Graphics display-devicesmod-1

Computer Graphics
Hardware and
Software
Lecture Notes,
CEng 477

What is Computer
Graphics?


Different things in different contexts:
−

−

tools used to make such pictures, software and
hardware, input/output devices.

−



pictures, scenes that are generated by a
computer.

the whole field of study that involves these tools
and the pictures they produce.

Use of computer to define, store, manipulate,
interrogate and present pictorial output.







How pictures are represented in computer
graphics?
How pictures are prepared for presentation?
How interaction within the picture is
accomplished?

Computer Graphics
Applications


Art, entertainment, and publishing
−
−

computer games

−

World Wide Web

−



movie production, animation, special effects

Book, magazine design, photo editing

CG and Image processing (syntesis vs.
analysis)



Simulations (education, training)



CAD architectural, circuit design etc.



Scientific analysis and visualization



Graphical User Interfaces

Display (Video
Display Device)



Most CG on video monitors



Still most popular: Cathode Ray Tube (CRT)



Other popular display types:
−

Liquid Crystal Display

−

Plasma display

−

Field Emission Displays

−

Light Emitting Diodes

−

3D display devices (hologram or page scan
methods)

CRT
3. when electron
beams contact
screen phosphor
emits light

1. cathode rays
emitted by the
electron gun
2. focusing and
deflection

4. light fades,
redraw required
in a small period
(refresh)

CRT types










Direct View Storage Tubes (not CRT, no need
for refresh, pictures stored as a permanent
charge on phosphor screen)
Calligraphic refresh CRT (line drawing or
vector random scan, need refreshing)
Raster-scan (point by point refreshing)
Refresh rate: # of complete images
(frames) drawn on the screen in 1 second.
Frames/sec.
Frame time: reciprocal of the refresh rate,
time between each complete scan. sec/frame

Vector Scan






Picture definition is stored as a set of linedrawing commands in a refresh buffer.
to display a picture, the system cycles
through the set of commands in the buffer
Designed for line drawing applications (CAD)

Raster Scan




Screen is a regular grid of samples called
pixels (picture element)
Screen is refreshed line by line
non-interlaced



interlaced, cycle 1

interlaced, cycle 2

interlaced, 2 cycles

Interlacing: Avoid flickering affect for small
refresh rates.
interlaced 50Hz: actually 25Hz









resolution: a 2D term that measures the
number of scan-lines and the number of
pixels on each line (maximum number of
points that can be displayed without overlap
on a CRT)
black and white display only binary pixels.
intensity of a pixel can be achieved by the
force of electron beam (gray scale)
color display?

Color Displays


Beam penetration method:
special phosphors emitting different colors for
different intensity of electron. Slow, limited colors.



Shadow mask method:
3 electron guns + a shadow mask grid. Intensities of
3 colors result in an arbitrary color pixel. (most TVs
and monitors)



black and white: 1 bit per pixel.



gray scale: 1 byte per pixel (256 gray levels)





true color: 3 bytes=24pits per pixel (224
colors)
indexed color frame buffer: each pixel uses 1
byte, an index entry in a colormap table
matching the color to the actual color.

Vector vs Raster Scan


raster scan monitors:
−
−

filled areas, patterns

−



inexpensive
refresh process is independent (constant for any
complex scene)

vector scan monitors:
−

Smooth lines. no need for scan conversion: lines
to pixels. (raster scan solution antialiasing)

−

sometimes memory and CPU efficient 1000 lines:
Vector scan: 2000 endpoints and 1000 operations
Raster scan: whole frame buffer 1000 scan
conversions.

LCD Displays










Thinner and lighter. No tube and electron
beams.
Blocking/unblocking light through polarized
crystals.
A matrix of LC cells
one for each pixel.
No refresh unless the
screen changes.
Color 3 cells per pixel.

Simple Raster Display
System




Frame buffer: stored pixel map of screen
Video controller just refreshes the frame
buffer on the monitor periodically.
Peripheral
Devices

CPU

System Bus

System
Frame
Memor
Buffer
y

Video
Controller

Monito
r









Inexpensive
Scan conversion of output primitives (lines,
rectangles etc.) done by the CPU. Slow.
As refresh cycle increases, memory cycles
used by the video controller increases.
Memory is less available to CPU.
Solution: Graphics Display Processor

Graphics Display
Processor





Scan conversion, output primitives, raster
operations (double buffering)
Separete frame
buffer

CPU

Peripheral
Devices

System Bus

Display
Processor

D. Proc. Frame
memory. Buffer

System
Memory

Video
Controller

Monito
r

Computer Graphics
Software


Rendering Primitives
−

Models are composed of, or can be converted to,
a large number of geometric primitives.

−

Typical rendering primitives directly supported in
hardware include:


Points (single pixels)



Line segments



Polygons (perhaps simple, triangle, rectangle)

−

Modeling primitives include these, but also




Piecewise polynomial (spline) surfaces



Implicit surfaces (quadrics, blobbies, etc.)



−

Piecewise polynomial (spline) curves

Other...

Software renderer may support modeling
primitives directly, or may convert them into
polygonal or linear approximations for hardware
rendering

Algorithms



A number of basic algorithms are needed:
−

Transformation: Convert representations of
models/primitives from one coordinate system to
another

−

Clipping/Hidden surface removal: remove
primitives and part of primitives that are not
visible on the display

−

Rasterization: Convert a projected screen space
primitive to a set of pixels.



Advanced algorithms:
−

Picking: select a 3D obejct by clicking an input
device over a pixel location.

−

Shading and illumination: Simulate the
interaction of light with a scene.

−

Animation: Simulate movement by rendering a
sequence of frames.

Application
Programming
Interfaces










X11: 2D rasterization
Postscript, PDF: 2D transformations, 2D
rasterization
Phigs+, GL, OpenGL, Direct3D: 3D pipeline
APIs provide access to rendering hardware
via conceptual model.
APIs abstract the hardware implementations
and algorithms in standard software calls.





For 3D interactive applications, we might
modify the scene or a model directly or just
the change the attributes like viewing
information.
We need to interface to input devices in an
event-driven, asynchronous and device
independent fashion. APIs and toolkits are
also defined for this task. GLUT, Qt, GTK,
MFC, DirectX, Motif, Tcl/Tk.

Graphics Rendering
Pipeline



Rendering:
image
3D
Scene





conversion from scene to

2D Image

Scene is represented as a model composed
of primitives. Model is generated by a
program or input by a user.
Image is drawn on an output device:
monitor, printer, memory, file, video frame.







Typically rendering process is divided into
steps called the graphics pipeline.
Some steps are implemented by graphics
hardware.
Programmable graphics accelerator, GPU:
programmable pipelines in graphics hardware



The basic forward projection pipeline:
Modeling
Transformations

Model

Viewing
Transformations

M1
Model

M2

3D World
Scene

V

3D View
Scene

Model
M3

WCS

MCS

VCS
Rasterization

P

Clip

Projection

Normalize

2D/3D Device
Scene

NDCS
2D Image

DCS
SCS

Graphics display-devicesmod-1

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