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From: lsmichae@immd4.informatik.uni-erlangen.de (Lars Michael)
Newsgroups: comp.sys.atari.st
Subject: SPECTRUM 512 information
Message-ID: <1991Mar1.152116.2195@informatik.uni-erlangen.de>
Date: 1 Mar 91 15:21:16 GMT
Organization: CSD., University of Erlangen, Germany
Lines: 52


Hi netlanders,

there was some confusion about the format of a SPECTRUM 512 picture.
I've done some work and here are my results:

The description of the file format of *.SPU and *.SPC in the ST
picture formats document made by David Baggett is correct.

I have some correction for the area of validity of the colors. 

In a SPECTRUM 512 picture each scan line has three color palettes,
that will make 48 colors. In the following I will number these three
palettes from 0 to 2 (first palette 0), e.g. color 45 will appear in 
palette 2. All three palettes build the one *virtual* palette.
All scan lines are independent from each other.

The ST has one hardware palette of 16 colors. Let me call this the
*real* palette.

Get the index <cnr> of a color of the virtual palette, the valid area 
will apear in the following table (left and right border are
inclusive).
				left border 	right border
Palette 0
0<cnr<=15:	cnr odd		0 		10*cnr-6
		cnr even	0		10*cnr-1

Palette 1
16<=cnr<=31:	cnr odd		10*(cnr-16)-5	10*cnr-6
		cnr even	10*(cnr-16)	10*cnr-1

Palette 2
32<=cnr<=47:	cnr odd		10*(cnr-16)-6	319
		cnr even	10*(cnr-16)	319

Color 0 in palette 0 has no valid area and is the border color.
SPECTRUM 512 sets the colors with number 0, 15, 16, 31, 32 and 47
to black [$000].

With the above table its not difficult to create a corresponding table
where you can get thy real palette from a x-value of a pixel.


								Lars

+-----------------------------------+----------------------------------+
|  Lars Michael                     |    | | |                         |
|  lsmichae@faui43.uni-erlangen.de  |    | | |   "Down with ATARI,     |
+-----------------------------------+   /  |  \   Long live the ST !"  |
|  "May the Schwartz be with you!"  |  /   |   \                       |
+-----------------------------------+----------------------------------+