Explanation of PGM and PPM image file format

  Seeing an image file in Linux
Using the viewer xv
Type "xv"
use the right mouse button to access the menu
   for loading, viewing, saving image files in various formats.
Using the viewer ee

Common image formats
 GIF and JPEG (see Supercomputer Applications).  
 For reading image data, two other formats, PGM and PPM are also used.
P2 - PGM grey scale image, stored in ASCII, one value per pixel
P3 - PPM color image, stored in ASCII, 3 RGB values per pixel
P5 - PGM grey scale image stored in binary (compressed) format
P6 - PPM color image stored in binary (compressed) format

 Sample PGM file
   First line tells the PGM format. P2 -- grey scale
   Lines beginning with # are comment lines
   Third line gives columns (width) and rows (height) of image
   Fourth line gives the max pixel value
   Rest is data

P2  
# CREATOR: XV Version 3.10a  Rev: 12/29/94 (PNG patch 1.2) 
97 128   
 45  45  49  57  65  65  61  85 125 138 121 117 117 121 121 121 117
109 146 215 219 219 223 206 182 190 194 194 190 194 198 198 198 202
198 194 190 190 194 198 198 202 202 202 198 198 194 194 194 194 198
198 198 198 198 194 194 194 198 198 198 202 198 198 194 190 194 194
194 198 198 190 190 210 215 215 210 206 170 146 146 142 130  65  45
 40  45  49  61  53  36  16  28  28  61 121 142  53  53  53  57  69
. . .

Sample PPM file
     Here's the same file in PPM (color) format. 
     Note the 3 values per pixel.
     To make it a gray file the rgb values are the
        same for each pixel.

P3   
# CREATOR: XV Version 3.10a  Rev: 12/29/94 (PNG patch 1.2) 
291 384   
255      
 45  45  45  45  45  45  49  49  49  57  57  57  65  65  65  85  85 
 85 121 121 121  . . .