Definitions

portable pixmap

Netpbm format

The portable pixmap file format (PPM), the portable graymap file format (PGM) and the portable bitmap file format (PBM) specify rules for exchanging graphics files. They provide very basic functionality and serve as a least-common-denominator for converting pixmap, graymap, or bitmap files between different platforms. Several applications refer to them collectively as the PNM format (portable anymap).

Netpbm usage

An important use of the Netpbm formats is in conjunction with the Netpbm package to convert between different file formats and to do simple image manipulation. The Netpbm package can, for example, use two successive conversion programs to turn the code from the PBM example given below into a bmp file:

pgmtoppm "#FFFFFF" j.pbm  > j.ppm
ppmtobmp j.ppm > j.bmp

Depending on the identification of the file format, portable pixmap systems can distinguish three similar file formats, each with two versions:

  • PBM - portable bitmap file format (P1/P4) - 1 bit per pixel
  • PGM - portable graymap file format (P2/P5) - 8 bits per pixel
  • PPM - portable pixmap file format (P3/P6) - 24 bits per pixel, 8 for red, 8 for green, 8 for blue

In each case, the lower-numbered version (P1, P2 or P3) refers to a human-readable, ASCII-based format similar to the one in the example above; and the higher-numbered version (P4, P5 or P6) refers to a binary format, not human-readable but more efficient at saving some space in the file, as well as easier to parse due to the lack of whitespace.

File format description

PBM example

Take the example of the letter "J" as a bitmap:
....X.
....X.
....X.
....X.
....X.
....X.
X...X.
.XXX..
......
......

The most basic (monochrome) PBM format represents it as follows:

P1
# This is an example bit map file j.pbm
6 10
0 0 0 0 1 0
0 0 0 0 1 0
0 0 0 0 1 0
0 0 0 0 1 0
0 0 0 0 1 0
0 0 0 0 1 0
1 0 0 0 1 0
0 1 1 1 0 0
0 0 0 0 0 0
0 0 0 0 0 0

The string P1 identifies the file format. The hash sign introduces a comment. The next two numbers give the width and the height. Then follows the matrix with the pixel values (in the monochrome case here, only zeros and ones).

Here is the resulting image: . Here it is again magnified 20 times:

The PGM and PPM formats (both ASCII and binary versions) have an additional parameter for the maximum value in a line between the X and Y dimensions and the actual pixel data.

PGM example

P2
# feep.pgm from NetPBM man page on PGM
24 7
15
0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0
0  3  3  3  3  0  0  7  7  7  7  0  0 11 11 11 11  0  0 15 15 15 15  0
0  3  0  0  0  0  0  7  0  0  0  0  0 11  0  0  0  0  0 15  0  0 15  0
0  3  3  3  0  0  0  7  7  7  0  0  0 11 11 11  0  0  0 15 15 15 15  0
0  3  0  0  0  0  0  7  0  0  0  0  0 11  0  0  0  0  0 15  0  0  0  0
0  3  0  0  0  0  0  7  7  7  7  0  0 11 11 11 11  0  0 15  0  0  0  0
0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0

PPM example

P3
#the P3 means colors are in ascii, then 3 columns and 2 rows, then 255 for max color, then RGB triplets
3 2
255
255 0 0
0 255 0
0 0 255
255 255 0
255 255 255
0 0 0

The image (expanded):

The above image was expanded without interpolation, using the imagemagick command

convert -sample %6400 tiny6pixel.ppm tiny6pixel.png
The P6 format of the same image will store a color with one byte. The file will be smaller but the color information will not be readable by humans:
P6
#any comment string
3 2
255
!@#$%^&*()_+|{}:"<

The PPM format is not compressed and so is extravagantly wasteful of space and bandwidth. For example, the above 192x128 PNG image has a file size of 552 bytes. When converted to a 192x128 PPM image, the file size is 73848 bytes. The PPM format is generally a vehicle to a more efficient format, notably the PNG (Portable Network Graphics) format. The PPM format can be converted to PNG, for example, without loss of information.

The PPM format is certainly simple to write from scratch. The following Python code makes the above example image. It can be adapted to making useful images by reading or constructing an array of numerical data, and programming a conversion of the data to color triplets.

  1. !/usr/bin/python

triplets=[[255, 0, 0], [0, 255, 0], [0, 0, 255], [255, 255, 0], [255, 255, 255], [0, 0, 0] ] width=3; height=2 comment='any comment string' ftype='P6' #use 'P3' for ascii, 'P6' for binary

ppmfile=open('testimage.ppm','wb') ppmfile.write("%sn" % (ftype)) ppmfile.write("#%sn" % comment ) ppmfile.write("%d %dn" % (width, height)) ppmfile.write("255n")

if ftype=='P3':

   for red,green,blue in triplets:
       ppmfile.write("%d %d %dn" % (red,green,blue))
elif ftype=='P6': #print 1 byte per color
   for red,green,blue in triplets:
       ppmfile.write("%c%c%c" % (red,green,blue))

ppmfile.close()

Similar code in C :

  1. include
  2. include
  3. include
  4. define xor(a, b) ((!(a && b)) && (a || b))

int main(int argc, char** argv){

 FILE* f;
 char* x;
 int i, j, k, l, m, r, c;
 if(argc != 4){
   printf("Usage: %s cols rows fname.ppmnn", *argv);
   return 1;
}
 f = fopen(argv[3], "w");
 c = atoi(argv[1]);
 r = atoi(argv[2]);
 x = (char*)malloc(r*c);
 for(i=0;i
   x[i] = 0;
}
 i = 0;
 i = 2;
 while(i < r*c/2){
   for(k=2*i;k
     x[k] = 1;
}
   i ++;
}
 fprintf(f, "P3n%dn%dn255n", c, r);
 for(l = 0; l < c; l++){
   for(m = 0; m < r; m++){
     if(!x[c*l+m])
       fprintf(f, "%d 255 %d", (256*l)/c, (256*m)/r);
     else
if(xor((l/10)%2,(m/10)%2)) fprintf(f, "%d 0 %d", (255*l*m)/(r*c), 255-(255*l*m)/(r*c)); else fprintf(f, "0 0 0");
       fprintf(f, "n");
}
   fprintf(f, "n");
}
 free(x);
 fclose(f);
 return 0;
}

16-bit extensions

The original definition of the PGM and the PPM binary formats (the P5 and P6 formats) did not allow bit depths greater than 8 bits. One can of course use the ASCII format, but this format both slows down reading and makes the files much larger. Accordingly, many programmers have attempted to extend the format to allow higher bit depths. Using higher bit depths encounters the problem of having to decide on the endianness of the file. Unfortunately it appears that the various implementations could not agree on which byte order to use, and some connected the 16-bit endianness to the pixel packing order; according to Netpbm, the de facto standard implementation of the PNM formats, the most significant byte is first.

References

  • Format details for the various pnm formats:

See also

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