Date:	Wed, 14 Sep 94 10:18:14 CET
From:	Piet=Mertens%users%LW AT cc3 DOT kuleuven DOT ac DOT be
Subject:	Re: palette problem
To:	djgpp AT sun DOT soe DOT clarkson DOT edu
Cc:	hodder AT geop DOT ubc DOT ca

Philip Hodder <hodder AT geop DOT ubc DOT ca> writes:

> Subject:        Palette problem with GRX 1.03m1 (& djgpp 112m1)
> Hi,
> 	I've been playing around with writing an image display program - this
> is for astronomical images so everything is greyscale (and 8 bit data also).
> I seem to have run into a problem when trying to display 64 grey levels 
using
> GRX 1.03m1 (you can only display 64 (6 bit) distinct grey levels on VGA).
> 
> I set up a palette of 256 colours:
> 
>   for (i=0; i<256; i++) { 
>     j = i / 4; 
>     pal[i] = GrAllocColor(j, j, j); 
>   }

You say you have only 64 cells in your color table, and you are allocating 256
cells (calling GrAllocColor() 256 times).
Once the color table is full, you will get wrong results. There are 
functions to query the number of free cells, see grx/src/colors.c
It is probably a good idea to allocate all available/needed cells first
and then set the colors.
	int cell;
	for (i=0; i<64; i++) {
	    if ((cell = GrAllocCell()) != GrNOCOLOR) 
	        GrSetColor(cell, level, level, level);
	    else 
	    	/* Color table used up */ 
		....
	}

> GrBlack() returns 0; GrWhite() returns 15 - so there are a total of 16 
colours
> instead of 64!

The 16 first cells were probably allocated earlier (before your call).

Hope this helps,

Piet
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
P. Mertens                            tel: +32.16..28.47.63
Department of Linguistics             fax: +32.16..28.50.25
Centre for Computational Linguistics  email: pmertens AT cc3 DOT kuleuven DOT ac DOT be
K.U.Leuven                                   Piet DOT Mertens AT arts DOT kuleuven DOT ac DOT be
Blijde-Inkomststraat 21               
3000 Leuven (Belgium)
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

- Raw text -