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rawtopgm - convert raw grayscale bytes into a portable graymap
rawtopgm [-bpp [1|2]] [-littleendian] [-maxval N] [-headerskip N]
[-rowskip N] [-tb|-topbottom] [width height] [imagefile]
Reads raw grayscale values as input. Produces a PGM file as output.
The input file is just a sequence of pure binary numbers, either one or
two bytes each, either bigendian or littleendian, representing gray
values. They may be arranged either top to bottom, left to right or
bottom to top, left to right. There may be arbitrary header
information at the start of the file (to which rawtopgm pays no
attention at all other than the header’s size).
Arguments to rawtopgm tell how to interpret the pixels (a function that
is served by a header in a regular graphics format).
The width and height parameters tell the dimensions of the image. If
you omit these parameters, rawtopgm assumes it is a quadratic image and
bases the dimensions on the size of the input stream. If this size is
not a perfect square, rawtopgm fails.
When you don’t specify width and height, rawtopgm reads the entire
input stream into storage at once, which may take a lot of storage.
Otherwise, rawtopgm ordinarily stores only one row at a time.
If you don’t specify imagefile, or specify -, the input is from
The PGM output is to Standard Output.
N is the maxval for the gray values in the input, and is also
the maxval of the PGM output image. The default is the maximum
value that can be represented in the number of bytes used for
each sample (i.e. 255 or 65535).
tells the number of bytes that represent each sample in the
input. If the value is 2, The most significant byte is first in
The default is 1 byte per sample.
says that the bytes of each input sample are ordered with the
least significant byte first. Without this option, rawtopgm
assumes MSB first. This obviously has no effect when there is
only one byte per sample.
rawtopgm skips over N bytes at the beginning of the stream and
reads the image immediately after. The default is 0.
This is useful when the input is actually some graphics format
that has a descriptive header followed by an ordinary raster,
and you don’t have a program that understands the header or you
want to ignore the header.
If there is padding at the ends of the rows, you can skip it
with this option. Note that rowskip need not be an integer.
Amazingly, I once had an image with 0.376 bytes of padding per
row. This turned out to be due to a file-transfer problem, but
I was still able to read the image.
Skipping a fractional byte per row means skipping one byte per
By default, rawtopgm assumes the pixels in the input go top to
bottom, left to right. If you specify -bt or -bottomfirst,
rawtopgm assumes the pixels go bottom to top, left to right.
The Molecular Dynamics and Leica confocal format, for example,
use the latter arrangement.
If you don’t specify -bt when you should or vice versa, the
resulting image is upside down, which you can correct with
This option causes rawtopgm to read the entire input stream into
storage at once, which may take a lot of storage. Ordinarly,
rawtopgm stores only one row at a time.
For backwards compatibility, rawtopgm also accepts -tb and
-topbottom to mean exactly the same thing. The reasons these
are named backwards is that the original author thought of it as
specifying that the wrong results of assuming the data is top to
bottom should be corrected by flipping the result top for
bottom. Today, we think of it as simply specifying the format
of the input data so that there are no wrong results.
pgm(5), rawtoppm(1), pnmflip(1)
Copyright (C) 1989 by Jef Poskanzer.
Modified June 1993 by Oliver Trepte, email@example.com
14 September 2000 rawtopgm(1)