Image Representation and Manipulation

CS302 Data Structures

Prof. George Bebis

http://www.cse.unr.edu/CVL

How are images represented?

8 bits/pixel

0

255

Color images

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)

CCD (Charged-Coupled Device) cameras

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

Image file formats

• Many image formats adhere to the following simple model:

– Header– Data (line by line, no breaks between lines)

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

Binary

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; };

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 imagefor(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); }

Reading/Writing PGM images

(1D array of unsigned char)

(1D array of unsigned char)

(2D array of int)

(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<char *>(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'); 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();

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); }

}

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