# Unit 3

Topics: Computer graphics, Color, Pixel Pages: 18 (3419 words) Published: August 18, 2013
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Formatted on September 11, 2000

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The Nature of Digital Images Vector Graphics Colour Systems Some Image Techniques Image And Graphics File Formats Digital Image Processing Image And Graphics Software Exercises 2 7 10 16 18 20 21 22

¯ To understand how computers process
images and graphics

¯ To understand how computers work with
colours

¯ To understand the differences between
images and graphics

Department of Computer Science

COMP3600/SCI2600 Multimedia Systems 3. Images and Graphics (200009) Slide: 1

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An image is a spatial representation of an object, a two-dimensional or three-dimensional scene or another image. Often the images reﬂect the intensity of lights. Most photographs are called continuous-tone images because the method used to develop the photograph creates the illusion of perfect continuous tone throughout the image. Images stored and processed by computers, displayed on computer screens, are called digital images although they often look like continuous-tone. This is because they are represented by a matrix of numeric values each represents a quantised intensity values. ½º½ × ÓÒ ÔØ×

The smallest element on a digital image is known as a pixel — a picture element. A digital image consists of a (usually rectangular) matrix of pixels.

Height

Height

Depth Width
Width

Depth

Department of Computer Science

COMP3600/SCI2600 Multimedia Systems 3. Images and Graphics (200009) Slide: 2

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The depth of an image is the number of bits used to represent each pixel. 1-bit black-and-white image, also called bitmap image.

4-bit

can represent 16 colours, used in low resolution screens(EGA/VGA) 8-bit can have 256 colours. The 256 colour images are often known as indexed colour images. The values are actually indexes to a table of many more different colours. For example, Colour 3 is mapped to (200, 10, 10). 8-bit grey 256 grey-levels. The image contains only brightness/intensity data without colour information.

0 3 1 2 3 253 254 255

(255, 0, 0) (255, 0, 10) (200, 0, 0) (200,10, 10) (10,10, 10) (20, 0, 50) (0,100, 100)

255 0 0 0 0 0 0 128

0 255 0 0 0 0 128 0

0 0 0 255 0 128 0 0

0 0 255 0 255 0 0 0

0 255 0 128 0 255 0 0

0 0 255 0 255 0 0 0

0 128 0 255 0 0 255 0

128 0 0 0 0 0 0 255

Department of Computer Science

COMP3600/SCI2600 Multimedia Systems 3. Images and Graphics (200009) Slide: 3

16-bit can have 65536 colours, also known as hi-colour in Windows systems. The 16 bits are divided into 5 bits for RED, 6 bits for GREEN and 5 bits for BLUE. 24-bit ¾¾ ½ ¾½ colours, true colour. Each byte is used to represent the intensity of a primary colour, RED, GREEN and BLUE. Each colour can have 256 different levels.

32-bit ¾¿¾ ¾ ¾ (4G). Usually, 3 bytes are used to represent the three primary colours and the fourth byte is used as the alpha channel.

RED 255 0 0 255 255 0 127 255 0

GREEN 0 255 0 255 0 255 127 255 0

BLUE 0 0 255 0 255 255 127 255 0

Colour Red Green Blue Yellow Magenta Cyan Light gray White Black

Department of Computer Science

COMP3600/SCI2600 Multimedia Systems 3. Images and Graphics (200009) Slide: 4

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Resolution measures how much detail an image can have. There are several resolutions relating to images. Image resolution is the number of pixels in an image. ¿¾¼

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Display (Monitor) resolution — refers to number of dots per inch (dpi) on a monitor. Windows systems usually have 96dpi resolution. Some high resolution video adapters/monitors support 120dpi. For example, a ¾ ¢ ¾½ image displayed on a monitor ¼¼ with 96dpi will be ¿¼¼ ¢ ¾ ½ . Output resolution — refers to number of dots per inch (dpi) on a (hard copy) output device. Many printers have 300dpi or 600 dpi resolution. High-quality imagesetters can print at a range...