The PGM
Image File Format
C. Furlong
WPI-ME/CHSLT
Description
PGM is a standard bitmap based format consisting of a 4 lines header, and
data stored in the unsigned char type, providing a maximum of 256
gray scale levels or 8-bit data per pixel.
The general structure
of a PGM image file
The header of a
PGM image file consists of:
-
First line containing
the signature of the image file and identifies the file as PGM
-
Second line is
the comment line
-
third line provides
information about the number and rows and columns of data stored
in the file, and
-
fourth line specifies
maximum gray level contained in the image
Data follows the
header information and is written in pixel values (in text or binary
format). Data is in raster order, which indicates that all data for
the first row of the image is written first, then for the second row, and
so on. The origin of the coordinate system for a PGM image is located
on the top left corner.
The following
is a 10 x 10 pixels PGM text (ASCII) image file containing a maximum
gray scale value of 255:
P5
#WPI-ME/CHSLT generated
image Ver.1.0 (0)
10 10
255
0 1 2 3 4 5 6 7 8 9 10 11
12 13 14 15 16 17 18 19 20 21 22 23 24 25
26 27 28 28 30 31 32 33
34 35 36 37 38 39 40 41 42 43 44 45 46 47
48 49 50 51 52 53 54 55
56 57 58 59 60 61 62 63 64 65 66 67 68 69 70
71 72 73 74 75 76 77 78
79 80 81 82 83 84 85 86 87 88 89 90 91 92 93
255 255 255 255 255 255
|
Header
is written in ASCII (or text) format and data in ASCII or binary format
PGM file images
are generally used for displaying/printing purposes and can be read by
many commercially available image packages, including a public domain UNIX
based named xv.
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