7361 Computer Graphics - Department of Computer Science

Transcript 7361 Computer Graphics - Department of Computer Science

Overview of Graphics Systems
Cathode-ray Tube (CRT)
- colors are represented using Red, Green, and Blue
components
- the CRT has a mechanism for displaying these
three components
- grayscale can be represented by making R, G, and
B equal
Display Technologies
Cathode Ray Tubes (CRTs)
•Most common display device
today
•Evacuated glass bottle
(last remaining vacuum tube)
•Heating element (filament)
•Electrons attracted to focusing
anode cylinder
•Vertical and Horizontal
deflection plates
•Beam strikes phosphor
coating on front of tube
Cathode Ray Tubes
Different phosphors have different :
•Color
-red
-green
-blue
•Persistence : how long they continue to
emit light
The image must be refreshed to avoid flicker, typically at least
60Hz, through 72H is easier on the eyes.
Frame Buffer
01001101
Frame Buffer
DAC
Monitor
Intensity of the raster scan beam is modulated according to
the contents of a Frame Buffer
Each element of the Frame Buffer is associate with a single
pixel on the screen
Raster Displays
•Electron beam traces over screen in raster
scan order
•Each left –to right trace is called a scan line
•Each spot on the screen is a pixel
• When the beam is turned off to sweep back,
that is a retrace, or a blanking interval
•B/W TVs are basically oscilloscopes
(with a hardwired scan pattern)
•Entire screen painted 30 times/sec
•Screen is traversed 60 times/sec
•Even/Odd lines on alternate scans
(called fields)
•Smooth motion on dynamic scenes
•High Resolution on static scenes
•Optimize bandwidth
Raster Display Simulation
Resolution
The maximum numbers of the points that can be displayed
without overlap on a CRT is referred to as the resolution.
Typical resolution on high-quality systems is 1280 x 1024.
The display’s “resolution” is determined by :
•Number of scan lines
•Number of pixels per scan line
•Number of bits per pixel
A system with 24 bit per pixel and a screen resolution 1024
by 1024 requires 3 MB of storage for the Frame Buffer.
Aspect Ratio
Frame aspect ratio = horizontal/vertical size.
An aspect ration of ¾ means that a vertical line plotted
with three points has the same length as a horizontal
line plotted with four points
TV
4/3
35mm film
3/2
RGB color model versus indexed color model
- in the RGB color model, the frame buffer
stores the RGB components for every pixel
in the indexed color model, the frame buffer
stores an index into a color-lookup table
Raster Display
Disadvantages
•Requires screen-sized memory array
Advantages
•Discrete spatial sampling (pixels)
•Allows solids to be
displayed
•Moire patterns result when shadowmask and dot-pitch frequencies are
mismatched
•Convergence (varying angles of
approach distance of e-beam across
CRT face)
•Limit on practical size (< 40 inches)
•Spurious X-ray radiation
•Occupies a large volume
•Leverages low-cost
CRT H/W (TVs)
•Whole Screen is
constantly updated
•Bright light-emitting
display technology
Gamma parameter
- intensity output is not a linear function of voltage input
- the shape of the curve may be represented by a power
function y = xa
-x is the input voltage
- a is the gamma parameter - typically 2.2
- y is the light intensity output
- http://www.libpng.org/pub/png/spec/PNGGammaAppendix.html
- http://www.inforamp.net/%7Epoynton/index.html
-concept and terminology originated w.r.t. film
Displaying images in Java - Example
We start by writing what is called a Java applet. This is a form of a
Java program that can be embedded in a HTML document and
accessed from the web. An example HTML document that calls our
example applet is shown below:
HTML>
<HEAD>
<TITLE>Demo Applet
</TITLE>
</HEAD>
<BODY>
</H1>
<CENTER>
<APPLET code="Demo.class" width=200 height = 200> </APPLET>
</CENTER>
<HR> </BODY>
</HTML>
Review of Raster Displays
•Display synchronized with CRT sweep
•Special memory for screen update
•Pixels are the discrete elements displayed
•Generally, updates are visible
High-end Graphics Display System
•Adds a second frame buffer
•Swaps during vertical blanking
•Updates are invisible
•Costly
Implementing a Memory Raster Object
•Maintains a copy of the screen (or some part of it)
in memory
•Relies on a fast copy
•Updates are nearly invisible
•Conceptual model of a physical object
Lets talk about Pixels
•Pixels stored as ints in the array called
pixel[]
•Java's standard model of a pixel
/** * This constructor creates an Raster initialized with the contents of an
image. */
public Raster(Image img) {
try {
PixelGrabber grabber = new PixelGrabber(img, 0, 0, -1, -1, true);
if (grabber.grabPixels()) {
width = grabber.getWidth();
height = grabber.getHeight();
pixel = (int []) grabber.getPixels(); } }
catch (InterruptedException e) {
}}
A Java Model of a Memory Raster
Here is the source for the Raster
object

7361 Computer Graphics - Department of Computer Science

Transcript 7361 Computer Graphics - Department of Computer Science

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