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MRF image format specification(0)				MRF image format specification(0)

       MRF - monochrome recursive format (compressed bitmaps)

       This program is part of Netpbm(1)

       MRF  is	a compressed format for bilevel (1-bit mono) images.  It achieves better compres-
       sion 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).

       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

       First,  here's  what  goes  where  in  an MRF file. I'll explain how the compression works

       Offset Description

       0      magic number - 'MRF1' (in ASCII)

       4      width (32-bit, MSB first (i.e. big-endian))

       8      height (same)

       12     reserved (single byte, must be zero)

       13     compressed data

       Note that there is no end-of-file marker in the file itself - the compressed data  carries
       on right up to EOF.

       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.

       The image is treated as a number of 64x64 squares, forming a grid large enough  to  encom-
       pass  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  unde-
       fined)  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

	   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

       (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.)

       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.

       If writing that sounds too much like hard work :-), you could always adapt pbmtomrf and/or
       mrftopbm.  That's the main reason their source code is public domain, after all.

       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 sim-
       ply leaving it blank. pbmtomrf does a little in	this  respect  but  could  definitely  be
       improved upon.

       mrftopbm's  -1 option causes it to include the edges, if any, in the output PBM.  This may
       help when debugging a compressor's edge optimization.

       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.

       mrftopbm(1) , pbmtomrf(1)

       Russell Marks.

netpbm documentation			       1991		MRF image format specification(0)
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