Image Representation and Manipulation

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Image Representation and Manipulation

• CS302 Data Structures
• Prof. George Bebis
• http//www.cse.unr.edu/CVL

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How are images represented?
8 bits/pixel
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Color images
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A Simple model of image formation

• The scene is illuminated by a single source.
• The scene reflects radiation towards the camera.
• The camera senses the light (i.e., through solid
  state cells for CCD cameras)

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CCD (Charged-Coupled Device) cameras

• Tiny solid state cells convert light energy into
  electric charge which is then digitized (A/D
  conversion).

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Image file formats

• Many image formats adhere to the following simple
  model
• Header
• Data (line by line, no breaks between lines)

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Image file formats (cont.)

• Header contains at least
• A signature or magic number (i.e., a short
  sequence of bytes for identifying the file
  format).
• The width and height of the image.

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Common image file formats

• PGM (Portable Gray Map)
• PNG (Portable Network Graphics)
• GIF (Graphic Interchange Format) -
• JPEG (Joint Photographic Experts Group)
• TIFF (Tagged Image File Format)
• FITS (Flexible Image Transport System)

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PGM format

• A popular format for grayscale images (8
  bits/pixel)
• Closely-related formats are
• PBM (Portable Bitmap), for binary images (1
  bit/pixel)
• PPM (Portable Pixelmap), for color images (24
  bits/pixel)
• ASCII or binary
  (raw) storage

ASCII
Binary
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Image Class

• class ImageType
• public
• ImageType() // constructor
• ImageType() // destructor
• void getImageInfo(int, int, int)
• void setImageInfo(int, int, int)
• void setPixelVal(int, int, int)
• void getPixelVal(int, int, int)
• // more functions ...
• private
• int N, M, Q //N rows, M columns
• int pixelValue

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Input / Output Functions

• C routine to read the header of a PGM image
• ReadImageHeader.cpp
  
• C routine to read a PGM image
• ReadImage.cpp
• C routine to write a PGM image
• WriteImage.cpp

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An example - Threshold.cpp

• void readImageHeader(char, int, int, int,
  bool)
• void readImage(char, ImageType)
• void writeImage(char, ImageType)
• void main(int argc, char argv)
• int i, j, M, N, Q
• bool type
• int val, thresh
• // read image header
• readImageHeader(argv1, N, M, Q, type)
• // allocate memory for the image array
• ImageType image(N, M, Q)

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Threshold.cpp (contd)

• // read image
• readImage(argv1, image)
• cout ltlt "Enter threshold "
• cin gtgt thresh
• // threshold image
• for(i0 iltN i)
• for(j0 jltM j)
• image.getVal(i, j, val)
• if(val lt thresh)
• image.setVal(i, j, 255)
• else
• image.setVal(i, j, 0)
• // write image
• writeImage(argv2, image)

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Reading/Writing PGM images
(1D array of unsigned char)
(2D array of int)
Use write
(1D array of unsigned char)
(2D array of int)
Use read
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Writing a PGM image to a file

• void writeImage(char fname, ImageType image)
• int N, M, Q
• unsigned char charImage
• ofstream ofp
• image.getImageInfo(N, M, Q)
• charImage (unsigned char ) new unsigned char
  MN
• // convert integer values to unsigned char
• int val
• for(i0 iltN i)
• for(j0 jltM j)
• image.getVal(i, j, val)
• charImageiMj(unsigned char)val

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Writing a PGM image... (contd)

• ofp.open(fname, iosout iosbinary)
• if (!ofp) cout ltlt "Can't open file " ltlt fname
  ltlt endl exit(1)
• ofp ltlt "P5" ltlt endl
• ofp ltlt M ltlt " " ltlt N ltlt endl
• ofp ltlt Q ltlt endl
• ofp.write( reinterpret_castltchar gt(charImage),
  (MN)sizeof(unsigned char))
• if (ofp.fail()) cout ltlt "Can't write image "
  ltlt fname ltlt endl exit(0)
• ofp.close()

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Reading a PGM image from a file

• void readImage(char fname, ImageType image)
• int i, j
• int N, M, Q
• unsigned char charImage
• char header 100, ptr
• ifstream ifp
• ifp.open(fname, iosin iosbinary)
• if (!ifp)
• cout ltlt "Can't read image " ltlt fname ltlt endl
• exit(1)

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Reading a PGM image from a file

• // read header
• ifp.getline(header,100,'\n')
• if ( (header0!80) // 'P'
• (header1!53) ) // '5'
• cout ltlt "Image " ltlt fname ltlt " is not PGM"
  ltlt endl
• exit(1)
• ifp.getline(header,100,'\n') // skip comments
• while(header0'')
• ifp.getline(header,100,'\n')
• Mstrtol(header,ptr,0) // read M, N
• Natoi(ptr)

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Reading a PGM image . (contd)

• ifp.getline(header,100,'\n')
• Qstrtol(header,ptr,0)
• charImage (unsigned char ) new unsigned char
  MN
• ifp.read( reinterpret_castltchar gt(charImage),
  (MN)sizeof(unsigned char))
• if (ifp.fail())
• cout ltlt "Image " ltlt fname ltlt " has wrong size"
  ltlt endl
• exit(1)
• ifp.close()

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Reading a PGM image(contd)

• // Convert unsigned characters to integers
• int val
• for(i0 iltN i)
• for(j0 jltM j)
• val (int)charImageiMj
• image.setVal(i, j, val)

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How do I see images on my computer?

• Unix/Linux xv, gimp
• Windows Photoshop
• Irfanview

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How do I convert an image from one format to
another?

• Use Save As option