PFM Format Description(5) 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 fol‐
lowed 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 determined
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)
