One of the formats used to encode an image is the Netpbm format (see [1]). The netpbm format supports rgb images (ppm), grayscale images (pgm) and black and white images (pbm).

Netpbm uses a "magic" number to describe these images:

• P1 (B&W image in ascii format)
• P2 (grayscale image in ascii format)
• P3 (rgb image in ascii format)
• P4 (B&W image in binary format)
• P5 (grayscale image in binary format)
• P6 (rgb image in binary format)

This program supports the following convertions:

• P2 to P1
• P3 to P2
• P5 to P4
• P6 to P5

Written in C, Ubuntu 16.04.3 LTS.

./netpbm < input_image > output_image
In the folder "input", there are images to test the program.

• make to compile
• make clean to the delete object files and the executable.

[1] https://en.wikipedia.org/wiki/Netpbm_format