# [Solved] Rotation around localx and a specified point.. What's wrong?

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Hello all! I made a camera class, it has a camera eyepoint in the scene graph, and calling movement methods of this class will move the eyepoint accordingly.. for example calling pan(dx,dy) will call eye.TranslateLocal(dx,dy,0).. etc. The problem is with the orbit method.. I want the camera to rotate around its target while looking at it. horizontal rotation is no problem, it's just rotation around the parent Z axis, like this:
    Public Sub Orbit(ByVal angle As Single)
mEye.LocalTransform *= Transform.Tranlation(-mLocalTarget) * Transform.RotationZ(-angle) * Transform.Tranlation(mLocalTarget)
End Sub


where the Transform class is similar to the matrix class of directx. now when I try to rotate vertically, I have to rotate around the target's location and the axis is the eyepoint's local x. first I tried to do this:
    Public Sub Tilt(ByVal angle As Single)
Dim relation As Vector3 = mLocalTarget - mEye.LocalPosition
mEye.LocalTransform = Transform.Tranlation(-mLocalTarget) * Transform.Tranlation(relation) * Transform.RotationX(angle) * Transform.Tranlation(-relation) * Transform.Tranlation(mLocalTarget) * mEye.LocalTransform

End Sub


but this made the eyepoint rotate around a point somewhere between it and the target... I then tried this:
    Public Sub Tilt(ByVal angle As Single)
Dim relation As Vector3 = mLocalTarget - mEye.LocalPosition
Dim z As New Vector3(0, 0, 1)

Dim rNorm As Vector3 = relation
rNorm.Normalize()

Dim left As Double = Math.Acos(Vector3.Dot(rNorm, z))

Dim res As Double = left + angle
If (res > 0) AndAlso (res < Math.PI) Then
Dim x As Vector3 = Vector3.Cross(z, relation)
x.Normalize()
Dim m As Matrix = Matrix.RotationAxis(x, angle)
relation = Vector3.TransformNormal(relation, m)
mEye.LocalRotationMatrix *= m
mEye.LocalPosition = mLocalTarget - relation
End If
End Sub


and this worked quite fine, untill it randomally starts to sway around with not obvious reason.. I spent days trying different ways and nothing worked. it's either a precision problem or something similar.. Please help!! I'm using managed directx9 [Edited by - Elias on May 26, 2007 1:01:01 PM]

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The problem was in the precision of the calculations. Rotation around the localX vector should never change it, but actually it does change it a VERY little bit, so using the new vector next time will cause more error.. and those errors accomulate making the camera sway around.

I had to use a way that doesn't make direct use of that vectore, so I used triangles!

The camera moves on a sphere which center is the target, so the start point, the end point and the center make a triangle with two equal segments. The length of each of them is the radius, and the angle between them is the angle of rotation.

I just had to find the angle at the base, and use it to calculate the translation that needs to be done, then rotate to acheive the required rotation.

Here's the code for this:
      Public Sub Tilt(ByVal angle As Single)'Calculate the radius of the sphere         Dim radiusVec As Vector3 = mLocalTarget - mEye.LocalPosition         Dim a As Double = CDbl(radiusVec.Length)'Calculate the length of the base where the equal segments are a and the base is c         Dim c As Double = a * Math.Sqrt(2.0# - 2.0# * Math.Cos(CDbl(angle)))'Calculate the base angle         Dim baseAngle As Double = 0.5# * (Math.PI - Math.Abs(CDbl(angle)))'Calculate the required translation         Dim dz As Single = CSng(c * Math.Cos(baseAngle))         Dim dy As Single = CSng(c * Math.Sin(baseAngle) * Math.Sign(angle))         mEye.TranslateLocal(0, dy, dz)'Finally, Rotate the camera (for some reason, this method doesn't cause error, or I would have had to do it manually as well)         mEye.RotateLocalX(angle)      End Sub

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