Graphics

Computer Graphics – Notes – Short Q & A’s

When it comes to scoring marks, short questions play a real significant role. Find answers like – aspect ratio, resolution, persistence and many more – rewritten in a simplified language.  Whats more these questions are picked from final question papers of various universities or schools.

What does resolution of a CRT depends on?

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

What is Persistence?

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.

Refresh Rate

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.

What is Resolution?

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.

What is Aspect Ratio?

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.

What is Refresh CRT?

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.

Horizontal Retrace

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.

Vertical Retrace

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.

Interlacing Scan or solution to flicker of the screen

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.

How long will it take to load 640 x 480 frame buffer with 12 bits per pixel. If 100000 bits can be transferred per second.

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

Find the aspect ratio of the raster screen using a 8 x 10 inches screen and 100 pixels per inch.

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.

sr_2018

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