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PFM Format
This document describes the PFM
graphic image file format as understood by
the Netpbm converters pamtopfm and pfmtopam.
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
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.
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