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.  This format appears to be the one Gimp
       calls PFM.

       Another  important  PFM  is  used  by  Adobe Photoshop and appears to be identical to this
       except that rows are ordered from top to bottom in the Adobe version.  If you  interchange
       images  between  systems  that  use these two formats, it will work except that your image
       gets flipped upside down.  You can compensate for that with pamflip -topbottom.

       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 delimiters 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 grouped by row, with the pixels in each row ordered left to right and the
       rows ordered bottom to top.

        Note: This is the opposite of Netpbm formats, which order rows top to bottom.  It is also
       the  opposite  of  the  format  Adobe  Photoshop calls PFM.  See the introduction for more
       information on this disparity.

       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.

DOCUMENT SOURCE

       This  manual page was generated by the Netpbm tool 'makeman' from HTML source.  The master
       documentation is at

              http://netpbm.sourceforge.net/doc/pfm.html