Libnetpbm Utility Functions
Updated: 27 August 2006
Table Of Contents
NAMElibpm ‐ netpbm utility functions
DESCRIPTION
These library functions are part of Netpbm.
This page documents functions in the Netpbm subroutine library
that are not
directly related to image data.
For introductory and general information using libnetpbm, see
Libnetpbm
User’s Guide.
The most commonly used libnetpbm functions are those that read
and write and
process Netpbm images. Those are documented in Libnetpbm
Netpbm Image
Processing Manual
To use these services, #include pam.h.
Functions
Initialization
Overview
void pm_init( int *argcP, char *argv[] );
Description
All Netpbm programs must call pm_init() just after startup,
before they
process their arguments. pm_init(), among other things, pro‐
cesses Netpbm
universal arguments and removes them from the argument list.
A program that isn’t a Netpbm program, but just uses libnetpbm
services,
need not invoke pm_init.
File Or Image Stream Access
Overview
FILE *pm_openr( char * name)
FILE *pm_openw( char * name );
FILE *pm_openr_seekable( const char * name );
FILE *pm_close( FILE * fp );
void pm_tell2( FILE * fileP, pm_filepos * fileposP, unsigned
int fileposSize
);
unsigned int pm_tell( FILE * fileP );
void pm_seek2( FILE * fileP, const pm_filepos * fileposP, un‐
signed int
fileposSize );
void pm_seek( FILE * fileP, unsigned long filepos );
char *pm_read_unknown_size( FILE * fp, long * nread );
Description
An image stream is just a file stream (represented in the
standard C library
as type FILE *).
These routines work on files > 2 GiB if the underlying system
does, using
the standard large file interface. Before Netpbm 10.15 (April
2003), though,
they would fail to open any file that large or process any
offset in a file
that could not be represented in 32 bits.
pm_openr() opens the given file for reading, with appropri‐
ate error
checking. A filename of ‐ is taken to mean Standard Input.
pm_openw() opens
the given file for writing, with appropriate error checking.
pm_close()
closes the file descriptor, with appropriate error checking.
pm_openr_seekable() appears to open the file just like
pm_openr(), but the
file thus opened is guaranteed to be seekable (you can use
ftell() and
fseek() on it). pm_openr_seekable() pulls this off by copying
the entire
file to a temporary file and giving you the handle of the tem‐
porary file, if
it has to. If the file you name is a regular file, it’s al‐
ready seekable sopm_openr_seekable() just does the same thing as pm_openr().
But if it is,
say, a pipe, it isn’t seekable. So pm_openr_seekable() reads
the pipe until
EOF into a temporary file, then opens that temporary file and
returns the
handle of the temporary file. The temporary file is seekable.
The file pm_openr_seekable() creates is one that the operat‐
ing system
recognizes as temporary, so when you close the file, by any
means, it gets
deleted.
You need a seekable file if you intend to make multiple passes
through the
file. The only alternative is to read the entire image into
memory and work
from that copy. That may use too much memory. Note that the
image takes less
space in the file cache than in a buffer in memory. As much as
96 times less
space! Each sample is an integer in the buffer, which is usu‐
ally 96 bits. In
the file, a sample may be as small as 1 bit and rarely more
than 8 bits.pm_tell2() returns a handle for the current position of the
image stream
(file), whether it be the header or a row of the raster. Use
the handle as
an argument to pm_seek2() to reposition the file there later.
The file must
be seekable (which you can ensure by opening it with
pm_openr_seekable()) or
this may fail.
The file position handle is of type pm_filepos, which is in‐
tended to be
opaque, i.e. used only with these two functions. In practice,
it is a file
offset and is 32 bits or 64 bits depending upon the capabili‐
ty of the
underlying system. For maximum backward and forward compati‐
bility, the
functions that take or return a pm_filepos have a fileposSize
argument for
the size of the data structure. In C, simply code size‐
of(pm_filepos) for
that.
pm_seek() and pm_tell are for backward compatibility only. Do
not use them
in new code. These functions are not capable of handle posi‐
tions in files
whose byte offset cannot be represented in 32 bits.
pm_tell2() and pm_seek2() replaced pm_tell() and pm_seek() in
Netpbm 10.15
(April 2003).
pm_read_unknown_size() reads an entire file or input stream of
unknown size
to a buffer. It allocates more memory as needed. The calling
routine has to
free the allocated buffer with free().
pm_read_unknown_size() returns a pointer to the allocated buf‐
fer. The nread
argument returns the number of bytes read.
Endian I/O
Entry Points
void pm_readchar( FILE * in, char * sP );
void pm_writechar( FILE * out, char s );
int pm_readbigshort( FILE * in, short * sP );
int pm_writebigshort( FILE * out, short s );
int pm_readbiglong( FILE * in, long * lP );
int pm_writebiglong( FILE * out, long l );
int pm_readlittleshort( FILE * in, short * sP );
int pm_writelittleshort( FILE * out, short s );
int pm_readlittlelong( FILE * in, long * lP );
int pm_writelittlelong( FILE * out, long l );
void pm_readcharu( FILE * in, char * sP );
void pm_writecharu( FILE * out, char s );
int pm_readbigshortu( FILE * in, short * sP );
int pm_writebigshortu( FILE * out, short s );
int pm_readbiglongu( FILE * in, long * lP );
int pm_writebiglongu( FILE * out, long l );
int pm_readlittleshortu( FILE * in, short * sP );
int pm_writelittleshortu( FILE * out, short s );
int pm_readlittlelongu( FILE * in, long * lP );
int pm_writelittlelongu( FILE * out, long l );
Description
pm_readchar(), pm_writechar(), pm_readbigshort(), pm_write‐
bigshort(),
pm_readbiglong(), pm_writebiglong(), pm_readlit‐
tleshort(),
pm_writelittleshort(), pm_readlittlelong(), and pm_writelit‐
tlelong() are
routines to read and write 1‐byte, 2‐byte, and 4‐byte pure bi‐
nary integers
in either big‐ or little‐endian byte order. Note that a "long
int" C type
might be wider than 4 bytes, but the "long" routines still
read and write 4
bytes.
pm_readbiglongu(), etc. (names ending in u) are the same ex‐
cept they work on
unsigned versions of the type.
The routines with declared return values always return 0. Be‐
fore Netpbm
10.27 (March 2005), they returned ‐1 on failure, including
EOF. Now, they
issue an error message to Standard Error and abort the program
if the I/O
fails or encounters EOF.
The 1‐byte routines were new in Netpbm 10.27 (March 2005). The
unsigned
versions were new somewhere around Netpbm 10.21 (2004).
Maxval Arithmetic
Entry Points
int pm_maxvaltobits( int maxval );
int pm_bitstomaxval( int bits );
unsigned int pm_lcm( unsigned int x, unsigned int y, un‐
signed int z,
unsigned int limit );
Description
pm_maxvaltobits() and pm_bitstomaxval() convert between a max‐
val and the
minimum number of bits required to hold it.
pm_lcm() computes the least common multiple of 3 integers. You
also specify
a limit and if the LCM would be higher than that limit,
pm_lcm() just
returns that limit.
Gamma Arithmetic
Entry Points
float pm_gamma( float intensity );
float pm_ungamma( float brightness );
Description
In graphics processing, there are two common ways of rep‐
resenting
numerically the intensity of a pixel, or a component of a pix‐
el.
The obvious way is with a number that is directly proportional
to the light
intensity (e.g. 10 means twice as many milliwatts per square
centimeter as
5). There are two problems with this:
* To the human eye, a 1 milliwatt per square centimeter dif‐
ference in a
bright image is much less apparent than a 1 milliwatt
per square
centimeter difference in a dark image. So if you have a
fixed number of
bits in which to store the intensity value, you’re wasting
resolution at
the bright end and skimping on it at the dark end.
* Monitor inputs and camera outputs aren’t directly propor‐
tional to the
light intensity they project or detect.
For these reasons, light intensities are often represented in
graphics
processing by an exponential scale. The transfer function is
called a gamma
function and the resulting numbers are called gamma‐cor‐
rected or
gamma‐adjusted. There are various gamma functions. The Netpbm
formats
specify that intensities are represented by gamma‐adjusted
numbers of a
particular gamma transfer function.
These functions let you convert back and forth between these
two scales,
using the same gamma transfer function that is specified in
the Netpbm
format specifications.
pm_gamma709 converts from an intensity‐proportional intensity
value to a
gamma‐adjusted intensity value (roughly proportional to
brightness, which is
the human subjective perception of intensity), using
the ITU‐R
Recommendation BT.709 gamma transfer function.
pm_ungamma709 is the inverse of pm_gamma709.
Messages
Overview
void pm_message( char * fmt, ... );
void pm_setusermessagefn(pm_usermessagefn * function);
Description
pm_message() is a printf() style routine to write an informa‐
tional message
to the Standard Error file stream. pm_message() suppresses the
message,
however, if the user specified the ‐quiet option on the com‐
mand line. See
the initialization functions, e.g. pnm_init(), for information
on the ‐quiet
option. Note that Netpbm programs are often used interactive‐
ly, but also
often used by programs. In the interactive case, it is nice
to issue
messages about what the program is doing, but in the program
case, such
messages are usually undesirable. By using pm_message() for
all your
messages, you make your program usable in both cases. Without
any effort on
your part, program users of your program can avoid the
messages by
specifying the ‐quiet option.
Netpbm distinguishes between error messages and information
messages;
pm_message() is just for informational messages. To issue an
error message,
see pm_errormsg().
pm_setusermessagefn registers a handler for informational mes‐
sages, called a
user message routine. Any library function (including pm_mes‐
sage()) that
wants to issue an informational message in the future will
call that
function with the message as an argument instead of writing
the message to
Standard Error.
The argument the user message routine gets is English text de‐
signed for
human reading. It is just the text of the message; there is no
attempt at
formatting in it (so you won’t see any newline or tab charac‐
ters).
To capture error messages in addition to informational mes‐
sages, see
pm_setusererrormsgfn().
You can remove the user message routine, so that the library
issues future
informational messages in its default way (write to Standard
Error) by
specifying a null pointer for function. Example:
static pm_usermessagefn logfilewrite;
static void
logfilewrite(const char * const msg) {
fprintf(mymsglog, "Netpbm message: %s", msg);
}
pm_setusermessagefn(&logfilewrite);
pm_message("Message for the message log");
System Utilities
* pm_system
* pm_tmpfile
Keyword Matching
Entry Points
void pm_keymatch();
Description
This subroutine is obsolete. It used to be used for command
line option
processing. Today, you can do better option processing more
easily with the
shhopt facility. See any recent program in the Netpbm pack‐
age for an
example. pm_keymatch() does a case‐insensitive match of str
against keyword.
str can be a leading substring of keyword, but at least min‐
chars must be
present.
_________________________________________________________________
Table Of Contents
* Functions
+ Initialization
+ File Or Image Stream Access
+ Endian I/O
+ Maxval Arithmetic
+ Messages And Errors
+ Keyword
