Computer Graphics- SCC 342

Chapter 2: Computer Graphics Hardware

Dr. Doaa Hegazy

Overview of a graphics system

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2.1 Video Display Devices

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Cathode-ray-tube (CRT)

CRT basics:• The screen output is stored in the frame buffer and

is converted into voltages across the reflection platesvia a digital-to-analog converter (DAG)

• Light is emitted when electrons hit phosphor• But light output from the phosphor decays

exponentially with time, typically in 10 – 60microseconds– Thus the screen needs to be redrawn or refreshed– Refresh rate is typically 60 Hz to avoid flicker (“twinkling”)– Flicker: when the eye can no longer integrate individual light pulses from a point on screen, e.g., due to low refresh rate

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2.1 Video Display Devices

Shadow-mask color CRTs• Three different colored phosphors (R, G, B)• dots are arranged in very small groups (triads) on

coating

We see a mixture of threecolorsThree electron guns (R, G,B) emit electron beams in acontrolled fashion so thatonly phosphors of theproper colors are excited

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2.1 Video Display Devices

2.1 Video Display Devices

• Raster – Scan Display

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2.1 Video Display DevicesThe Frame Buffer• Stores per-pixel information

– Depth of a frame buffer: number of bits per pixel– E.g. for color representation, 1 bit => 2 colors,8 bits => 256 colors,24 bits => true color (16 million colors)

• Color buffer is only one of many buffers, otherinformation, e.g., depth, can also be used

• Implemented with special type of memory instandard PCs or on a graphics card for fastredisplay

• Part of standard memory in earlier systems

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2.1 Video Display Devices

Raster-scan basics:• The screen is a rectangular array

of picture elements, or pixels• Resolution: determines the

details you can seenumber of pixels in an image,

• e.g.,1024×768, 1280x1024, 1366 x 768, etc.

• also in ppi or dpi – pixel or dot per inch

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Raster-Scan Pattern:• Horizontal scan rate: # scan lines per second• Interlaced (TV) vs. non-interlaced displays

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2.1 Video Display Devices

2.1 Video Display DevicesRandom-Scan Display• Also called Vector-scan display• Pictures are generated as line drawings

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Flat-Panel Display• A class of video devices that have reduced

volume, weight and power• Two categories: – Emissive displays: convert electrical energy to light

(e.g. Plasma panels and Light-emitting diode (LED)– Nonemissive displays: use optical effect to convert

light from a certain source into graphics patterns (e.g. Liquid-crystal display (LCD)

2.1 Video Display Devices

2.2 Raster-Scan Systems

• Simple raster-graphics system• Video controller (display processor)

controls operations of display device.

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• Raster system with fixed portion of main memory reserved for the frame buffer.

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2.2 Raster-Scan Systems

2.2 Raster-Scan Systems

• Video controller

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2.2 Raster-Scan Systems

• Raster – scan display processor- Digitizes picture definition into a set of pixel values for storage in the frame buffer (scan conversion).

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• Additional functions for Raster – scan display processor- generating line styles- display color areas- transformations- interface with interactive devices.

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2.2 Raster-Scan Systems

• Input devices include• Pointing/locator devices:• indicate location on screen

– Mouse/trackball/spaceball– Data tablet– Joystick– Touch pad and touch screen

• Keyboard device: send character input• Choice devices: mouse buttons, function keys

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2.4 Input Devices