[Coco] VEF specification

[Willard Goosey]

>From: "Boisy G. Pitre" <boisy at boisypitre.com>
>Date: Tue, 10 Aug 2004 20:22:44 -0500
>
>I'm looking for the specification on VEF, the CoCo 3 VDG Extended File 
>format.  Instead of trying to construct it from looking at source, and 
>possibly missing a detail, does there exist a document which explicitly 
>defines the format of a VEF file?

Yep.  The MVCanvas docs, by Mike Haaland of Hyper-Tech Software, 1989,
updated 03/22/90.  

He says:

FILE FORMAT
----------------

NOTE: "char" below refers to a Byte.

VEF picture format:

A two byte VEF Header.  Header Values are in Hexadecimal

Standard   Squashed  Screen Type  Screen size   Colors
-------------------------------------------------------
0000       8000       08          320x200       16
0001       8001       07          640x200        4
0003       8003       06          320x200        4
0004       8004       05          640x200        2

16 bytes palette information

Picture Data:
----------------

If picture is a Standard (UnCompressed) VEF

     Raw Data -- just put it on the screen

If picture is compressed (Squashed)

     char len;	   Number of bytes to read and decode to restore 1/2 scan
		   line of screen data.  1/2 scan line is 80 bytes on a
		   type 7 or 8 screen, 40 bytes on types 5 or 6 screen.
		   (40 and 80 are Decimal values)
     char count;   compressed/uncompressed byte count

     if data is compressed:

     char data;

	if data is uncompressed:

     char data[count];

The header data will only be found once at the beginning of the file.

Now to decode the Compressed Data.  Read the first byte and call it
len.  It's the length of the compressed 1/2 scan line.  Now read 'len'
bytes into an array.  All we have to do now is to decode the data we
just read into the buffer.  Get the first byte in the buffer and see
if the high bit is set.  It's our 'count' byte.  If the high bit is
set it's compressed.  Now subtract 128 from our 'count' (clear the
high bit) and put the next byte in the buffer 'count' times.

If the high bit is not set, get the next 'count' bytes from the buffer
and put them on the screen.

This continues until you've used 'len' bytes that are in the buffer.
When the buffer has been decoded, read the next 'len' byte from the
file and do it again.  This will be repeated 400 times in the file.

For compressed data:
--------------------------
                   (In Decimal)
    char count,    1 to 127 - High bit set - means its compressed
         data;     this will be the screen data, put it on the screen
                   'count' times.

For uncompressed data:
--------------------------
                   (In Decimal)
    char count,    1 to 127 - Uncompressed number of bytes to follow.
         data[count];


Willard says:  That's it!  I've only dealt with uncompressed VEFs
myself.  
>
>Boisy
>
>
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-- 
Willard Goosey  goosey at sdc.org
Socorro, New Mexico, USA
"I've never been to Contempt!  Isn't that somewhere in New Mexico?"
   --- Yacko