From: "Jeff" <sjford AT sprintmail DOT com>
Newsgroups: comp.os.msdos.djgpp
Subject: Re: Tank Movement
Date: Fri, 9 Oct 1998 21:42:02 -0400
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To: djgpp AT delorie DOT com
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Reply-To: djgpp AT delorie DOT com

Chris Brooker wrote ...
>Hi,
>Sorry if my code sucks, I a mquite new to C.
>I am busy writing a Real Time Strategy ...

>double dir_degrees(int x1, int y1, int y2, int x2)
>{
> int v1, v2;
> double dir;
> v1 = y1 - y2;
> v2 = x1 - x2;
> if (v2 == 0)
> v2++; //Stops division by zero errors
> dir = 57 * atan(v1 / v2);
> return dir;
>}
>

The atan() function can only return values between -90 and 90 degrees, which
is two out of four quandrants. You need to check the signs of v1 and v2 to
determine if the angle is between -180 and -90 or  between 90 and 180
degrees and add or subtract 90 degrees accordingly.

If x and y coordinates are standard cartesian, with "right" as positive x
and "top" as positive y:

if x>0 and y>0, the angle must be 0 to 90 degrees.  No adjustment.
if x>0 and y<0, angle is -90 to 0.  No adjustment.
if x<0 and y>0 angle is  90 to 180. Add 180 degrees to return value.
if x<0 and y<0 angle is -180 to -90.  Subtract 180 degrees from return
value.

You may need to experiment to get the right adjustment as I have not tested
it.  Good luck.

-Jeff