.TL
MRF image format specification
Table Of Contents
.SH 1
MRF format
.LP
Updated: 1991
.br
.SH 2
NAME
.LP
MRF - monochrome recursive format (compressed bitmaps)
.SH 2
DESCRIPTION
.LP
.LP
This program is part of Netpbm.
.LP
MRF is a compressed format for bilevel (1-bit mono) images.  It
achieves better compression for some such images than either GIF or
PNG. (It's also very easy to implement (about the same difficulty as
RLE, I'd say) and an MRF reader needs no tables/buffers, which may
make it useful for tiny machines).
.LP
In case the above hasn't made it sufficiently clear, I'll make this
next point explicitly: MRF cannot represent color at all. Nor
can it represent grayscale.  It's a specifically mono format.  (If you
want to compress a color or grayscale image, my advice is to use
JPEG2000).
.LP
First, here's what goes where in an MRF file. I'll explain how the
compression works afterward.
.RS
.IP "Offset"
Description
.IP "0"
magic number - "MRF1" (in ASCII)
.IP "4"
width (32-bit, MSB first (i.e. big-endian))
.IP "8"
height (same)
.IP "12"
reserved (single byte, must be zero)
.IP "13"
compressed data
.RE
.LP
Note that there is no end-of-file marker in the file itself - the
compressed data carries on right up to EOF.
.LP
The way the picture is compressed is essentially very simple, but
as they say, the devil is in the detail.  So don't be put off if it
sounds confusing.
.LP
The image is treated as a number of 64x64 squares, forming a grid
large enough to encompass it. (If an image is (say) 129x65, it'll be
treated in the same way as a 192x128 one. On decompression, the extra
area which was encoded (the contents of this area is undefined) should
be ignored.) Each of these squares in turn (in left-to-right,
top-to-bottom order) is recursively subdivided until the smallest
completely black or white squares are found. Some pseudocode (eek!)
for the recursive subdivision routine should make things clearer:
.DS L
    if square size > 1x1 and square is all one color, output 1 bit
    if whole square is black, output a 0 bit and return
    if whole square is white, output a 1 bit and return
    output a 0 bit
    divide the square into four quarters, calling routine for
    each in this order: top-left, top-right, bottom-left, bottom-right
.DE
.LP
(Note that the "output a 0 bit" stage is not reached for squares
of size 1x1, which is what stops it recursing infinitely.  I mention
this as it may not be immediately obvious.)
.LP
The whole of the compressed data is made up of the bits output by
the above routine. The bits are packed into bytes MSB first, so for
example outputting the bits 1,0,0,0,0,0,0,0 would result in a 80h byte
being output. Any `unused' bits in the last byte output are undefined;
these are effectively after EOF and their value is unimportant.
.LP
If writing that sounds too much like hard work :-), you could
always adapt \fBpbmtomrf\fR and/or \fBmrftopbm\fR.  That's the main
reason their source code is public domain, after all.
.LP
Above, I said the contents of any extra area encoded (when a bitmap
smaller than the grid of squares is compressed) is undefined.  This is
deliberate so that the MRF compressor can make these unseen areas
anything it wants so as to maximize compression, rather than simply
leaving it blank. \fBpbmtomrf\fR does a little in this respect but
could definitely be improved upon.
.LP
\fBmrftopbm\fR's \fB-1\fR option causes it to include the edges, if
any, in the output PBM.  This may help when debugging a compressor's
edge optimization.
.LP
Note that the "F" in the name "MRF" comes from "format," which is redundant
because it is the name of a format.  That sort of makes "MRF format" sound
as stupid as "PIN number," but it's not really that bad.
.SH 2
SEE ALSO
.LP
\fBmrftopbm\fR,
\fBpbmtomrf\fR
.SH 2
AUTHOR
.LP
Russell Marks.
.br
\l'5i'
.SH 2
Index
.LP
.IP \(bu
NAME
.IP \(bu
DESCRIPTION
.IP \(bu
SEE ALSO
.IP \(bu
AUTHOR
.LP