PFM Format Description(5) File Formats Manual PFM Format Description(5)NAME
PFM - PFM graphic image file format
DESCRIPTION
This document describes the PFM graphic image file format as understood by the Netpbm converters pamtopfm(1)
and pfmtopam(1)
There are multiple similar formats known as PFM in the world, none of them authoritatively documented. The format described here is one
that Bryan Henderson deduced from a program he found somewhere that dealt with a 'PFM' format.
The PFM format is inspired by the Netpbm formats, and you will see lots of similarity. It is not, however, an official Netpbm format. Its
goal is not consistent with those of Netpbm formats.
The format
A PFM image is a stream of bytes. The stream consists of a header followed immediately by a raster. These two components are described
below. There are no delimeters before or after the sections as described.
PFM header
The PFM header is 3 consecutive 'lines' of ASCII text. After each line is a white space character. That character is typically a newline
character, hence the term 'line,' but doesn't have to be.
pamtopfm uses a newline in the PFM it generates.
Identifier Line
The identifier line contains the characters 'PF' or 'Pf'. PF means it's a color PFM. Pf means it's a grayscale PFM.
Dimensions Line
The dimensions line contains two positive decimal integers, separated by a blank. The first is the width of the image; the second is the
height. Both are in pixels.
Scale Factor / Endianness
The Scale Factor / Endianness line is a queer line that jams endianness information into an otherwise sane description of a scale. The
line consists of a nonzero decimal number, not necessarily an integer. If the number is negative, that means the PFM raster is little
endian. Otherwise, it is big endian. The absolute value of the number is the scale factor for the image.
The scale factor tells the units of the samples in the raster. You use somehow it along with some separately understood unit information
to turn a sample value into something meaningful, such as watts per square meter.
PFM raster
The raster is a sequence of pixels, packed one after another, with no delimiters of any kind. They are in standard Western reading order:
left to right and top to bottom within the image.
Each pixel consists of 1 or 3 samples, packed one after another, with no delimiters of any kind. 1 sample for a grayscale PFM and 3 for a
color PFM (see the Identifier Line of the PFM header).
Each sample consists of 4 consecutive bytes. The bytes represent a 32 bit string, in either big endian or little endian format, as deter-
mined by the Scale Factor / Endianness line of the PFM header. That string is an IEEE 32 bit floating point number code. Since that's the
same format that most CPUs and compiler use, you can usually just make a program use the bytes directly as a floating point number, after
taking care of the endianness variation.
netpbm documentation PFM Format Description(5)
Check Out this Related Man Page
PGM Format Specification(5) File Formats Manual PGM Format Specification(5)NAME
pgm - Netpbm grayscale image format
DESCRIPTION
This program is part of Netpbm(1)
The PGM format is a lowest common denominator grayscale file format. It is designed to be extremely easy to learn and write programs for.
(It's so simple that most people will simply reverse engineer it because it's easier than reading this specification).
A PGM image represents a grayscale graphic image. There are many pseudo-PGM formats in use where everything is as specified herein except
for the meaning of individual pixel values. For most purposes, a PGM image can just be thought of an array of arbitrary integers, and all
the programs in the world that think they're processing a grayscale image can easily be tricked into processing something else.
The name "PGM" is an acronym derived from "Portable Gray Map."
One official variant of PGM is the transparency mask. A transparency mask in Netpbm is represented by a PGM image, except that in place of
pixel intensities, there are opaqueness values. See below.
The format definition is as follows. You can use the libnetpbm(1)
C subroutine library to conveniently and accurately read and interpret the format.
A PGM file consists of a sequence of one or more PGM images. There are no data, delimiters, or padding before, after, or between images.
Each PGM image consists of the following:
o A 'magic number' for identifying the file type. A pgm image's magic number is the two characters 'P5'.
o Whitespace (blanks, TABs, CRs, LFs).
o A width, formatted as ASCII characters in decimal.
o Whitespace.
o A height, again in ASCII decimal.
o Whitespace.
o The maximum gray value (Maxval), again in ASCII decimal. Must be less than 65536, and more than zero.
o A single whitespace character (usually a newline).
o A raster of Height rows, in order from top to bottom. Each row consists of Width gray values, in order from left to right. Each
gray value is a number from 0 through Maxval, with 0 being black and Maxval being white. Each gray value is represented in pure
binary by either 1 or 2 bytes. If the Maxval is less than 256, it is 1 byte. Otherwise, it is 2 bytes. The most significant byte
is first.
A row of an image is horizontal. A column is vertical. The pixels in the image are square and contiguous.
Each gray value is a number proportional to the intensity of the pixel, adjusted by the ITU-R Recommendation BT.709 gamma transfer
function. (That transfer function specifies a gamma number of 2.2 and has a linear section for small intensities). A value of zero
is therefore black. A value of Maxval represents CIE D65 white and the most intense value in the image and any other image to which
the image might be compared.
Note that a common variation on the PGM format is to have the gray value be 'linear,' i.e. as specified above except without the
gamma adjustment. pnmgamma takes such a PGM variant as input and produces a true PGM as output.
In the transparency mask variation on PGM, the value represents opaqueness. It is proportional to the fraction of intensity of a
pixel that would show in place of an underlying pixel. So what normally means white represents total opaqueness and what normally
means black represents total transparency. In between, you would compute the intensity of a composite pixel of an 'under' and
'over' pixel as under * (1-(alpha/alpha_maxval)) + over * (alpha/alpha_maxval). Note that there is no gamma transfer function in
the transparency mask.
Strings starting with '#' may be comments, the same as with PBM(1)
Note that you can use pamdepth to convert between a the format with 1 byte per gray value and the one with 2 bytes per gray value.
There is actually another version of the PGM format that is fairly rare: 'plain' PGM format. The format above, which generally considered
the normal one, is known as the 'raw' PGM format. See pbm(1)
for some commentary on how plain and raw formats relate to one another and how to use them.
The difference in the plain format is:
- There is exactly one image in a file.
- The magic number is P2 instead of P5.
- Each pixel in the raster is represented as an ASCII decimal number (of arbitrary size).
- Each pixel in the raster has white space before and after it. There must be at least one character of white space between any two
pixels, but there is no maximum.
- No line should be longer than 70 characters.
Here is an example of a small image in the plain PGM format.
P2
# feep.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
There is a newline character at the end of each of these lines.
Programs that read this format should be as lenient as possible, accepting anything that looks remotely like a PGM.
All characters referred to herein are encoded in ASCII. 'newline' refers the character known in ASCII as Line Feed or LF. A 'white space'
character is space, CR, LF, TAB, VT, or FF (I.e. what the ANSI standard C isspace() function calls white space).
COMPATIBILITY
Before April 2000, a raw format PGM file could not have a maxval greater than 255. Hence, it could not have more than one byte per sample.
Old programs may depend on this.
Before July 2000, there could be at most one image in a PGM file. As a result, most tools to process PGM files ignore (and don't read) any
data after the first image.
SEE ALSO pnm(1) , pbm(1) , ppm(1) , pam(1) , libnetpbm(1) , programsthatprocessPGM(1) ,
AUTHOR
Copyright (C) 1989, 1991 by Jef Poskanzer.
netpbm documentation 03 October 2003 PGM Format Specification(5)