Chapter 2 : Imaging and Image Representation

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Chapter 2 Imaging and Image Representation

• Computer Vision Lab.
• Chonbuk National University

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Contents

• 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
• 2.2 Image Device
• 2.3 Problems in Digital Images
• 2.4 Picture Function and Digital Image
• 2.5 Digital Image
• Formats

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2.1 Sensing Light

• Simple model of common photography

(Sun or Flash bulb)
Sense via chemical on film
Reflects radiation Toward camera
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2.2 Imaging devices

• CCD Camera

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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 Ir,c
• I array name
• r row
• c column

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2.2 Imaging devices

• The Human Eye

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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 (0255)
• In C program, unsigned char I512512
• ltDefgt 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)
• ltDefgt digital image Ir,c represented by a
  discrete 2D array of intensity samples, each of
  which is represented using a limited precision

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2.4 Picture functions and digital images

• Coordinate systems

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2.4.2 Image Quantization and Spatial Measurement

• ltDefgt 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
• ltDefgt gray scale image Monochrome digital image
  Ir,c with one intensity value per pixel
• ltDefgt multi-spectral image 2D image Mx,y has a
  vector of values at each spatial point or pixel
  (If image is color, vector has 3 elements)
• ltDefgt binary image digital image with all pixel
  values 0 or 1
• ltDefgt labeled image digital image Lr,c whose
  pixel values are symbols from finite alphabet
  (Related concepts thematic image and
  pseudo-colored image)

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2.4.2 Image Quantization and Spatial Measurement

• ltdefgt nominal resolution size of scene element
  that images to a single pixel on the image plane
  
• ltdefgt resolution number of pixels (e.g.,
  640480)

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2.4.2 Image Quantization and Spatial Measurement

• Use appropriate resolution
• Too little produce poor recognition
• Too much slow down algorithm and waste memory

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2.4.2 Image Quantization and Spatial Measurement

• Spatial quantization effects impose limits on
  measurement accuracy and detectability

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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

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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.

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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 consumers
• (exciting area from signal processing to object
  recognition)

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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

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2.5.5 Run-Coded Binary Images

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

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2.5.6 PGM Portable Gray Map

• Simplest file format
• Family format PBM/PGM,PPM
• Image header encoded in ASCII

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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
  encoding 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
  regions
• Compression scheme DCT(Discrete Cosine
  Transform) followed by Huffman coding

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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

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2.5.12 Comparison of Formats
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2.6 Richness and Problems of real imagery
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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

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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

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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 related 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