Angle Vectoring

no-reply@allegro.cc (Don Freeman) — Mon, 26 Jun 2006 20:59:24 +0000

Ok...I'd first like to say hello again to everyone ...it's been a while since I've been able to get back here.

Here's my question:
How do you find the angle between two points. I am trying to find this angle so I can vector the object to the target, such as a homing missle. The structure would have at least:

typedef struct _Object
{
float x, y, angle, vel;
}Object;

I've looked at a couple of math equations, but I may be doing something wrong or I am just totally confused.

To find the vector:
V = V
To find the length of the vector: (Also just the distance from (0,0)-(Vx,Vy) )
Vlen = sqrt(V.x*V.x+V.y*V.y);

I think you use the DOT Product to find the angle, but when I try it...it doesn't seem to work for me:
DotProduct = -cos(angle) = / |U| * |V|
where |U| & |V| is the length of U & V respectfully.

The goal is to find the angle from (player.x, player.y) to (target.x, target.y) so I can determine where to turn left / right or keep on current angle to reach target.

Thanks in advance.
(Please...any math problems...please explain. I get confused with some of the expressions and stuff)

no-reply@allegro.cc (Arthur Kalliokoski) — Mon, 26 Jun 2006 21:46:26 +0000

Here's some console prog stuff to demonstrate radians to polar coords and vice versa

1 
2 // polar coordinates to rectangular
3 
4#include 
5#include 
6#include 
7 
8double x,y;
9 
10double angle,radius;
11 
12int main(int argc, char **argv)
13{
14 
15angle = strtod(argv[1],0);
16radius = strtod(argv[2],0);
17 
18//convert degrees to radians
19angle = angle * M_PI/180.0;
20 
21y = sin(angle) * radius;
22x = cos(angle) * radius;
23 
24printf("\nx is %f y is %f",x,y);
25 
26return 0;
27} //end
28 
29 //rectangular coordinates to polar
30 
31#include 
32#include 
33#include 
34 
35double x,y;
36 
37double x2,y2;
38 
39double angle,radius;
40 
41double tmpangle;
42 
43int main(int argc, char **argv)
44{
45 
46x = strtod(argv[1],0);
47y = strtod(argv[2],0);
48 
49radius = sqrt(x*x+y*y);
50 
51//if( (x*y) < 0.0) radius = radius * -1.0;
52 
53x2 = asin(fabs(x)/radius);
54 
55//this would work just as well as the above, they come out the same
56//y2 = acos(y/radius) * 180.0/M_PI;
57 
58 
59tmpangle = x2;
60if(y<0)
61{
62if(x >= 0) tmpangle = M_PI * 2.0 - x2;
63else
64tmpangle = x2 + M_PI;
65}
66else
67if(x<0) tmpangle = M_PI - x2;
68 
69x2 = tmpangle * 180.0/M_PI;
70 
71printf("\n%f degrees at %f unit distance",x2,radius);
72 
73return 0;
74}

gcc math lib is probably necessary, -lm.

no-reply@allegro.cc (Don Freeman) — Mon, 26 Jun 2006 21:51:30 +0000

Thanks! I knew I was missing something...but I couldn't think of what it was! Had to convert radians to polar coords...just didn't know how! Thanks again! Now...where did I leave off with this program....::)

no-reply@allegro.cc (Krzysztof Kluczek) — Mon, 26 Jun 2006 21:54:10 +0000

atan2
EDIT: It should work better than the code above, as atan2 handles all cases intenally.

no-reply@allegro.cc (Niunio) — Tue, 27 Jun 2006 00:26:15 +0000

Or use dot product.

Amarillion has a great tutorial about sin, cos and all that stuff. It should be at pixwiki, but now bafsoft server seems down so I can't find it. Anyway, I have a copy of most of pixelate tuts so I've attached the tutorial, wich includes both atan2 and dot product examples.

no-reply@allegro.cc (Arthur Kalliokoski) — Wed, 28 Jun 2006 23:44:02 +0000

atan2, huh! I don't remember why I did the above code (years ago), maybe it was so I could do it in asm as well, or maybe my compiler didn't have an atan2 then. I thought it wasn't supported by all compilers but dj info says it's ANSI. Learn something new every day.

no-reply@allegro.cc (Andrei Ellman) — Fri, 30 Jun 2006 02:10:31 +0000

The dot product is overkill for 2 dimensions. I'd use atan2.

AE.

1
2	// polar coordinates to rectangular
3
4	#include
5	#include
6	#include
7
8	double x,y;
9
10	double angle,radius;
11
12	int main(int argc, char **argv)
13	{
14
15	angle = strtod(argv[1],0);
16	radius = strtod(argv[2],0);
17
18	//convert degrees to radians
19	angle = angle * M_PI/180.0;
20
21	y = sin(angle) * radius;
22	x = cos(angle) * radius;
23
24	printf("\nx is %f y is %f",x,y);
25
26	return 0;
27	} //end
28
29	//rectangular coordinates to polar
30
31	#include
32	#include
33	#include
34
35	double x,y;
36
37	double x2,y2;
38
39	double angle,radius;
40
41	double tmpangle;
42
43	int main(int argc, char **argv)
44	{
45
46	x = strtod(argv[1],0);
47	y = strtod(argv[2],0);
48
49	radius = sqrt(xx+yy);
50
51	//if( (xy) < 0.0) radius = radius -1.0;
52
53	x2 = asin(fabs(x)/radius);
54
55	//this would work just as well as the above, they come out the same
56	//y2 = acos(y/radius) * 180.0/M_PI;
57
58
59	tmpangle = x2;
60	if(y<0)
61	{
62	if(x >= 0) tmpangle = M_PI * 2.0 - x2;
63	else
64	tmpangle = x2 + M_PI;
65	}
66	else
67	if(x<0) tmpangle = M_PI - x2;
68
69	x2 = tmpangle * 180.0/M_PI;
70
71	printf("\n%f degrees at %f unit distance",x2,radius);
72
73	return 0;
74	}