1. Sampling, Quantization, Color Models & Indexed Color
CS450 Lecture 2
Sampling, Quantization,
Color Models & Indexed Color

2. Digital Image Sources Digital Cameras Scanned Film & Photographs
Digitized TV Signals
Computer Graphics
Radar & Sonar
Medical Imaging Devices (X-Ray, CT)
The Internet

3. Digitizing Images Images are digitized using a two step process:
sampling the continuous tone image
quantizing pixels

4. Sampling

5. Quantization
pixel’s samples are averaged

6. Quantization Example

7. Image Resolution 68 x 104 136 x 208 272 x 416 less detail more detail
less storage more storage

8. Digital Cameras Low-Res 640x480 (TV grade) Medium Low-Res 1536x1024
Medium-Res 2048x1536
Medium Hi-Res 2560x1920
Hi-Res 3000x2000 up to 10 Megapixel

9. Dynamic Range The number of quantized pixel values: 256 levels

10. Images - 2D array of values
Binary Images (pixel values = 0,1)
Grayscale Images (pixel values = 0-255)
Color Images
Each pixel has three color components
For example, (red, green, blue) or RGB
Each color component is 0-255

11. Color Images
3 Images Overlayed
Blue
Green
Red

12. Histograms: What’s in the image?
What is a histogram?
Simple numeric example
Histogram 1 3 21 31 21 4 2 1 2

13. Color Image Histograms
Histogram for each color

14. RGB Additive Color Model
RED
GREEN
BLUE
bright values => high amounts of that color
dark values => low amounts of that color

15. CMYK Subtractive Color Model
CYAN
MAGENTA
Bright => use less of
that ink color
Dark => use lots of
YELLOW
BLACK

16. HSB Visual Color Model
HSB: how artists perceive color properties

17. HSB Visual Color Model HSB: how artists perceive color properties 360o
Hue
Select Hue

18. HSB Visual Color Model HSB: how artists perceive color properties 360
Saturation o
Hue
Select Hue
- then click in box for saturation, brightness

19. HSB Visual Color Model HSB: how artists perceive color properties 360
Saturation
Brightness o
Hue
Select Hue
- then click in box for saturation, brightness

20. Digital Image Formats GIF (Graphic Interchange Format)
indexed color (up to 256 colors)
compressed
used in Web applications
JPEG (Joint Photographic Experts Group)
lossy compression with variable controls
also used in Web applications

21. Digital Image Formats PNG (Portable Network Graphics) BMP
designed for online viewing (e.g., Web)
patent-free replacement for GIF
lossless compression
BMP
MS Windows image format

22. Indexed Color
“Indexed Color” can be used to reduce the size of a color image file
255
= 27 bytes
= 18 bytes 1 2

23. Indexed Color Images are derived from full color images
are smaller or more compact in storage
are composed of pixels selected from a limited palette of colors or shades