Image Representation and Read/Write CS479/679 – Prof. Bebis

How are images represented in the computer?

Color images

Image file formats Many image formats adhere to the following simple model: –Header –Data

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.

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)

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 raw

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

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

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);

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); }

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

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; }

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 (charImage), (M*N)*sizeof(unsigned char)); if (ofp.fail()) { cout << "Can't write image " << fname << endl; exit(0); } ofp.close(); }

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); }

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);

Reading a PGM image …. (cont’d) ifp.getline(header,100,'\n'); // read Q Q=strtol(header,&ptr,0); charImage = (unsigned char *) new unsigned char [M*N]; ifp.read( reinterpret_cast (charImage), (M*N)*sizeof(unsigned char)); if (ifp.fail()) { cout << "Image " << fname << " has wrong size" << endl; exit(1); } ifp.close();

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); }

Color Images struct RGB { RGB(int, int, int); RGB& operator=(RGB); int r, g, b; } RGB;

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

ReadImage - PPM charImage = (unsigned char *) new unsigned char [3*N*M]; ifp.read( reinterpret_cast (charImage), (3*M*N)*sizeof(unsigned char)); RGB val; for(i=0; i < N; i++) for(j=0; j < 3*M; j+=3) { val.r = (int)charImage[i*3*M+j]; val.g = (int)charImage[i*3*M+j+1]; val.b = (int)charImage[i*3*M+j+2]; image.setPixelVal(i, j/3, val); }

WriteImage - PPM charImage = (unsigned char *) new unsigned char [3*M*N]; for(i=0; i<N; i++) for(j=0; j<3*M; j+=3) { image.getPixelVal(i, j/3, val); charImage[i*3*M+j]=(unsigned char)val.r; charImage[i*3*M+j+1]=(unsigned char)val.g; charImage[i*3*M+j+2]=(unsigned char)val.b; } ofp.write( reinterpret_cast (charImage), (3*M*N)*sizeof(unsigned char));

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

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