Angles from vectors

Try using the dot product of the vectors.

From 3d math primer for graphics and game development, a book I HIGHLY recommend:

quote:

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p. 59

The dot product is related to the angle between two vectors. The dot product is equal to the product of the magnitudes of the vectors and the cosine of the angle between the vectors.:

a dot b = ||a|| * ||b|| * cos(theta)

Solving for theta:

theta = arccos((a dot b) / ||a|| * ||b||)

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Now, to do that math, know that ||v|| is the magnitude of the vector v. The magnitude in 2d math is basically sqrt(pow(x, 2) + pow(y, 2)), better known as the distance formula. There are two vectors in these equations, a and b.

To take the dot product of two vectors a and b (written as a dot b), take the sum of the products of the corresponding elements in the vectors.

a = {1, 2}
b = {3, 4}

a dot b = (1 * 3) + (2 * 4) = 11

I hope I haven't given completely wrong advice, please someone correct me if I'm wrong!

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aut viam inveniam aut faciam

MoonStar Projects

[edited by - Ronin Magus on February 20, 2003 8:21:23 PM]

I hate to be annoying, but I really want to solve the problem with atan! I just dont understand why it wont work...what I''m seeing now is that it works, but only for 45 degrees of Quadrant I...heres my code:

nTargetAngle = RAD_TO_DEG(atanf(sx/-sy));

RAD_TO_DEG is just a simple macro that multiplies the result by 180/pi

Brian

tan(x) = sin(x)/cos(x)
or
tan(x) = opp/adjacent, so I presume your sx and sy are are difference in the x and y values, respectively.

do this...given the points A and B:

opp = B.y - A.y
adj = B.x - A.x
angle = RAD_2_DEG(atan(opp/adj))

Now, the angle returned should be in the 1st quadrant, or 0 < angle < 90 degrees, so you''ll have to watch for the sign of both opp and adj to know what quadrant you''re in.

Seeing as how you''re taking your first geometry class, you must never have touched trigonmetry. Either way, I applaud your efforts...

Hope that helps, best of luck

I''m doing that exactly, and its giving me the following values as I move the two points: 1, skips to 45, 63-90...its skipping a whole lot. any ideas? is my triangle backwards maybe?

brian

Every 2d game I''ve played has has some sort of following/tracking algorithm...how can this be so hard? I cant find any information on it either. If anyone knows anything please help me, this is driving me nuts!

Brian

This is one possible way to get the rotation:

roty = atan(((x1-x2)/(z1-z2)))*(180.0f/D3DX_PI);
if(z2 > z1)
{
    roty+=180;
}
 

-CProgrammer
My Website

CP, shouldnt those z''s be y''s? if not, i''m in 2d, and I have no z''s in the equation...

Brian

Sorry didn''t know it was 2s. However you can just exchange the z''s with y''s. My code example is actually 2d just as you want it. I just used it in a 3d app with a perspective from the top. I used it to rotate the NPC''s so they look at the player and offcourse follow him(a little extra code for that) when the player gets too close.

So use this to rotate an object to make it look at another object in your 2d-game. Making the object follow should be easy having the angle.

rot = atan(((x1-x2)/(y1-y2)))*(180.0f/D3DX_PI);
if(y2 > y1)
{    
    rot+=180;
}