C program to write image in PGM format

PGM is a portable grayscale map. If we want to store a 2D array in C as an image in PNG, JPEG or any other image format, we have to do a lot of work to encode the data in some specified format before writing to the file.

The Netpbm format provides a simple and portable solution. Netpbm is an open source graphics package, basically used on Linux or Unix platforms. It also runs under Microsoft Windows systems.

Every file starts with a two-byte magic number. This magic number is used to identify the type of file. Types include PBM, PGM, PPM, etc. It also identifies the encoding (ASCII or binary). A magic number is a capital P followed by a number.

ASCII encoding allows for human readability and easy transfer to other platforms; the binary format is more efficient in terms of file size, but may have native byte order issues.

How to write PGM files?

• Set magic number P2
• Add spaces (space, tab, CR, LF)
• Add width, format is decimal ASCII characters
• Add blank
• Add height, format is ASCII decimal character
• Add blank
• Enter the maximum grayscale value, also use ASCII decimal format
• Add spaces
• Width x height grayscale value, each value is in ASCII decimal (range between 0 and maximum), separated by spaces from top to bottom.

Sample code

#include  main() { int i, j; int w = 13, h = 13; // This 2D array will be converted into an image The size is 13 x 13 int image[13][13] = { { 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15 }, { 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31}, { 47, 47, 47, 47, 47, 47, 47, 47, 47, 47, 47, 47, 47}, { 63, 63, 63, 63, 63, 63, 63, 63, 63, 63, 63, 63, 63}, { 79, 79, 79, 79, 79, 79, 79, 79, 79, 79, 79, 79, 79}, { 95, 95, 95, 95, 95, 95, 95, 95, 95, 95, 95, 95, 95 }, { 111, 111, 111, 111, 111, 111, 111, 111, 111, 111, 111, 111, 111}, { 127, 127, 127, 127, 127, 127, 127, 127, 127, 127, 127, 127, 127}, { 143, 143, 143, 143, 143, 143, 143, 143, 143, 143, 143, 143, 143}, { 159, 159, 159, 159, 159, 159, 159, 159, 159, 159, 159, 159, 159}, { 175, 175, 175, 175, 175, 175, 175, 175, 175, 175, 175, 175, 175}, { 191, 191, 191, 191, 191, 191, 191, 191, 191, 191, 191, 191, 191}, { 207, 207, 207, 207, 207, 207, 207, 207, 207, 207, 207, 207, 207} }; FILE* pgmimg; pgmimg = fopen("my_pgmimg.pgm", "wb"); //write the file in binary mode fprintf(pgmimg, "P2
"); // Writing Magic Number to the File fprintf(pgmimg, "%d %d
", w, h); // Writing Width and Height into the file fprintf(pgmimg, "255
"); // Writing the maximum gray value int count = 0; for (i = 0; i < h; i++) { for (j = 0; j < w; j++) { fprintf(pgmimg, "%d ", image[i][j]); //Copy gray value from array to file } fprintf(pgmimg, "
"); } fclose(pgmimg); }

PGM image is shown below

Output

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