Topic       : ATARI ST Picture formats
Author      : David Baggett/Jan K⌐upka
Version     : atari_gf.hyp 0.01 (17/09/96)
Subject     : Documentation/File formats
Nodes       : 52
Index Size  : 1328
HCP-Version : 3
Compiled on : Atari
@charset    : atarist
@lang       : 
@default    : 
@help       : 
@options    : -i -s +y +z
@width      : 75Spectrum 512    *.SPU

80 words        first scan line of picture (unused) -- should be zeroes
15920 words     picture data (screen memory) for scan lines 1 through 199
9552 words      3 palettes for each scan line (the top scan line is
                not included because Spectrum 512 can't display it)
51104 bytes     total

  Note that the Spectrum 512 mode's three palette changes per scan
line allow more colors on the screen than normally possible, but a
tremendous amount of CPU time is required to maintain the image.

  The Spectrum format specifies a palette of 48 colors for each scan line.
To decode a Spectrum picture, one must be know which of these 48 colors
are in effect for a given horizontal pixel position.

  Given an x-coordinate (from 0 to 319) and a color index (from 0 to 15),
the following C function will return the proper index into the Spectrum
palette (from 0 to 47):

/*
 *  Given an x-coordinate and a color index, returns the corresponding
 *  Spectrum palette index.
 *
 *  by Steve Belczyk; placed in the public domain December, 1990.
 */
int FindIndex(x, c)
        int x, c;
{
        int x1;

        x1 = 10 * c;

        if (1 & c)              /* If c is odd */
                x1 = x1 - 5;
        else                    /* If c is even */
                x1 = x1 + 1;






        if (x >= x1 && x < x1 + 160) c = c + 16;
        else if (x >= x1 + 160) c = c + 32;

        return c;
}