Libnetpbm User's Guide

The Libnetpbm programming library is part of Netpbm.

Example

Here is an example of a C program that uses libnetpbm to read a Netpbm image input and produce a Netpbm image output.

   /* Example program fragment to read a PAM or PNM image
      from stdin, add up the values of every sample in it
      (I don't know why), and write the image unchanged to
      stdout. */

   #include <pam.h>

   struct pam inpam, outpam;
   unsigned int row;

   pnm_init(&argc, argv);

   pnm_readpaminit(stdin, &inpam, PAM_STRUCT_SIZE(tuple_type));

   outpam = inpam; outpam.file = stdout;

   pnm_writepaminit(&outpam);

   tuplerow = pnm_allocpamrow(&inpam);

   for (row = 0; row < inpam.height; row++) {
       unsigned int column;
       pnm_readpamrow(&inpam, tuplerow);
       for (column = 0; column < inpam.width; ++column) {
           unsigned int plane;
           for (plane = 0; plane < inpam.depth; ++plane) {
               grand_total += tuplerow[column][plane];
           }
       }
       pnm_writepamrow(&outpam, tuplerow); }

   pnm_freepamrow(tuplerow);

Guide To Using Libnetpbm

libnetpbm classes

In this section, we cover only the PAM functions in libnetpbm. As described in the introduction to libnetpbm, there are four other classes of image processing functions (PBM, PGM, PPM, PNM). They are less important, since you can do everything more easily with the PAM functions, but if you're working on old programs or need the extra efficiency those older functions can sometimes provide, you can find them documented as here: PBM Function Manual, PGM Function Manual,PPM Function Manual, and PNM Function Manual.

In case you're wondering, what makes the PAM functions easier to use is:

• Each function handles all the formats. It does so without converting to a common format, so your program can treat the different formats differently if it wants. However, the interface makes it easy for your program to ignore the differences between the formats if that's what you want.
• The PAM function parameter lists convey most information about the image with which you're working with a single pam structure, which you can build once and use over and over, whereas the older functions require you to pass up to 5 pieces of image information (height, width, etc.) as separate arguments to every function.

THE pam STRUCTURE

The PAM functions take most of their arguments in the form of a single pam structure. This is not an opaque object, but just a convenient way to organize the information upon which most the functions depend. So you are free to access or set the elements of the structure however you want. But you will find in most cases it is most convenient to call pnm_readpaminit() or pnm_writepaminit() to set the fields in the pam structure before calling any other pam functions, and then just to pass the structure unchanged in all future calls to pam functions.

The fields are:

size

The storage size in bytes of this entire structure.

len

The length, in bytes, of the information in this structure. The information starts in the first byte and is contiguous. This cannot be greater than size. size and len can be used to make programs compatible with newer and older versions of the Netpbm libraries.

file

The file.

format

The format code of the raw image. This is PAM_FORMAT unless the PAM image is really a view of a PBM, PGM, or PPM image. Then it's PBM_FORMAT, RPBM_FORMAT, etc.

plainformat

This is a boolean value and means: The format above is a plain (text) format as opposed to a raw (binary) format. This is entirely redundant with the format member and exists as a separate member only for computational speed.

height

The height of the image in rows.

width

The width of the image in number of columns (tuples per row).

depth

The depth of the image (degree of or number of samples in each tuple).

maxval

The maxval of the image. See definitions in pam.

bytes_per_sample

The number of bytes used to represent each sample in the image file. See the format definition in pam. This is entirely redundant with maxval. It exists as a separate member for computational speed.

tuple_type

The tuple type of the image. See definitions in pam. Netpbm does not define any values for this except the following, which are used for a PAM image which is really a view of a PBM, PGM, or PPM image: PAM_PBM_TUPLETYPE, PAM_PGM_TUPLETYPE, PAM_PPM_TUPLETYPE.

allocation_depth

The number of samples for which memory is allocated for any tuple associated with this PAM structure. This must be at least as great as 'depth'. Only the first 'depth' of the samples of a tuple are meaningful.

The purpose of this is to make it possible for a program to change the type of a tuple to one with more or fewer planes.

0 means the allocation depth is the same as the image depth.

PLAIN VERSUS RAW FORMAT

The PNM formats each come in two varieties: the older plain (text) format and the newer raw (binary) format. There are different format codes for the plain and raw formats, but which of the two formats the pnm and pam functions write is independent of the format code you pass to them.

The pam functions always write raw formats. If you specify the format code for a plain format, a pam function assumes instead the raw version of that format.

The pnm functions choose between plain and raw based on the forceplain parameter that every write-type pnm function has. If this boolean value is true, the function writes the plain version of the format specified by the format code. If it is false, the function writes the raw version of the format specified by the format code.

We are trying to stamp out the older plain formats, so it would be a wise choice not to write a program that sets forceplain true under any circumstance. A user who needs a plain format can use the pnmtoplainpnm program to convert the output of your program to plain format.

Reference

The Libnetpbm Netpbm Image Processing Manual describes the the libnetpbm functions for processing image data.

The Libnetpbm Utility Manual describes the functions that are not specifically related to the Netpbm image formats.