1 Perception
Dec 22, 2015
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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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