1. COMS 161 Introduction to Computing
Title: Digital Images
Date: February 23, 2005
Lecture Number: 17

2. Announcements
Homework
Due 2/25/05

3. Review The nature of images How digital images are
Natural vs. artificial images
How digital images are
Organized
Created
Stored
Processed

4. Outline Color Images Color Models Storing Images RGB CMYK HSB
Color index

5. Color Images
High-quality digital cameras, scanners, and other image acquisition devices capture millions of colors
There are several different color models
Methods to represent colors
Color models map colors onto 2D, 3D, and 4D Cartesian space
Color space
Color components are called color channels

6. RGB Color Model RGB color model is additive
RGB color model represents color as a combination of three channels: RED, GREEN, and BLUE
RGB color model is additive
RGB color is in most color video displays
Red, Green, and Blue are the primary colors
Other colors are secondary (mixed)
Colors are emitted in the RGB Color Model

7. RGB Color Images

8. RGB Color Images RGB images contain data representing each channel
1 byte for each channel
3 bytes/pixel
200 X 200 RGB image requires
200 X 200 X 3 bytes
120,000 bytes

9. RGB Color Space Origin (0, 0, 0) is black Opposite corner is white
No color
Opposite corner is white
All colors
Solid cube of colors

10. CMYK Color Model Cyan, Magenta, Yellow & Black
Primarily used in color printing
cyan, magenta, and yellow (CMY) are primary colors
red, green, and blue are secondary colors
Black is added to improve printing
CMYK is a subtractive color model
Ink absorbs light, the color we see is the color of the reflected light, not the absorbed color

11. CMYK Color Model
Shine a white light on a an area and the color seen is cyan
The red component of the white light is absorbed leaving only cyan to be reflected
Cyan absorbs red
Magenta absorbs green
Yellow absorbs blue
CMY is the anti-color space of RGB
C = 255 – R, M = 255 – G, Y = B

12. CMYK Color Model
CMYK image

13. CMYK Images
CYAN channel

14. CMYK Images
MAGENTA channel

15. CMYK Images
YELLOW channel

16. CMYK Images
BLACK channel

17. CMYK Color Space White is (0,0,0) Black is (1,1,1)
Black is the absence of color

18. HSB Color Model RGB and CMYK are not the way we think of color
Paint your room
Think of the color you want it to be
How bright we want that color to be
If it should be pastel or vivid

19. HSB Color Model HSB resembles how artists conceive color properties
Hue: corresponds to what we think of as color
Saturation: purity of the hue
Highly saturated colors are vivid
Lowly saturated colors are washed out
Grayed out
Pastels
Brightness: relative amount of whiteness or lightness of a color

20. HSB Color Model HSB color is depicted by a 3-D polar coordinate system
Increasing brightness adds white to the hue
Decreasing brightness adds black to the hue

21. HSB Color Model

22. Storing Digital Images
A single hue depicted with a series of changing brightness and saturation values

23. Indexed Color Images
Most digital color images do NOT use all possible colors that can be represented in their color model
RGB model
8 bits of R, G, and B
224 colors = 16 million different colors
In fact, the human eye cannot distinguish this many different colors!!

24. Indexed Color Images
Indexed color images store different colors in a look-up table
Pixel values are indices into the look-up table
Red
Yellow
Tan
Purple
Sky Blue .
255 2 1

25. Indexed Color Images
INDEXED COLOR images are smaller or more compact in storage
Composed of pixels selected from a limited palette of colors or shades