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  • 1. Image Formation Fundamentals CS491E/791E
  • 2. How are images represented in the computer?
  • 3. Color images
  • 4. Image formation • There are two parts to the image formation process: – The geometry of image formation, which determines where in the image plane the projection of a point in the scene will be located. – The physics of light, which determines the brightness of a point in the image plane as a function of illumination and surface properties.
  • 5. A Simple model of image formation • The scene is illuminated by a single source. • The scene reflects radiation towards the camera. • The camera senses it via chemicals on film.
  • 6. Pinhole camera • This is the simplest device to form an image of a 3D scene on a 2D surface. • Straight rays of light pass through a “pinhole” and form an inverted image of the object on the image plane. fX x= Z fY y= Z
  • 7. Camera optics • In practice, the aperture must be larger to admit more light. • Lenses are placed to in the aperture to focus the bundle of rays from each scene point onto the corresponding point in the image plane
  • 8. Image formation (cont’d) • Optical parameters of the lens – lens type – focal length – field of view • Photometric parameters – type, intensity, and direction of illumination – reflectance properties of the viewed surfaces • Geometric parameters – type of projections – position and orientation of camera in space – perspective distortions introduced by the imaging process
  • 9. Image distortion
  • 10. What is light? • The visible portion of the electromagnetic (EM) spectrum. • It occurs between wavelengths of approximately 400 and 700 nanometers.
  • 11. Short wavelengths • Different wavelengths of radiation have different properties. • The x-ray region of the spectrum, it carries sufficient energy to penetrate a significant volume or material.
  • 12. Long wavelengths • Copious quantities of infrared (IR) radiation are emitted from warm objects (e.g., locate people in total darkness).
  • 13. Long wavelengths (cont’d) • “Synthetic aperture radar” (SAR) imaging techniques use an artificially generated source of microwaves to probe a scene. • SAR is unaffected by weather conditions and clouds (e.g., has provided us images of the surface of Venus).
  • 14. Range images • An array of distances to the objects in the scene. • They can be produced by sonar or by using laser rangefinders.
  • 15. Sonic images • Produced by the reflection of sound waves off an object. • High sound frequencies are used to improve resolution.
  • 16. CCD (Charged-Coupled Device) cameras • Tiny solid state cells convert light energy into electrical charge. • The image plane acts as a digital memory that can be read row by row by a computer.
  • 17. Frame grabber • Usually, a CCD camera plugs into a computer board (frame grabber). • The frame grabber digitizes the signal and stores it in its memory (frame buffer).
  • 18. Image digitization • Sampling means measuring the value of an image at a finite number of points. • Quantization is the representation of the measured value at the sampled point by an integer.
  • 19. Image digitization (cont’d)
  • 20. Image quantization(example) • 256 gray levels (8bits/pixel) 32 gray levels (5 bits/pixel) 16 gray levels (4 bits/pixel) • 8 gray levels (3 bits/pixel) 4 gray levels (2 bits/pixel) 2 gray levels (1 bit/pixel)
  • 21. Image sampling (example) original image sampled by a factor of 4 sampled by a factor of 2 sampled by a factor of 8
  • 22. Digital image • An image is represented by a rectangular array of integers. • An integer represents the brightness or darkness of the image at that point. • N: # of rows, M: # of columns, Q: # of gray levels – N= 2n, M= 2 m, Q= 2q (q is the # of bits/pixel) – Storage requirements: NxMxQ (e.g., N=M=1024, q=8, 1MB) f (0,0) f (0,1) ... f (0, M − 1) f (1,0) f (1,1) ... f (1, M − 1) ... f ( N − 1,0) ... ... f ( N − 1,1) ... ... f ( N − 1, M − 1)
  • 23. Image file formats • Many image formats adhere to the simple model shown below (line by line, no breaks between lines). • The header contains at least the width and height of the image. • Most headers begin with a signature or “magic number” - a short sequence of bytes for identifying the file format.
  • 24. Common image file formats • • • • • • GIF (Graphic Interchange Format) PNG (Portable Network Graphics) JPEG (Joint Photographic Experts Group) TIFF (Tagged Image File Format) PGM (Portable Gray Map) FITS (Flexible Image Transport System)
  • 25. Comparison of image formats
  • 26. 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
  • 27. ASCII vs Raw format • ASCII format has the following advantages: – Pixel values can be examined or modified very easily using a standard text editor. – Files in raw format cannot be modified in this way since they contain many unprintable characters. • Raw format has the following advantages: – It is much more compact compared to the ASCII format. – Pixel values are coded using only a single character !
  • 28. Image Class class ImageType { public: ImageType(); ~ImageType(); // more functions ... private: int N, M, Q; //N: # rows, M: # columns int **pixelValue; };
  • 29. 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; int M, N, Q; bool type; int val; int thresh; // read image header readImageHeader(argv[1], N, M, Q, type); // allocate memory for the image array ImageType image(N, M, Q); // read image readImage(argv[1], image);
  • 30. Threshold.cpp (cont’d) cout << "Enter threshold: "; cin >> thresh; // threshold image for(i=0; i<N; i++) for(j=0; j<M; j++) { image.getVal(i, j, val); if(val < thresh) image.setVal(i, j, 255); else image.setVal(i, j, 0); } // write image writeImage(argv[2], image); }
  • 31. Reading/Writing PGM images
  • 32. 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 [M*N]; // convert the integer values to unsigned char int val; for(i=0; i<N; i++) for(j=0; j<M; j++) image.getVal(i, j, val); charImage[i*M+j]=(unsigned char)val; }
  • 33. Writing a PGM image... (cont’d) ofp.open(fname, ios::out); if (!ofp) { cout << "Can't open file: " << fname << endl; exit(1); } ofp << "P5" << endl; ofp << M << " " << N << endl; ofp << Q << endl; ofp.write( reinterpret_cast<char *>(charImage), (M*N)*sizeof(unsigned char)); if (ofp.fail()) { cout << "Can't write image " << fname << endl; exit(0); } ofp.close(); }
  • 34. 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, ios::in); if (!ifp) { cout << "Can't read image: " << fname << endl; exit(1); } // read header ifp.getline(header,100,'n'); if ( (header[0]!=80) || /* 'P' */ (header[1]!=53) ) { /* '5' */ cout << "Image " << fname << " is not PGM" << endl; exit(1); }
  • 35. Reading a PGM image …. (cont’d) ifp.getline(header,100,'n'); while(header[0]=='#') ifp.getline(header,100,'n'); M=strtol(header,&ptr,0); N=atoi(ptr); ifp.getline(header,100,'n'); Q=strtol(header,&ptr,0); charImage = (unsigned char *) new unsigned char [M*N]; ifp.read( reinterpret_cast<char *>(charImage), (M*N)*sizeof(unsigned char)); if (ifp.fail()) { cout << "Image " << fname << " has wrong size" << endl; exit(1); } ifp.close();
  • 36. Reading a PGM image…(cont’d) // // Convert the unsigned characters to integers // int val; for(i=0; i<N; i++) for(j=0; j<M; j++) { val = (int)charImage[i*M+j]; image.setVal(i, j, val); } }
  • 37. How do I “see” images on the computer? • Unix: xv, gimp • Windows: Photoshop
  • 38. How do I display an image from within my C++ program? • Save the image into a file with a default name (e.g., tmp.pgm) using the WriteImage function. • Put the following command in your C++ program: system(“xv tmp.pgm”); • This is a system call !! • It passes the command within the quotes to the Unix operating system. • You can execute any Unix command this way ….
  • 39. How do I convert an image from one format to another? • Use xv’s “save” option • It can also convert images
  • 40. How do I print an image? • Load the image using “xv” • Save the image in “postscript” format • Print the postscript file (e.g., lpr -Pname image.ps)
  • 41. Image processing software • • • • • CVIPtools (Computer Vision and Image Processing tools) Intel Open Computer Vision Library Microsoft Vision SDL Library Matlab Khoros • For more information, see – http://www.cs.unr.edu/~bebis/CS791E – http://www.cs.unr.edu/CRCD/ComputerVision/cv_resources.html

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