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]=='#')
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