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signed angle between vectors

whenever i want to find the angle between two vectors, i use the dot product method to find it and it gives me the smaller of the two possible angles. how am i to get the other?

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  • Josh CodeJosh Code Member Posts: 675
    : whenever i want to find the angle between two vectors, i use the dot product method to find it and it gives me the smaller of the two possible angles. how am i to get the other?
    :

    cos (x) = cos(-x)

    To get the other angle, you can just change the sign of the first angle or subtract it from 2*pi radiens (360 degrees).
  • amit_gurjaramit_gurjar Member Posts: 6
    hi josh,
    thanks all the same. but my query was how am i to find the oriented angle in one direction from, say the negative X-axis. the angle that i get is always the smaller of the two angles and how am i to get the other? this other may not always be the one i want. am i clear? :P
    : : whenever i want to find the angle between two vectors, i use the dot product method to find it and it gives me the smaller of the two possible angles. how am i to get the other?
    : :
    :
    : cos (x) = cos(-x)
    :
    : To get the other angle, you can just change the sign of the first angle or subtract it from 2*pi radiens (360 degrees).
    :

  • amit_gurjaramit_gurjar Member Posts: 6
    hi josh,
    thanks all the same. but my query was how am i to find the oriented angle in one direction from, say the negative X-axis. the angle that i get is always the smaller of the two angles and how am i to get the other? this other may not always be the one i want. am i clear? :P
    : : whenever i want to find the angle between two vectors, i use the dot product method to find it and it gives me the smaller of the two possible angles. how am i to get the other?
    : :
    :
    : cos (x) = cos(-x)
    :
    : To get the other angle, you can just change the sign of the first angle or subtract it from 2*pi radiens (360 degrees).
    :

  • Josh CodeJosh Code Member Posts: 675
    : hi josh,
    : thanks all the same. but my query was how am i to find the [b]oriented angle[/b] in one direction from, say the negative X-axis. the angle that i get is always the smaller of the two angles and how am i to get the other? this other may not always be the one i want. am i clear? :P
    : : : whenever i want to find the angle between two vectors, i use the dot product method to find it and it gives me the smaller of the two possible angles. how am i to get the other?
    : : :
    : :
    : : cos (x) = cos(-x)
    : :
    : : To get the other angle, you can just change the sign of the first angle or subtract it from 2*pi radiens (360 degrees).
    : :
    :

    What do you mean by "oriented angle"?


    v1 v2 = |v1||v2| cos theta

    where theta is the angle separating the 2 vectors

    Ignoring the sign of the cos ratio, there would be 4 possible angles in a full cycle. 1 for each quadrant.

    theta, 180 degrees - theta, 180 degrees+theta, 360 degrees - theta


    Does that answer your question?
  • SephirothSephiroth Fayetteville, NC, USAMember Posts: 1,035
    I am currently teaching myself this same stuff, and I found a SWEET tutorial on it. The link is below and I hope it helps you. It has my monsters properly rotating, although my collision-detection system is still non-existant! Guess I need to get a bit more knowledge.

    http://www.gamedev.net/reference/articles/article1832.asp

    -[italic][b][red]S[/red][purple]e[/purple][blue]p[/blue][green]h[/green][red]i[/red][purple]r[/purple][blue]o[/blue][green]t[/green][red]h[/red][/b][/italic]

  • amit_gurjaramit_gurjar Member Posts: 6
    hi josh,
    what i want is this:
    i have a tangent to a curve and want to find the angle that the tangent makes with the x-axis as the curve progresses. now this angle has to be the signed angle. how am i to obtain the 'signed angle' as the tangent makes different angles at different instances? hence, the problem is not for one parrticular dot product obtained angle. am i clear now? thanks a heap forsparing some time to help me out. thanks for the link to the site u quoted sephiroth!
    have a great time!
    amit
    : : hi josh,
    : : thanks all the same. but my query was how am i to find the [b]oriented angle[/b] in one direction from, say the negative X-axis. the angle that i get is always the smaller of the two angles and how am i to get the other? this other may not always be the one i want. am i clear? :P
    : : : : whenever i want to find the angle between two vectors, i use the dot product method to find it and it gives me the smaller of the two possible angles. how am i to get the other?
    : : : :
    : : :
    : : : cos (x) = cos(-x)
    : : :
    : : : To get the other angle, you can just change the sign of the first angle or subtract it from 2*pi radiens (360 degrees).
    : : :
    : :
    :
    : What do you mean by "oriented angle"?
    :
    :
    : v1 v2 = |v1||v2| cos theta
    :
    : where theta is the angle separating the 2 vectors
    :
    : Ignoring the sign of the cos ratio, there would be 4 possible angles in a full cycle. 1 for each quadrant.
    :
    : theta, 180 degrees - theta, 180 degrees+theta, 360 degrees - theta
    :
    :
    : Does that answer your question?
    :

  • amit_gurjaramit_gurjar Member Posts: 6
    hi josh,
    what i want is this:
    i have a tangent to a curve and want to find the angle that the tangent makes with the x-axis as the curve progresses. now this angle has to be the signed angle. how am i to obtain the 'signed angle' as the tangent makes different angles at different instances? hence, the problem is not for one parrticular dot product obtained angle. am i clear now? thanks a heap forsparing some time to help me out. thanks for the link to the site u quoted sephiroth!
    have a great time!
    amit
    : : hi josh,
    : : thanks all the same. but my query was how am i to find the [b]oriented angle[/b] in one direction from, say the negative X-axis. the angle that i get is always the smaller of the two angles and how am i to get the other? this other may not always be the one i want. am i clear? :P
    : : : : whenever i want to find the angle between two vectors, i use the dot product method to find it and it gives me the smaller of the two possible angles. how am i to get the other?
    : : : :
    : : :
    : : : cos (x) = cos(-x)
    : : :
    : : : To get the other angle, you can just change the sign of the first angle or subtract it from 2*pi radiens (360 degrees).
    : : :
    : :
    :
    : What do you mean by "oriented angle"?
    :
    :
    : v1 v2 = |v1||v2| cos theta
    :
    : where theta is the angle separating the 2 vectors
    :
    : Ignoring the sign of the cos ratio, there would be 4 possible angles in a full cycle. 1 for each quadrant.
    :
    : theta, 180 degrees - theta, 180 degrees+theta, 360 degrees - theta
    :
    :
    : Does that answer your question?
    :

  • ZaffireZaffire Member Posts: 22
    [b][red]This message was edited by Zaffire at 2002-10-21 18:58:47[/red][/b][hr]
    : hi josh,
    : what i want is this:
    : i have a tangent to a curve and want to find the angle that the tangent makes with the x-axis as the curve progresses. now this angle has to be the signed angle. how am i to obtain the 'signed angle' as the tangent makes different angles at different instances? hence, the problem is not for one parrticular dot product obtained angle. am i clear now? thanks a heap forsparing some time to help me out. thanks for the link to the site u quoted sephiroth!
    : have a great time!
    : amit
    : : : hi josh,
    : : : thanks all the same. but my query was how am i to find the [b]oriented angle[/b] in one direction from, say the negative X-axis. the angle that i get is always the smaller of the two angles and how am i to get the other? this other may not always be the one i want. am i clear? :P
    : : : : : whenever i want to find the angle between two vectors, i use the dot product method to find it and it gives me the smaller of the two possible angles. how am i to get the other?
    : : : : :
    : : : :
    : : : : cos (x) = cos(-x)
    : : : :
    : : : : To get the other angle, you can just change the sign of the first angle or subtract it from 2*pi radiens (360 degrees).
    : : : :
    : : :
    : :
    : : What do you mean by "oriented angle"?
    : :
    : :
    : : v1 v2 = |v1||v2| cos theta
    : :
    : : where theta is the angle separating the 2 vectors
    : :
    : : Ignoring the sign of the cos ratio, there would be 4 possible angles in a full cycle. 1 for each quadrant.
    : :
    : : theta, 180 degrees - theta, 180 degrees+theta, 360 degrees - theta
    : :
    : :
    : : Does that answer your question?
    : :
    :
    :
    if you want to find the angle between a vector and the x-axis, you could always just use a unit vector in the direction of the x-axis,
    i.e. theta = acos[(a b) / (|a||v|)] where a is your vector and b = <1, 0, 0>. that gives you the smaller angle. to get the larger one, just subtract theta from 2 * pi or 360 degrees.

    hope that helps!
  • WudanWudan Member Posts: 66
    : whenever i want to find the angle between two vectors, i use the dot product method to find it and it gives me the smaller of the two possible angles. how am i to get the other?
    :

    Hi Amit.
    Not sure exactly what you mean, as there can only be one angle between two vectors. But I think I know what you mean. you could try negating one of the vectors and doing the dot product again. This will give you a 'big angle ' I hope its the one you want :)
  • Chris BrownChris Brown USAMember Posts: 4,624 ✭✭

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