How to read a PGMB format image in C

PGM or Portable Gray Map file is a grayscale image where each pixel is encoded with 1 or 2 bytes. It contains header information and pixel grayscale values in a matrix form.

Approach: The idea is followed to read PGMB format image is as follows:

• Open a PGMB (binary format PGM image).
• Extract the pixel information, which can be then used for further processing.
• The header information is stored in ASCII format can be read using any text editor but the pixel information is stored in a binary formation and the text editor will show that as some gibberish text.

Below is the sample pgm image.

:

• magic number for identifying the file type:
  • Binary PGM File (PGMB): "P5"
  • ASCII PGM File (PGMA): "P2"
• Width in ASCII decimal format
• Height in ASCII decimal format
• Maximum Gray Value, ASCII decimal format, between 0-255
• Can contain comments, denoted by beginning the line with a '#'
• All separated with white spaces (blanks space, tabs, CRs, LFs)

Header information in gfg_logo.pgm:

P5
# sample PGMB image
# gfg_logo.pgm
200 200
255

After the header information, there is a grid of dimensions height * weight containing the grayscale pixel values of the image in binary format.

:

• Open the image in the read binary, rb mode.
• Check if any comments are present and ignore them.
• Read the Magic Number.
• Read any comments/blank line/white space.
• Read width and height separated by white space.
• Read the max gray value, before and after any white space/comment.
• Read the grid (width * height) of pixel values, separated by white spaces.

Below is the program for the above approach:

Output:

Explanation:

• Create a structure for storing the PGMB image details and allocate memory for the same.
• Take the file name input either as a command-line argument or by hard coding it in the program.
• The openPGM() function processes the input image files and takes the memory pointer and filename are input.
• The ignoreComments() function is used to skip any comments in the file. Comments are usually present in the header portion and thus, we check for them after reading every property.
• In openPGM() function, read the file header information, ie. file type, height, weight, etc as given above.
• Then allocate memory as per the height of the image and for each row, allocate memory for the width of the image.
• The fread() method reads the gray values and stores them in the allocated memory for the 2d character matrix of the pgm structure.
• printImageDetails() is used to print the values retrieved from the PGMB image file.