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 com‐
pression 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 com‐
pressed 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-bot‐
tom 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 compressed) is undefined. This is
deliberate so that the MRF compressor can make these unseen areas any‐
thing it wants so as to maximize compression, rather than simply leav‐
ing it blank. pbmtomrf does a little in this respect but could defi‐
nitely 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 optimiza‐
tion.
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 ALSOmrftopbm(1), pbmtomrf(1)AUTHOR
Russell Marks.
netpbm documentation 1991 MRF image format specification(0)
