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why doesn't my D3DXVec3Dot(...) equal 1.57..?

Started by mickey Oct 21, 2002 at 11:37 AM 6 replies 2k views

Original Post

Oct 21, 2002 at 11:37 AM

hiya, D3DXVECTOR3 v1(1.0f, 0.0f, 0.0f); D3DXVECTOR3 v2(0.0f, 1.0f, 0.0f); float answer = D3DXVec3Dot(&v1, &v2); why does answer equals zero? also, am doing that to check for collision, is that okey? many thanks,

[edited by - mickey on October 21, 2002 12:42:08 PM]

niyaw

Oct 21, 2002 at 11:50 AM

you''re taking a dot product of two perpendicular vectors, which will always be zero. to find a magnitude of the sum of the vectors, which is what you seem to be after, use sqrt(d3dxvec3lengthsq(&v1)+d3dxvec3lengthsq(&v2)).

mickey

Oct 21, 2002 at 8:06 PM

eh i''d really like to know why isn''t the answer 90 degrees since dot product gives us the angle?

CrazedGenius

Oct 21, 2002 at 8:22 PM

the dot product does not give you the angle.
http://mathworld.wolfram.com/DotProduct.html

Author, "Real Time Rendering Tricks and Techniques in DirectX", "Focus on Curves and Surfaces"

HunterTKilla

Oct 21, 2002 at 8:24 PM

Using Dot Product Here Are The Results:
Result < 0 = Acute Angle (<90 Degree)
Result > 0 = Obtuse Angle (>90 Degree)
Result = 0 = Perpendicular (90 Degree)

mickey

Oct 22, 2002 at 2:17 AM

i always come across ''the dot product of two vectors can give us the angle in between'' shoot, i didn''t realize that the result(dot product) of this is what will help us to get the angle. all along i thought the dot product was the angle!

hehe thanks guys!

Nypyren

Oct 22, 2002 at 2:27 AM

Let's see... isn't it...

A.Dot(B) == A.Length() * B.Length() * cos(angle_between_them)

?

If that's true, then:

angle = acos(A.Dot(B) / (A.Length()*B.Length()) );

... I think.

[edited by - Nypyren on October 22, 2002 3:28:57 AM]

mickey

Oct 22, 2002 at 5:41 AM

uh did i get this thing right... this method of getting the dot product is not useful? dot = v1 * v2 = (vx1 * vx2) + (vy1 * vy2) ? we should be using the other method which nypyren said?

also, do i have to normalized the vector before getting their dot product because the dot product should only be between -1.0 to +1.0?

thanks,

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