Computer Graphics – Notes – Short Q & A’s

Kind of Phosphor is used in CRT
Deflection System
Focusing System
Intensity to be displayed

The time it takes for a screen emitted light to become 1/10th of its original intensity. For example if the intensity of the light is 200 units when it was emitted from CRT Screen and it decays to 20 units in 5 seconds. So Persistence is 5 seconds. We can see more the persistence less refresh of the screen is required.

Number of times the phosphor gets refreshed is called refresh rate. Its measured in Hertz. Most common refresh rate is 60Hz. Notice here and try to establish a link between persistence and refresh rate. If persistence of a screen is low then it would require higher refresh rate to display the screen without flicker. Well opposite is also true – More persistence and less would be refresh rate.

The maximum number of the pixels that can be displayed on the screen without overlap. For example- if the resolution of the screen is 600 x 400 that means 24000 pixels can be displayed on the screen without overlap.

To draw a same length line vertically and horizontally on the screen pixels would be needed, the ratio of number of pixels needed to draw same length line vertically and horizontally is called aspect ratio. Standard format for this is 4:3. The ratio can be horizontal to vertical or vertical to horizontal.

Refresh CRT means to redraw the picture on the screen by hitting the same pixel on the screen with electron gun repeatedly. It helps in phosphor glowing continuously.

When electron gun returns to the left of the screen after refreshing the scan line. In other words every time the horizontal retrace happens refreshing of new scan line starts.

Screen is also known as Frame, so every time the refreshing of the frame is done the electron gun comes back to top left corner of the screen to start a new frame. This movement of the screen is known as Vertical Retrace. Generally electron gun takes 1/80th or 1/60th of second to finish up one frame.

This technique reduces the flicker on the screen. Divide the frame in two parts – Even and Odd scan lines
The even scan lines are refreshed first and then odd scan lines in the second round by the electron gun. The image produced such a way is called interlaced image.

The total time taken = (640 x 480 x 12) / 100000 = 36.8 Seconds

The resolution of the picture is also defined as number of pixels per inch on the screen.
Size of the image = 8 x 10 inch
Resolution = 100 pixels per inch
Resolution of the screen = 800 x 1000
Aspect Ratio : 800/1000 = 4/5 there fore 4:5.