How do you use this dot product??? every where says you can use cos to find the angle.... but how does a scalar value represent 2 axis of rotation... I dont know why I cant get this... every time I think about it... I''m looking to find 2 return values for axis rotation...

It doesn''t represent 2 axis of rotation, it gives you the angle between two vectors, no more, no less...


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today we''re a step onward!

No, it gives you the cosine of the angle between two vectors. In one interpretation.
It also gets a point''s distance from a plane centered at origin.

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Original post by Vaporisator
It doesn''t represent 2 axis of rotation, it gives you the angle between two vectors, no more, no less...

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to me this doesnt make sense... how does a single value represent the angle between vector A and vector B in 3D space... if you where looking down the Z axis then yeah the angle between them in XY is the dot procutct but how does it also represent the other axis...

I mean sure if you have a vectorXYZ (1,0,0) A and vectorXYZ (0,1,0) then the angle would be 90degs CCW... but what if B was 0,1,1) how CAN a single value also give the angle for the ZY axis...

hmmm... is it like... if you were to make a triangle using 0 A B then its the angle between A and B from 0 looking down the normal???

It''s the angle between the vectors in the plane in which the two vectors act. Think about it, the two vectors define a plane, the angle retrieved from the dot product is in this plane.

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Oh, hehe, you replied just before me. Your analogy of creating a triangle and the angle being based in that plane is correct.

Death of one is a tragedy, death of a million is just a statistic.

way.... I get it now... I was looking for axis angles... didnt think about angle on its own plane...

dThe DotProduct itself doesnt you the cosine of the angle.

a.b = |a||b|cos(THETA)
cos(THETA) = (a.b) / (|a||b|)
THETA = Arcos((a.b) / (|a||b|))

Seemed to me no one told crakinshot this.

ahh man... this is a tad confussing at first.... the Cross product of 2 vectors returns the normal vector... which I believe is perpendicular to the plane... but becuase I''m also a 3d modeler I see normals as being a vector from the center of the plane moving outward.. the way the plane is facing...

so now I have two different conflicting versions of what a plane normal is...

math was never made to be easy was it...

quote:

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a.b = |a||b|cos(THETA)
cos(THETA) = (a.b) / (|a||b|)
THETA = Arcos((a.b) / (|a||b|))

Seemed to me no one told crakinshot this.

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yeah I know bout how you get the angle thanks... I just needed to know what angle is was refering too... hehe...