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



NAME
       MRF - monochrome recursive format (compressed bitmaps)


DESCRIPTION
       This program is part of Netpbm(1).

       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).

       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).

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



       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 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:

           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 com-
       pressed) 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. 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.



SEE ALSO
       mrftopbm(1), pbmtomrf(1)


AUTHOR
       Russell Marks.



netpbm documentation                 1991    MRF image format specification(0)