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Subject: Re: Tank Movement
Date: 7 Oct 1998 01:32:55 GMT
Organization: Texas Instruments
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In article <6vebp7$sap$1 AT fir DOT prod DOT itd DOT earthlink DOT net>,
	"JEFF" <jeff AT spam DOT com> writes:
> 
> Chris Brooker wrote in message <36147a32 DOT 10039817 AT ct-news DOT iafrica DOT com>...
>>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 "up" 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

A much easier approach - use atan2(), which takes care of the quadrant
mapping for you, e.g.,

double dir_degrees(int x1, int y1, int y2, int x2)
{
 int v1, v2;
 double dir;
 v1 = y1 - y2;
 v2 = x1 - x2;
 dir = 57 * atan( v1, v2 );
 return dir;
}