From: ron DOT t DOT lewis AT bbs DOT st DOT net DOT au (Ron T Lewis)
Newsgroups: comp.os.msdos.djgpp
Subject: Re: Background colour fo
Date: 24 Apr 98 13:25:00
Organization: Soft-Tech +61-7-3869-2666
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Hello Mike,
           Sorry if this is off topic.

xmerm05 AT manes DOT vse DOT cz  on 04-23-98 Spoke to All  about Re: Background 
colour for

x>From: Michal Mertl <xmerm05 AT manes DOT vse DOT cz>
x>Subject: Re: Background colour for text using Allegro
x>Organization: Prague University of Economics

x>On 16 Apr 1998, Mike Collins wrote:
x>> I am trying to use Allegro to write a simple typing tutor.

x>> Initially, the screen is white with black text on it that has to be 
x>> read and typed. To show at what speed a line should be typed, a 
x>> coloured bar moves along the text line making a background for the 
x>> text, changing the original white background to, for example, 
x>> green. I would like to do this without rewriting the text, but 
x>> everything I have tried simply hides the text under my colour bar.

x>Do you mean to draw the text only once? If so you can't do that with
x>any Allegro function. 

Not quite true..

If he sets up his palette correctly say:-

 0=black , 1=white, 254=yellow, 255=red.

use a function like:-

// Reverse the color field of given block
void my_xor_block(BITMAP *bmp, int x, int y, int w, int h)
{
int x1=x+w;
int y1=y+h;
xor_mode(TRUE);
show_mouse(NULL);
rectfill(bmp, x, y, x1, y1,255);
show_mouse(screen);
xor_mode(FALSE);
}

To change the text color you pass the paramaters of the screen area and 
presto the text colors are altered to the values of the colors at the 
opposing palette position.

In the above case the black would turn to red and the white would turn 
yellow.  

if we XOR (as in the above function) with 255 the folowing results. 
color  0 = 255
       1 = 254
       2 = 253
       . . ...
       127 = 128
       128 = 127
       ... . ...
       254 = 1
       255 = 0

To restore the screen simply call the routine again..

Of course if you aren't concerned what color is displayed you needn't 
alter the palette at all and simply find an XOR value that returns 
something close..

Hope that helps...

(OH! Well, Lifes like that Sumtimes) With regards Ron T Lewis
EMail........ ron DOT t DOT lewis AT bbs DOT st DOT net DOT au
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