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PerceptionPerception

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PerceptionPerception

“The consciousness or awareness of objects or other data through the

medium of the senses.”

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PerceptionPerception

In Multimedia Systems 1 we have already covered the physics of sound and its perception

Here we will deal with visual perception It is important when designing

multimedia to have a fundamental grasp of visual perception and the physiological aspects of sight

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Electromagnetic SpectrumElectromagnetic Spectrum

Visible light is only one component of a wide range of waveforms known as the electromagnetic spectrum

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Speed of LightSpeed of Light

All electromagnetic waves travel at the speed of light (c)

c = 300,000,000ms-1

c = f

300,000,000ms-1 / 900,000,000Hz = 0.333m

0.333m is the wavelength () of what?

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Visible LightVisible Light

Visible light has a wavelength from 400 – 700 nanometers

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Colour VisionColour Vision

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Colour VisionColour Vision

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Measurement of LightMeasurement of Light

Light is often measured in terms of its:Radiance (watts)– Total amount of energy

emittedLuminance (lumens) – Light strength that is

perceived by the human eyeBrightness – Subjective measure of how bright

an object appears to be

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Colour VisionColour Vision

Most colours can be made by combining different amounts of Red, Green and Blue (RGB) light

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Colour VisionColour Vision

There are different models for measuring colour depending on the sourceActive sources such as monitors and

projectors produce colours by emitting different wavelengths of light

Passive sources such as printed documents produce colours by absorbing wavelengths and reflecting others

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Passive DisplaysPassive Displays

Passive displays have the primary colours Cyan, Magenta and Yellow (CMY)

This known as the subtractive colour system as these colours absorb (subtract) the colours Red, Green and Blue respectively

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Passive DisplaysPassive Displays

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Active DisplaysActive Displays

Active displays emit combinations of Red, Blue and Green light

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Colour GamutColour Gamut

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Colour SensitivityColour Sensitivity

The eye is not equally sensitive to all colours

Intensity is the weighted sum of RGB components:

For a particular colour the intensity is:0.299 x Red

+ 0.587 x Green

+ 0.114 x Blue

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Psychophysical Colour Models

Psychophysical Colour Models

Based on human perception of colours.HLS

• Hue – The pure dominant colour• Lightness • Saturation - How much the colour is diluted by white

light

HSV• Hue• Saturation• Intensity (Value)

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Representing Digital ImagesRepresenting Digital Images

Digital images are composed of pixels (or picture elements)

Picture resolution is the number of pixels or samples used to represent the image

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Representing Digital ImagesRepresenting Digital Images

Digital images are often referred to as as bitmaps

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Representing Digital ImagesRepresenting Digital Images

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Representing Digital ImagesRepresenting Digital Images

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True Colour Image Data Model

True Colour Image Data Model

Y

X

Pixel

Image

R G B

Colour

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True ColourTrue Colour

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Palette Colour Image Data Model

Palette Colour Image Data Model

Y

X

Pixel

Image

Index

R G B

Palette

R G B

Colour

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Palette ColourPalette Colour

165 16 49

R G B

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True Colour Vs. Palette Colour

True Colour Vs. Palette Colour

True Colour:High qualityExpensive hardwareRequire more memory (normally 3 times)

Palette ColourLimited number of coloursCheap hardwareEnables easier implementation of certain

techniques

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ResolutionResolution

Image resolution determines: the amount of detail storage requirements

Here is a digitised image with a resolution of:300 x 466139,800 pixels

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ResolutionResolution

Here is the same digitised image, but with a resolution of:75 x 1158625 pixels

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ResolutionResolution

Here is it is again with a resolution of:43 x 672881

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Resolution and Digital Cameras

Resolution and Digital Cameras

Digital camera resolution ranges from approximately 1 – 6 megapixels

A 2 megapixel camera will have a resolution of 1600x12001,920,000 pixels

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Aspect ratioAspect ratio

Aspect Ratio expresses the resolution of an image as: number of horizontal pixels x the number of vertical

pixels It is a format attribute

Typical ratios: 320 x 240, 640 x 480, 800 x 600, and 1024 x 768,

1600 x 1200, 1792 x 1344, 2048 x 1536 – all of which are 4:3

1280 x 1024 (5:4)768 x 576 (4:3) standard PAL TV format1024 x 576 (16:9) wide screen TV

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FinFin

Fin