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Thread: Rotating around a World Space Axis - How Works?

  1. #1
    Junior Member Newbie
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    Apr 2012
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    Rotating around a World Space Axis - How Works?

    I have a cube that I am successfully rotating by mouse drags with the following algorithm:

    1) Initialize rotation matrix to identity

    2) Multiply world-space y-axis (0, 1, 0) by inverted rotation matrix to get a vector in object space corresponding to the y-axis. Then rotate a bit around this vector given the mouse drags.

    3) Same thing for x-axis.

    4) Each update, apply rotation matrix to object.

    This works fine, I am just having trouble understanding the theory behind why you can get the y-axis in object space by multiplying the y-axis in world space by the inverted rotation matrix. Can anyone explain this in very simple terms - I must be missing a key concept.

    Thanks!

  2. #2
    Senior Member OpenGL Guru Dark Photon's Avatar
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    Re: Rotating around a World Space Axis - How Works?

    Quote Originally Posted by subkey
    I am just having trouble understanding the theory behind why you can get the y-axis in object space by multiplying the y-axis in world space by the inverted rotation matrix. Can anyone explain this in very simple terms - I must be missing a key concept.
    (MODELING TRANSFORM) * (object space vec) = (world-space-vec), i.e.
    (MODELING) * object = world

    multiply both sides by the inverse modeling transform:

    (MODELING)-1 * (MODELING) * object = (MODELING)-1 * world, i.e.
    object = (MODELING)-1 * world

    The rotation is your MODELING transform.

    (...or at least it is your MODELING transform with any translation removed.)

  3. #3
    Junior Member Newbie
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    Re: Rotating around a World Space Axis - How Works?

    @Dark Photon: That helps a *lot*, thanks so much! :]

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