From:	Nate Eldredge <neldredge AT hmc DOT edu>
Newsgroups:	comp.os.msdos.djgpp
Subject:	Re: Allegro screen writing.
Date:	Thu, 16 Sep 1999 13:50:49 -0700
Organization:	Harvey Mudd College
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See below.

JAB wrote:
> 
>  My program won't run. I don't know what's wrong but it's something to
> do with me miss-using allegro. The problem is something
> do with the putpixel() bit of code. I think it's to do with the
> global "screen" thing.
> 
>  This is a cut of the relavent (ie screen/allegro) related code from my
> void main.
> 
> void main(void)
> {
>   BITMAP *bmp = screen;
>   if (allegro_init()!=0){printf("Allegro load up error\n");return;}
>   [snip]
> 
>   /* Set up screen */
>   set_color_depth(24);
>   if (set_gfx_mode(GFX_AUTODETECT, VDU_X, VDU_Y, 0, 0) != 0)
>   {
>     printf("Error setting video mode\n");
>     return;
>   }
> 
>   [snip]
> }
> 
>  Here is my routine to plot the ready done image (it's in the ToDiplay
> array) in main memory to the on screen memory
>  pixel by pixel.
> 
> void display(void)
> {
>   register unsigned int x,y,tempnum;
>   for(y=0;y<=VDU_Y;y++)
>   {
>     for(x=0;x<=VDU_X;x++)
>     {

Arrays in C are zero-based, remember?  So if VDU_X and VDU_Y are the x
and y sizes of the screen, you're overrunning your ToDisplay buffer, and
also the screen.  This is bad.

You probably want the test `x < VDU_X'.

>       putpixel(screen, x, y, ToDisplay[tempnum]); /* This is the line
> the debugger points me to */
>       /* ToDisplay is a 1D array of the screen info to be put on screen
> */
>       tempnum++;
>     }
>   }
> }
> 
>  What is wrong with the putpixel line? Is there something I should have
> done with "screen"?
>  I know the way I'm doing things isn't the best for PC. Writing to
> things to a main memory array then plotting directly
> to the screen from that. But the program wasn't written for the PC, I
> just need to get it running, no matter how crude,
>  and in a short space of time. Maybe I'll come back to it latter and
> change all the lines plotting to ToDisplay, but I've
> not got time now.
> 
>  Thanks for any help.
> 
> --
> Catch ya later
> 
> JAB
> 
> Sent via Deja.com http://www.deja.com/
> Share what you know. Learn what you don't.

-- 

Nate Eldredge
neldredge AT hmc DOT edu

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