1. COMS 161 Introduction to Computing
Title: Digital Images
Date: February 25, 2005
Lecture Number: 18

2. Announcements
Homework
Due 2/25/05

3. Review Color Images Color Models Storing Images RGB CMYK HSB
Color index

4. Outline Storing Images in Files File format Format types
Gif
Jpeg
Lossy compression
Lossless compression

5. Storing Digital Images
Digital images are converted to files for storage and transfer
File type
A special format for ordering and storing the bytes that make up the image
There are many different file or format types to represent digital images
Different formats are not often compatible

6. Storing Digital Images
Applications specify the image formats it accepts
Most formats have a
Header
Image data

7. Storing Digital Images
Headers contain
Number of
rows (height)
columns (width)
bands
bits per pixel
file type (magic number)
type of compression

8. Storing Digital Images
GIF (Graphic Interchange Format)
Indexed color image (up to 256 colors)
Compressed
Used in Web applications
JPEG (Joint Photographic Experts Group)
Lossy compression with variable controls
Also used in Web applications

9. Compressing Images Some image file types compress the image data
Lossless
Lossy compression

10. Compressing Images JPEG (pronounced "jay-peg")
A standardized image compression mechanism
Stands for Joint Photographic Experts Group
The original name of the committee that wrote the standard
Designed for compressing
Full-color or gray-scale images of natural, real-world scenes
Handles only still images
A related standard (MPEG) for motion pictures 7

11. Compressing Images JPEG is "lossy“
Decompressed image is an approximation to the original image
Compression exploits known limitations of the human eye
Small color changes are perceived less accurately than small changes in brightness
The degree of lossiness can be varied by adjusting compression parameters
Trade-off between file size and image quality
Decoders can trade off decoding speed against image quality 7

12. Compressing Images JPEG advantages Disadvantages Smaller files
Data storage
Network transmission
Can store RGB 24-bit color images
gif images are only 8-bit
Disadvantages
Longer to view a file
Decompress the data 7