Introduction to Multimedia Computing

Transcription

1 COMP 319 Lecture 02 Introduction to Multimedia Computing Fiona Yan Liu Department of Computing The Hong Kong Polytechnic University

2 Learning Outputs of Lecture 01 Introduction to multimedia technology What is multimedia History of multimedia technology Reference reading: Chapter 1 Introduction to html What is html Html document with web generation Reference reading: Chapter 1 Introduction to COMP319 main.html 2

3 Outline of Lecture 02 Fundamentals of image data representation ti Standard image Gray level image Bitmap and bitplane Color image Light and human vision Different color models 24 bit color image and 8 bit color image Histogram and color lookup tables Some popular p image file formats Reference reading Chapter 3 & Chapter 4 3

4 Outline of Lecture 02 Fundamentals of image data representation ti Standard image Gray level image Bitmap and bitplane Color image Light and human vision Different color models 24 bit color image and 8 bit color image Histogram and color lookup tables Some popular p image file formats Reference reading Chapter 3 & Chapter 4 4

5 Fundamentals of Image Data Representation Images consist of pixels The smallest discrete component of an image on the screen Image resolution The number of pixels in a digital image Standard images Illustrate algorithms and compare the performance Lena: for gray level image generally Baboon: for color image generally 5

6 Standard Images Lena: Image Resolution is 256 * 256 Baboon: Image Resolution is 512* 512 6

7 Outline of Lecture 02 Fundamentals of image data representation ti Standard image Gray level image Bitmap and bitplane Color image Light and human vision Different color models 24 bit color image and 8 bit color image Histogram and color lookup tables Some popular p image file formats Reference reading Chapter 3 & Chapter 4 7

8 Gray Level Image Image is represented using luminance information only 8 bit gray level image Each pixel has a gray value between 0 and bit color Image Gray Image 8

9 Binary Image Each pixel is stored as a single bit 0/1. Also referred as 1 bit image Use for the pictures containing simple graphics or text 9

10 Bitmap of Gray level Image The two dimensional array of pixel values that represent the images/graphics 10

11 Bitplane of 8 bit image Bitplanes Consider the 8 bit image as a set of 1 bit bitplanes Each plane consists of a 1 bit representation ti of the image 11

12 Outline of Lecture 02 Fundamentals of image data representation ti Standard image Gray level image Bitmap and bitplane Color image Light and human vision Different color models 24 bit color image and 8 bit color image Histogram and color lookup tables Some popular p image file formats Reference reading Chapter 3 & Chapter 4 12

13 Light and Prism Light is an electromagnetic wave White light contains all the colors of a rainbow Sir Isaac Newton's experiment 13

14 Light and Spectral Power Distribution Most light sources produce contributions over many wavelengths SPD shows the relative amount of light energy Spectral power distribution of daylight 14

15 The Color of the Light The color of the light is characterized by the wavelength of the light Short wavelengths produce a blue sensation, long wavelengths produce a red one Spectral power distribution of daylight 15

16 Human Vision Humans cannot detect all light Visible light is an electromagnetic wave in the range 400 nm to 700 nm Human vision is formed Sensor: Eye Most sensitive to red(r), green(g), and blue(b). Processor: Brain RGB R,G,B R G, G B, and B R 16

17 Human Retina Human retina consists of an array of rods and three kinds of cones Rod Cones Detect gray level information Three kinds of cones are used to detect R,G,B, The proportions of R,G,B cones are 40:20:1 The eye is most sensitive to light in the middle of the visible spectrum 17

18 RGB Color Model Three primaries Red, green and blue Additive color model When two light beams impinge on a target, their colors add 18

19 24 bit Color Image of RGB Model R G B Blue 8 bit 8 bit 8bit 24-bit color Image Red Channel Channel Green Channel 19

20 24 bit Color Image Each pixel is represented by three bytes, usually RGB. Supports 256*256*256, totally 16.8 million possible combined color. Storage: for image resolution of 640*480, needs 921.6KB Some 24 bit color images are stored as 32 bit image Extra byte of data for special effect information 20

21 Color Lookup Tables (CLTs) Use index or code value instead of 24 bit color information for each pixel Color lookup the table works well for small combinations 21

22 Image Histogram Refers to the probability bilit mass function of the image intensities The number of containers is equal to six 6 Original data: The number of containers is equal to three

23 8 bit Color Image Divide id the RGB cube into equal slides in each dimension R: 3 bit; G: 3 bit; B: 2 bit; Edge artifacts 24-bit color image 8-bit color image 23

24 L*a*b* Color Model Perceptual uniform Color differences a human perceives as equal correspond to Euclidean distances RGB color model is not perceptual uniform L*a*b* color model is perceptual uniform L*: luminance a*: red/green balance b*: green/blue balance 24

25 RGB and CIE L*a*b 25

26 HSV Color Model Approximation of perceptual uniform Hue: position in the color spectrum Saturation: the vividness of a color Value: the brightness of the color 26

27 YUV Color Model Be used din JPEG Y: luminance value Luma Y : (gamma corrected) U and V: chrominance The difference between a color and a reference white at the same luminance U = B Y V = R Y 27

28 Gamma Correction Human s response is not linear to the driving voltage of display device 28

29 YUV Color Model 29

30 YC b C r color model YC b C r color model is closely related to the YUV transformation Be used in MPEG 30

31 YIQ Color Model YIQ is used in NTSC color TV broadcasting Y in YIQ is the same as in YUV I and Q are a rotated version of U and V U and V are rotated by

32 YIQ color model I and Q in YIQ model U and V in YUV model 32

33 Subtractive Color Model Additive i color model When two light beams impinge on a target, their colors add Subtractive ti color model For ink deposited on paper, opposite situation holds RGB color model CMY color model 33

34 CMY Color Model and CMYK Color Model CMY color model consist of Cyan(C), Magenta(M) and Yellow(Y) Subtractive color primaries Transformation from RGB to CMY The transformation is invertible CMYK color model Add one color: real black Using black ink is cheaper and better than using the mixing colored ink 34

35 Outline of Lecture 02 Fundamentals of image data representation ti Standard image Gray level image Bitmap and bitplane Color image Light and human vision Different color models 24 bit color image and 8 bit color image Histogram and color lookup tables Some popular p image file formats Reference reading Chapter 3 & Chapter 4 35

36 Some Popular Image File Formats BMP BitMap Mainly use RGB color model Gif Graphics Interchange Format Important tformats because of fits historical i connection to the WWW and HTML PDF Portable Document Format Include compression JPEG Joint Photographic Experts Group Currently the most important common file format 36

37 Announcement of Further Arrangement Lab Thur. 16:30 17:20 PQ604B Tue. 9:30 10:20 PQ606 Quiz The first quiz will be next Thur. Sept. 18 8:30 9:00 Cover Lecture 1 2 Form of the questions True or False Answer the questions Calculation Sept. 11, 2014 Lecture 02: Color Model and Human Vision 37