Message-Id:	<199612300146.CAA24905@math.amu.edu.pl>
Comments:	Authenticated sender is <grendel AT ananke DOT amu DOT edu DOT pl>
From:	"Mark Habersack" <grendel AT ananke DOT amu DOT edu DOT pl>
Organization:	Home, sweet home (Poznan, Poland)
To:	"Joshua Boyd" <budpro AT aol DOT com>, djgpp AT delorie DOT com
Date:	Mon, 30 Dec 1996 02:45:51 +0100
MIME-Version:	1.0
Subject:	Re: Could somebody help me with this program.
Reply-to:	grendel AT ananke DOT amu DOT edu DOT pl

Once upon a time (on 28 Dec 96 at 20:38) Joshua Boyd said:

> I am having trouble with this program.  It is supposed to draw an x
> in the middle of the screen.  I am trying to write my own graphics
> library, witch is what is contained in the file graphics.h, then the
> small program is contained in temp.c.  Any help would be
> apprecieated.  This was written using djgpp.
You don't write what problems exactly you have, but I have spotted 
three of them:

1. (depends on point of view):

void putpixel (int x, int y, int color) {
        virt_screen[x*320+y]=color;
}

In this function you reverse the coordinate system of the PC GFX 
screen: (0,0) is located at the top-left corner and X axis goes 
right with Y axis going down. The calculation of the pixel's location 
should be [x+320*y].

2. You probably want the "Hello world" strings

        printf("Hello World");
        while (getch() != 'a') {}
        printf("Hello World");
        while (getch() != 'a') {}

        vga256 ();

to appear before entering the GFX mode. The reason why you can't see 
them is that stdout (the standard output stream which 'printf()' 
uses) is buffered (like all the 'standard' streams). You have two 
ways to resolve the problem: either flush the stream by calling 
'fflush(stdout)' or put it into unbufferred mode. The second method 
is better:

       setvbuf(stdout, NULL, _IONBF, 0);
       printf("Hello World");

this resolves the problem.

3. The reason why you don't see anything on the GFX screen is that 
you're putting the pixels with a color value of '0', which in PC 
means 'background color'. No matter what is your background color you 
will never see anything when putting pixel in that color on the 
'bare' background.


     putpixel (158,98,0);

should be. e.g.

     putpixel (158,98,15);

After applying these changes you will see your gfx on screen (you 
will have to recalculate the coordinates also ;-))

###########################################################
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