Outline

1
Outline

• Image formats and basic operations
• Image representation
• Basic operations

2
Four Stages of Visual Processing

• Image-based stage
• Surface-based stage
• Object-based stage
• Category-based stage

3
Problems in Computer Vision

• Classification
• Edge detection
• Segmentation
• Recognition
• Motion analysis

4
Digital Image Formation

• How to acquire a digital image Digital cameras
• The continuous spatial space is digitized into
  small cells
• The color of a cell is all the photons fall into
  the cell within a very short period of time
• Charge Coupled Device  - CCD cameras
• An electronic memory which is able to be charged
  by light

5
Image Representation

• Mathematically, an image can be represented by a
  2-D matrix
• Each entry (i,j) represents the value at the
  corresponding location, which is called a pixel
• The value of a pixel can have different types,
  depending on the image types
• Unsigned char
• Int
• Float
• A vector (Color image, for example)

6
Image Sequence Representation

• Mathematically, an image sequence is represented
  as a 3-D matrix
• At any time, it is a 2-D matrix
• A commonly used format is animated gif

7
Image as a File

• Image formats
• TIFF
• GIF
• JPEG
• PBM

8
Image as a File cont.

• Image file consists of two parts
• Header
• Type of the image
• Size of the image
• Color map
• Data
• Uncompressed
• Compressed

9
PBM Format

• Portable Bitmap Utilities
• Header
• A two-letter magic number that identifies the
  format type
• File parameters
• Height, width, maximum value of a pixel of the
  image
• Data
• In ASCII or binary, depending on the format type

10
Portable Gray Map

• Binary PGM format
• P5
• Width
• Height
• Max
• Pixels as binary values

11
Portable Pixel Map

• Binary PPM format
• P6
• Width
• Height
• Max

12
Programs to Read/Write Images

• These functions are based on my program
• type-def.h defines the structures
• imageio.c defines the functions to read and write
  images of different types
• util.c defines functions to dynamically allocate
  memory needed for an image

13
Image Sequence

• There are many different kinds of formats for
  movie or video sequence
• avi
• mpeg
• An easy way to generate a video sequence is to
  concatenate images together
• Animated gif is such a format

14
Visualization of Images

• xv
• A utility for X-window system
• Can display and convert many different kinds of
  image formats
• ImageMagick package
• convert
• display
• animate
• available at http//www.simplesystems.org/ImageMag
  ick/