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diff --git a/pfm.html b/pfm.html new file mode 100644 index 00000000..c08c3a53 --- /dev/null +++ b/pfm.html @@ -0,0 +1,82 @@ +<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 3.2//EN"> +<HTML><HEAD><TITLE>PFM Format Description</TITLE></HEAD> +<BODY> +<H1>PFM Format</H1> +<p> +This document describes the PFM graphic image file format as understood by +the Netpbm converters <a href="pamtopfm.html"><b>pamtopfm</b></a> and +<a href="pfmtopam.html"><b>pfmtopam</b></a>. + +<p>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. + +<p>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. + +<h2>The format</h2> + +<p>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. + +<h3>PFM header</h3> + +<p>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. + +<p><b>pamtopfm</b> uses a newline in the PFM it generates. + +<h4>Identifier Line</h4> + +<p>The identifier line contains the characters "PF" or +"Pf". PF means it's a color PFM. Pf means it's a grayscale +PFM. + +<h4>Dimensions Line</h4> + +<p>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. + +<h4>Scale Factor / Endianness</h4> + +<p>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. + +<p>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. + + +<h3>PFM raster</h3> + +<p>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. + +<p>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). + +<p>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. + +</BODY> +</HTML> + |