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PGM Format

PGM Format. CS474/674 – Prof. Bebis. How are images represented?. Color images. 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)

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PGM Format

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  1. PGM Format CS474/674 – Prof. Bebis

  2. How are images represented?

  3. Color images

  4. 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)

  5. PGM Image file format(Portable Graymap) • It adheres to the following simple model: • Header • Data (line by line, no breaks between lines).

  6. PGM Image file format (cont.) • Header contains at least: • A signature or “magic number” (i.e., a short sequence of bytes for identifying the file format). • The columns (M) and rows (N) of the image. • The number of quantization levels (L).

  7. PGM and related image file formats • PGM is 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 raw storage #Col #Row #Q (usually on a separate line) ASCII Raw

  8. Image Class • class ImageType { • public: • ImageType(); // constructor • ImageType(ImageType&); // copy-constructor • ~ImageType(); // destructor • void getImageInfo(int&, int&, int&); • void setImageInfo(int, int, int); • void getVal(int, int, int&); • void setVal(int, int, int); • // more functions ... • private: • int N, M, Q; //N: # rows, M: # columns • int **pixelValue; • };

  9. Input / Output Functions • C++ routine to read the header of a PGM image: ReadImageHeader.cpp • C++ routine to read a PGM image from a file: ReadImage.cpp • C++ routine to write a PGM image to a file: WriteImage.cpp

  10. 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(argv[1], N, M, Q, type); • // allocate memory for the image array • ImageType image(N, M, Q);

  11. Threshold.cpp (cont’d) • // read image • readImage(argv[1], image); 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, 0); • else • image.setVal(i, j, 255); • } • // write image • writeImage(argv[2], image); • }

  12. Reading/Writing PGM images (2D array of int) (1D array of unsigned char) Use “write” (1D array of unsigned char) (2D array of int) Use “read”

  13. 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 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; • }

  14. Writing a PGM image... (cont’d) • ofp.open(fname, ios::out | ios::binary); • 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(); • }

  15. 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 | ios::binary); • if (!ifp) { • cout << "Can't read image: " << fname << endl; • exit(1); • }

  16. Reading a PGM image from a file • // 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); • } • ifp.getline(header,100,'\n'); // skip comments • while(header[0]=='#') • ifp.getline(header,100,'\n'); • M=strtol(header,&ptr,0); // read M, N • N=atoi(ptr);

  17. Reading a PGM image …. (cont’d) • ifp.getline(header,100,'\n'); // assumes Q is stored on a separate line • 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();

  18. Reading a PGM image…(cont’d) • // Convert 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); • } • }

  19. How do I “see” images on my computer? • Unix/Linux: xv, gimp • Windows: Photoshop Irfanview

  20. How do I convert an image from one format to another? • Use “Save As” option

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