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 CCD Astrocamera format DOT for CCDDOTV Software
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 Preamble
----------
The early CCD cameras were for astronomy buffs with a lot of
money to spare, monochrome and quite small. Image processing
software could hardly be found and it was even harder to bring
the money together to buy it. Some amateurs wrote dedicated
programs and saved the processed pictures in newly invented file
formats.

The CCDTOOLS [1] - a suite of astro CCD image processing 
software - were developed by myself in the early 90s. A DOS
application only, but quite comprehensiv, very fast, extremly
cheap (gratis) and thus rather popular.

At first, I could not do cross section presentations and
invented the Dotogram instead. Later, when I had implemented the
cross section presentation, I kept the Dotogram because it is at
times a supperior to show the data. The file format used is
simple enough. A DOT file may contain a Dotogram or a cross
section picture.


 The DOT format
----------------
A Dotogram is 640 pixels wide and 350 pixels high and has
224'000 dots. Since it is a two color picture, one bit suffices
to store a dot. The 640 column bits are combined into 80 bytes
and there are 350 lines made up that way. The bits are rotated
from right to left into the byte. Actually, the 28'000 bytes are
a dump of the EGA video RAM at segment A000 hex.

The DOT format has the file extension DOT. There is a 5 byte
file head and data compression is employed. The encoding type is
a blend of Hewlett Packards Run Length Encoding (RLE) as found
in printer software [2], and Run Length Limitation Encoding
(RLLE) initially developed by Mark Zachmann of ZSoft [3,4],
later to be found in Microsofts Paintbrush PCX image file
format.

+--------------+-------------------------+
¦ 5 bytes Head ¦ n bytes RLCE image data ¦
+--------------+-------------------------+

The DOT file can contain image data for pictures of 350 lines
with 640 columns each. The dots are organized from left to
right, top to bottom.


 File Head
-----------
Bytes 0 & 1: Integer value (Intel format, i.e. low-high)
             containing the file size.
Bytes 2 & 3: Integer value containing the number of pixel values
             in the file.
Byte 4:      First Counter.


 Run Length Count Encoding (RLCE)
----------------------------------
RLCE combines the advantages of RLE and RLLE while evading
their disadvantages.

Byte 4, the 5th byte in the file, is the first counter. Bits 0
thru 6 make up the counter. A value of 0 is prohibitted and
flags a corrupted file. Bit 7 is special.

If bit 7 is 0: If this is the very first counter (byte 4 in the
file), this means that all data following it are uncompressed
pixel values. If it is any other counter, bit 7 = 0 means that
the following number of bytes as is the value of bit 0 thru 6 of
this counter byte, must be copied. The next counter byte will
follow immediately after the number of bytes to be copied.

Bit 7 = 1 means that the following byte has to be replicated as
many times, as is the value of bit 0 thru 6 of this counter
byte. The byte after the byte following the counter is the next
counter.

The file size is between 705 and 28'500 bytes.


 REFERENCES
------------
[1] Wernli, Hans-Ruedi; Die CCD Astrokamera für den Amateur,
    1995, Birkhäuser. ISBN 3-7643-5218-3.

[2] Hewlett Packard, Deskjet 500/500C Technical Reference
    Manual, February 1992, Part Number C2114-90008.

[3] Pohl, Martin; Eriksdottar Holger; Die wunderbare Welt der
    Grafikformate; 1991, Wolfram's, ISBN 3-925328-05-x.

[4] Born, Günter; Referenzhandbuch Dateiformate, 1992,
    Addison-Wesley, ISBN 3-89319-446-0.


 Author
--------
hans-rudolf.wernli@bluewin.ch
http://mypage.bluewin.ch/horo/
05. August 2002