Chapter 2 :Imaging and Image Representation

Computer Vision Lab.Chonbuk National Univers

ity

Contents

2.1 Sensing Light

2.2 Image Device

2.3 Problems in Digital Images*

2.4 Picture Function and Digital Image

2.5 Digital Image Formats *

2.6 Richness and Problems of Real Imagery

2.7 3D Structure from 2D Images

2.8 Five Frames of Reference

2.9 Other Types of Sensors *

2.1 Sensing Light

• Simple model of common photography

(Sun or Flash bulb)

Reflects radiationToward camera

Sense via chemical on film

2.2 Imaging devices

• CCD Camera

2.2 Imaging devices

• Frame Buffer – High speed image store

available : Actually Store several Images or their derivatives

• Digital Image refer to pixel values as I[r,c]– I : array name– r : row– c : column

2.2 Imaging devices

• The Human Eye

2.4 Picture functions and digital images

• Concepts of analog image and digital images• Digital image : 2D rectangular array of discrete values

– Image space and intensity range are quantized into a discrete set of values

– Permitting the image to be stored in 2D computer memory structure– Common intensity range: 8bit (0~255) – In C program, unsigned char I[512][512]

<Def> analog image: F(x,y) which has infinite precision in spatial parameters x and y and infinite precision in intensity at each spatial point (x, y)

<Def> digital image: I[r,c] represented by a discrete 2D array of intensity samples, each of which is represented using a limited precision

2.4 Picture functions and digital images

• Coordinate systems

2.4.2 Image Quantization and Spatial Measurement

<Def> picture function: f(x,y) of a picture as a function of two spatial variables x and y that are real values defining points of picture and f(x, y) is usually also real value

<Def> gray scale image: Monochrome digital image I[r,c] with one intensity value per pixel

<Def> multi-spectral image: 2D image M[x,y] has a vector of values at each spatial point or pixel (If image is color, vector has 3 elements)

<Def> binary image: digital image with all pixel values 0 or 1

<Def> labeled image: digital image L[r,c] whose pixel values are symbols from finite alphabet (Related concepts thematic image and pseudo-colored image)

2.4.2 Image Quantization and Spatial Measurement

<def> nominal resolution: size of scene element that images to a single pixel on the image plane

<def> resolution: number of pixels (e.g., 640*480)

2.4.2 Image Quantization and Spatial Measurement

• Use appropriate resolution– Too little produce poor

recognition– Too much slow down

algorithm and waste memory

2.4.2 Image Quantization and Spatial Measurement

• Spatial quantization effects impose limits on measurement accuracy and detectability

2.5 Digital Image Formats

• Dozen of different formats still in use• Raw data: encode image pixels in row-by-row (raster order)• Most recently developed standard formats contain a header with

non-image information necessary to label the data to decode it

2.5.1/2 Image File header & Data

• Image File header– Need to make an image file self-describing so that

image-processing tools can work with them– Should contain :

• image dimension, type, data , title• color table, coding table • history

• Image Data– Nowadays, multimedia format including image data

along with text, graphics, music, etc.

2.5.3 Data Compression

• Reduce the size of an image (30 percent or even 3 percent of raw size)

• Copression can be lossless or lossy – Lossless compression : original image recovered exactly– Lossy compression : loss of quality is perceived (but, not always)

• To implement compression– Include overhead (compression method and parameter)– Loss or change of a few bit having little or no affect on consumer

s (exciting area from signal processing to object recognition)

2.5.4 Commonly Used Formats

• For colleague, Image data base, scanned documents– GIF, JPG, PS, TIFF etc.

• Image/Graphics file formats are still evolving

2.5.5 Run-Coded Binary Images

• Efficient for binary or labeled images• Reduce memory space • Speed up image operations

2.5.6 PGM : Portable Gray Map

• Simplest file format

• Family format: PBM/PGM,PPM

• Image header encoded in ASCII

2.5.9 JPEG Image File Format

• JPEG (Joint Photographic Experts Group)• Provide practical compression of high-quality color• Stream oriented and allow realtime hardware for encodin

g and decoding• Up to 64K X 64K pixels of 24 bits• Header contain thumbnail image (up to 64k)• To achieve high compression, flexible but lossy coding

scheme: Unnoticeable degration(1/20)• Compression work well when has large constant regio

ns• Compression scheme : DCT(Discrete Cosine Transfor

m) followed by Huffman coding

2.5.11 MPEG format for video

• Stream-oriented encoding scheme for video, text, and graphics• MPEG stands for Motion Picture Experts Group• MPEG-1

– Primary design for multimedia systems– Data rate

• Compression audio : 0.25 Mbits/s • Compression video : 1.25 Mbits/s

• MPEG-2– Data rate up to 15Mbits/s – Handle high definition TV rates

• Compression scheme takes advantage of both spatial redundancy (used in JPEG) and temporal redundancy, general 1/25, 1/200 possible

• Motion JPEG compression is not good

2.5.12 Comparison of Formats

2.6 Richness and Problems of real imagery

2.8 Five frames of reference

• Pixel Coordinate Frame– Each point has integer

pixel coordinates– Using only image I,

cannot determine which object is actually larger in 3D

2.8 Five frames of reference

• Object Coordinate Frame O– Used to model ideal objects in both computer

graphics and computer vision– Remains the same regardless of how block is posed

related to world

• Camera Coordinate Frame C– Often needed for egocentric (camera centric) view– Represent just in front of the sensor

2.8 Five frames of reference

• Real Image Coordinate Frame F– Coordinate [xf, yf, f]– F : focal length– xf, yf : not description of pixels in the image array but r

elated to the pixel size and pixel position of optical axis in the image

– Frame F contians the picture function digital image in the pixel array I

• World Coordinate Frame W– Needed to relate objects in 3D