Newsgroups:	comp.os.msdos.djgpp
From:	pscargill AT cix DOT compulink DOT co DOT uk ("Peter Scargill")
Subject:	Re: ALLEGRO - 256 shades of grey?? How?
Message-ID:	<EIGLHA.43r@cix.compulink.co.uk>
Organization:	Compulink Information eXchange
References:	<3 DOT 0 DOT 16 DOT 19971019191838 DOT 35af0f86 AT hem1 DOT passagen DOT se>
Date:	Wed, 22 Oct 1997 15:24:46 GMT
Lines:	65
To:	djgpp AT delorie DOT com
DJ-Gateway:	from newsgroup comp.os.msdos.djgpp

I hope this will be the final comment on this extended thread which I;m 
ashamed to say I started. Thanks to everyone who contributed something 
useful to the thread, and here is the end result.

You CAN'T have more than 64 greys using standard VGA registers, not 
directly, but you CAN make it look that way. The code below is a 
combination of my own idea and code from the archives. I have allocated 
the first 16 colours to be the standard IBM colours, and then allocated 
238 shades of grey, using a method taken from the archives of staggering 
the green and blue somewhat. This works more or less perfectly. There are 
2 unallocated colours at the top which I've madce black and white.

I have been experimenting with this having read the arguments presented 
in this thread and I can tell you for sure that not only can you tell the 
difference between a 16-grey palette and a 256-grey palette but you can 
also discern the loss of quality when you move down from 256 to 64 
shades. 

The solution below allows you to use other objects in normal vga colours 
and yet still put up perfectly acceptable grey images. Incidentally, 
thanks for all the help folks with the BT848 which I may add Brooktree or 
their new bosses failed to help with at all, providing only the most 
useless of DOS libraries and offering no support at all, not to mention 
holding onto some source code. This particular chip can grab images using 
a restricted palette - yes, you guessed it, colours 16-253 !!! Here's the 
code, works a treat within Allegro and thanks to everyone for their help.

void set_our_palette(void)
{
int i;
RGB rgb;                // for the palette
    // 16 standard vga colors first
    rgb.r=00; rgb.g=00; rgb.b=00; set_color(0,&rgb);
    rgb.r=00; rgb.g=00; rgb.b=31; set_color(1,&rgb);
    rgb.r=00; rgb.g=31; rgb.b=00; set_color(2,&rgb);
    rgb.r=00; rgb.g=31; rgb.b=31; set_color(3,&rgb);
    rgb.r=31; rgb.g=00; rgb.b=00; set_color(4,&rgb);
    rgb.r=31; rgb.g=00; rgb.b=31; set_color(5,&rgb);
    rgb.r=31; rgb.g=31; rgb.b=00; set_color(6,&rgb);
    rgb.r=31; rgb.g=31; rgb.b=31; set_color(7,&rgb);
    rgb.r=15; rgb.g=15; rgb.b=15; set_color(8,&rgb);
    rgb.r=00; rgb.g=00; rgb.b=63; set_color(9,&rgb);
    rgb.r=00; rgb.g=63; rgb.b=00; set_color(10,&rgb);
    rgb.r=00; rgb.g=63; rgb.b=63; set_color(11,&rgb);
    rgb.r=63; rgb.g=00; rgb.b=00; set_color(12,&rgb);
    rgb.r=63; rgb.g=00; rgb.b=63; set_color(13,&rgb);
    rgb.r=63; rgb.g=63; rgb.b=00; set_color(14,&rgb);
    rgb.r=63; rgb.g=63; rgb.b=63; set_color(15,&rgb);

     for (i=0;i<253-16;i++)                     // set 237 shades of grey
     {
      rgb.r=rgb.g=rgb.b=i/4;
      rgb.r+=i&1;
      rgb.g+=(i&2)>>1;
      set_color(i+16,&rgb);
     }
     rgb.r=rgb.g=rgb.b=0;  set_color(254,&rgb); // these two are spare
     rgb.r=rgb.g=rgb.b=63; set_color(255,&rgb); // 
}


----------------------------------------
 Regards, Pete.
 Email: pete AT quantech DOT co DOT uk
 WWW:   www.quantech.co.uk 

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