1 Perception

2 “The consciousness or awareness of objects or other data through the medium of the senses.”

3 Perception  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

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

5 Speed 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?

6 Visible Light  Visible light has a wavelength from 400 – 700 nanometers

7 Colour Vision

8

9 Measurement of Light  Light is often measured in terms of its:  Radiance (watts)– Total amount of energy emitted  Luminance (lumens) – Light strength that is perceived by the human eye  Brightness – Subjective measure of how bright an object appears to be

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

11 Colour Vision  There are different models for measuring colour depending on the source  Active 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

12 Passive 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

13 Passive Displays

14 Active Displays  Active displays emit combinations of Red, Blue and Green light

15 Colour Gamut

16 Colour 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: x Red x Green x Blue

17 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)

18 Representing 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

19 Representing Digital Images  Digital images are often referred to as as bitmaps

20 Representing Digital Images

21 Representing Digital Images

22 True Colour Image Data Model

23 True Colour

24 Palette Colour Image Data Model

25 Palette Colour R G B

26 True Colour Vs. Palette Colour  True Colour:  High quality  Expensive hardware  Require more memory (normally 3 times)  Palette Colour  Limited number of colours  Cheap hardware  Enables easier implementation of certain techniques

27 Resolution  Image resolution determines:  the amount of detail  storage requirements  Here is a digitised image with a resolution of:  300 x 466  139,800 pixels

28 Resolution  Here is the same digitised image, but with a resolution of:  75 x 115  8625 pixels

29 Resolution  Here is it is again with a resolution of:  43 x 67  2881

30 Resolution and Digital Cameras  Digital camera resolution ranges from approximately 1 – 6 megapixels  A 2 megapixel camera will have a resolution of 1600x1200  1,920,000 pixels

31 Aspect 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 format  1024 x 576 (16:9) wide screen TV

32 Fin