Monitor
The values in the framebuffer are converted from a
digital (1s and 0s representation, the bits) to an
analog signal that goes out to the monitor. A video
card’s RAMDAC performs this operation, once per
frame. This is done automatically (not controlled by
your code), and the conversion can be done while
writing to the framebuffer.

Image Quality Issues
• Screen resolution
• Color
• Blank space between
the pixels
• Intentional image
degradation
• Brightness
• Contrast
• Refresh rate
• Sensitivity of display
to viewing angle

Pixels
• Pixel - The most basic addressable image
element in a screen
– CRT - Color triad (RGB phosphor dots)
– LCD - Single color element
• Screen Resolution - measure of number of
pixels on a screen (m by n)
– m - Horizontal screen resolution
– n - Vertical screen resolution

Other meanings of resolution
• Pitch - Size of a pixel, distance from center
to center of individual pixels.
• Cycles per degree - Addressable elements
(pixels) divided by twice the FOV measured
in degrees.
• The human eye can resolve 30 cycles per
degree (20/20 Snellen acuity).

Video Formats
• NTSC - 525x480, 30f/s, interlaced
• PAL - 625x480, 25f/s, interlaced
• VGA - 640x480, 60f/s, noninterlaced
• SVGA – 800x600, 60f/s noninterlaced
• RGB - 3 independent video signals and
synchronization signal, vary in resolution and
refresh rate
• Time-multiplexed color - R,G,B one after another
on a single signal, vary in resolution and refresh
rate

Raster Displays
• Cathode Ray Tubes (CRTs), most “tube”
monitors you see. Very common, but big
and bulky.
• Liquid Crystal Displays (LCDs), there are
two types transmissive (laptops, those
snazzy new flat panel monitors) and
reflective (wrist watches).

Cathode Ray Tubes (CRTs)
Heating element on the yolk.
Phosphor coated screen
Electrons are boiled off the
filament and drawn to the
focusing system.
The electrons are focused into
a beam and “shot” down the
cylinder.
The deflection plates “aim”
the electrons to a specific
position on the screen.

CRT Phosphor Screen
• The screen is coated with
phosphor, 3 colors for a color
monitor, 1 for monochrome.
• For a color monitor, three
guns light up red, green, or
blue phosphors.
• Intensity is controlled by the
amount of time at a specific
phosphor location.

scan line - one row on the screen
interlace vs. non-interlace - Each frame is either
drawn entirely, or as two consecutively drawn
fields that alternate horizontal scan lines.
vertical sync (vertical retrace) - the motion of
the beam moving from the bottom of the image
to the top, after it has drawn a frame.
refresh rate - how many frames are drawn per
second. Eye can see 24 frames per second. TV
is 30 Hz, monitors are at least 60 Hz.
Beam Movement

• Refresh rate is important, but remember it is
different than your program’s update rate.
• Why is higher, better?

Vector Displays
• Unlike CRTs, vector
displays have a single gun
that is controlled to draw
lines. Think of having a
VERY FAST drawing pen.
• Pros: Diagrams/only draw
what you need
• Cons: No fill objects/Slows
with complexity

CRTs (cont.)
• Strong electrical fields and high voltage
• Very good resolution
• Heavy, not flat

Liquid Crystal Displays (LCDs)
• Also divided into pixels, but without an
electron gun firing at a screen, LCDs have
cells that either allow light to flow through,
or block it.

Liquid Crystal Displays
• Liquid crystal displays use small flat chips
which change their transparency properties
when a voltage is applied.
• LCD elements are arranged in an n x m
array call the LCD matrix
• Level of voltage controls gray levels.
• LCDs elements do not emit light, use
backlights behind the LCD matrix

LCDs (cont.)
• Color is obtained by placing filters in front of
each LCD element
• Usually black space between pixels to separate
the filters.
• Because of the physical nature of the LCD
matrix, it is difficult to make the individual
LCD pixels very small.
• Image quality dependent on viewing angle.

Advantages of LCDs
• Flat
• Lightweight
• Low power consumption

Projection Displays
• Use bright CRT or LCD
screens to generate an
image which is sent
through an optical
system to focus on a
(usually) large screen.

Basic Projector Designs
(Images from Phillips Research)
Reflective Projection System Transmittive Projection
System

Transmitive Projectors
CRT Based
• One color CRT tube (red, blue,
green phosphors) displays an
image with one projection lens.
• One black-and-white CRT with
a rapidly rotating color filter
wheel (red, green, blue filters) is
placed between the CRT tube
and the projection lens.
• Three CRT tubes (red, green,
blue) with three lenses project
the images. The lenses are
aligned so that a single color
image appears on the screen.
CRT-based projectors are usually
heavy and large compared to
other technologies

Transmitive Projectors
• LCD Based
– Use a bright light to illuminate an LCD
panel, and a lens projects the image
formed by the LCD onto a screen.
• Small, lightweight compared to CRT
based displays

Reflective Projectors
• In reflective projectors, the image is formed
on a small, reflective chip.
• When light shines on the chip, the image is
reflected off it and through a projection lens to
the screen.
• Recent innovations in reflective technology
have been in the the following areas:
– Microelectromechanical systems (MEMS)
• Digital micromirror device (DMD, DLP)
• Grating light valve (GLV)
– Liquid crystal on silicon (LCOS)

Advantages/Disadvantages
of Projection Display
• Very large screens can provide large FoV
and can be seen by several people
simultaneously.
• Image quality can be fuzzy and somewhat
dimmer than conventional displays.
• Sensitivity to ambient light.
• Delicate optical alignment.

Displays in Virtual Reality
• Head-Mounted Displays
(HMDs)
– The display and a position
tracker are attached to the
user’s head
• Head-Tracked Displays
(HTDs)
– Display is stationary,
tracker tracks the user’s
head relative to the display.
– Example: CAVE,
Workbench, Stereo monitor

3D Glasses
3D Display
3D Object

Graphics Software
• How to talk to the hardware?
• Special purpose software
– Excel
– AutoCAD
– Medical Visualization
• Programming API
• Advantages?
• Please Read Section 2.9
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Overview of graphics systems

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