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1. UNIT-1
Basics of Computer Graphics
By: Medhavi Pandey

2. OUTLINE
⮚Display Devices
Types of display devices:
• CRT
• Color CRT Monitors
1. Beam Penetration
2. Shadow Mask
• Direct View Storage Tube (DVST)
• Flat Panel Display
• Raster Scan
• Random Scan

3. CRT
• A CRT is an evacuated glass tube.
• An electron gun at the rear of the tube produces a beam of
electron which is directed towards the front of the tube (screen)
by a high voltage.
• The inner side of the screen is coated with phosphor substance
which gives off light when it is stroked by electron.
• The control grid voltage determines the velocity achieved by the
electrons before they hit the phosphor.

4. CRT Diagram

5. CRT
• The control grid voltage determines how many electrons are actually in the
electron beam.
• Thus, control grid controls the intensity of spot where the electron beam is
strike the screen.
• The focusing system concentrates the electron beam so that the beam
converges to a small point when it hits the phosphor coating.

6. CRT
• The deflection system of the CRT consists of two pairs of parallel plates,
referred to as the vertical and horizontal deflection plates.
• The voltage applied to vertical deflection of the electron beam and voltage
applied to the horizontal deflection plates controls the horizontal deflection of
the electron beam.
• There are two techniques used for producing images on the CRT screen :
Random Scan (Vector scan) and Raster Scan.

7. Color CRT Monitors
• A CRT monitor displays color pictures by using a combination of
phosphors that emit different color lights.
Methods :-
1. Beam Penetration
2. Shadow Mask

8. Beam Penetration Method
• Two layers of phosphor (red and green) are coated onto the inside of
the CRT screen.
• The display color depends on how far the electron beam penetrates
into the phosphor layers.
• The speed of the electrons, and the screen color at any point, is
controlled by the beam acceleration voltage.
• The beam penetration method:
• Used with random scan monitors.
• Only four colors are possible (red, green, orange, and yellow).
• Quality of pictures is not as good as with other methods.

9. Shadow Mask Method
• The color CRT has:
• Three color phosphor dots (red, green and blue) at each point on the screen
• Three electron guns, each controlling the display of red, green and blue light.

10. Shadow Mask Method
• When the three beams pass through a hole in the shadow mask, they
activate a dot triangle, which appears as a small color spot on the screen
• We obtain color variations by varying the intensity levels of the three
electron beams
• High-quality raster-graphics systems have 24 bits per pixel in the frame
buffer and nearly 17 million color choices for each pixel(full-color or true-
color system).

11. Shadow Mask Method
• We obtain color variations by varying the intensity levels of the three
electron beam.
• Advantages:
• Used in raster scan system (including color TV).
• Designed as RGB monitors.
• High quality raster graphics system have 24 bits per pixel in the frame
buffer (a full color system or a true color system).

12. Direct View Storage Tube
• It is rarely used now-a-days as part of display system.
• In DVST there is no refresh buffer; the images are created by drawing
vectors or line segment with relatively slow moving electron beam.
• It is one of the display devices in which an electron flood gun and writing
gun is present.
• The flood gun floods electrons to a wire grid on which already the writing
gun has written some image.

13. DVST Diagram

14. Direct View Storage Tube
• The electrons from the flood gun will be repelled back by the
negatively charged wire grid which has been charged so by the
writing electron beam.
• The part of the wire grid which has not been charged -ve will allow
the electrons to pass through and the electrons will collide on the
screen and produce the image.

15. Direct View Storage Tube
Advantages:-
• Refreshing CRT is not required.
• Complex picture can be displayed in high resolution without flicker
• It has flat screen.
Disadvantages –
• Do not display color and the selected parts of a picture cannot be erased.
• Erasing and redrawing take several seconds for complex pictures.

16. Flat Panel Display
• Flat Panel Display:
• It refers to a class of video devices that have a:-
• reduced volume, smaller, thinner
• reduced weight and
• reduced power requirements compared to a standard CRT
• Examples are LCD, Plasma display panel, LED ,panel and thin CRT.

17. Flat Panel Display
• Current usage:
• Small and big TV monitors
• Pocket video games
• Laptop computers
• Advertisement boards in
• elevators and showrooms.
• Portable monitors.

18. Raster Scan System
• In a raster scan system, the electron beam is swept across the screen,
one row at a time from top to bottom.

19. Raster Scan System
• As the electron beam moves across each row, the beam intensity is
turned on and off to create a pattern of illuminated spots.
•Picture definition is stored in a memory area called the refresh buffer
or frame buffer.
•Refresh buffer or frame buffer: This memory area holds the set of
intensity values for all the screen points.

20. Raster Scan System
• Stored intensity values then retrieved from refresh buffer and
“painted” on the screen one row (scan line) at a time.

21. Raster Scan System
• Intensity range for pixel positions depends on the capability of the
raster system.
• A black-and-white system: each screen point is either on or off, so
only one bit per pixel is needed to control the intensity of screen
positions.
• On a black-and-white system with one bit per pixel, the frame buffer
is called bitmap.
• For system with multiple bits per pixel, the frame buffer is called
pixmap.

22. Raster Scan System
• Sometimes, refresh rates are described in unit of cycles per second, or
Hertz (HZ).
• Refreshing on raster scan displays is carried out at the rate 60 to 80
frame per second

23. Raster Scan System
• Horizontal retrace: The return to the left of the screen, after
refreshing each scan line.

24. Raster Scan System
• Vertical retrace: At the end of each frame (displayed in 1/80th to 1/60th
of a second) the electron beam returns to the top left corner of the
screen to begin the next frame.

25. Interlacing
• On an older, 30 frame per-second, non-interlaced display, some
flicker is noticeable.
• With interlacing, each of the two passes can be accomplished in
1/60th of a second.
• An Effective technique to avoid a flicker.

26. Raster image
• The quality of a raster image is determined by the total number pixels
(resolution), and the amount of information in each pixel (color
depth).

27. Raster image
• Raster graphics cannot be scaled to a higher resolution without loss
of apparent quality.

28. Random Scan?