Question

I am trying to make a very simple object rotate around a fixed point in 3dspace.

Basically my object is created from a single D3DXVECTOR3, which indicates the current position of the object, relative to a single constant point. Lets just say 0,0,0.

I already calculate my angle based on the current in game time of the day.

But how can i apply that angle to the position, so it will rotate?

:(?

Sorry im pretty new to Directx.

Answer 1

So are you trying to plot the sun or the moon?

If so then one assumes your celestial object is something like a sphere that has (0,0,0) as its center point.

Probably the easiest way to rotate it into position is to do something like the following

D3DXMATRIX matRot;
D3DXMATRIX matTrans;
D3DXMatrixRotationX( &matRot, angle );
D3DXMatrixTranslation( &matTrans, 0.0f, 0.0f, orbitRadius );
D3DXMATRIX matFinal = matTrans * matRot;

Then Set that matrix as your world matrix.

What it does is it creates a rotation matrix to rotate the object by "angle" around the XAxis (ie in the Y-Z plane); It then creates a matrix that pushes it out to the appropriate place at the 0 angle (orbitRadius may be better off as the 3rd parameter in the translation call, depending on where your zero point is). The final line multiplies these 2 matrices together. Matrix multiplications are non commutative (ie M1 * M2 != M2 * M1). What the above does is move the object orbitRadius units along the Z-axis and then it rotates that around the point (0, 0, 0). You can think of rotating an object that is held in your hand. If orbitRadius is the distance from your elbow to your hand then any rotation around your elbow (at 0,0,0) is going to form an arc through the air.

I hope that helps, but I would really recommend doing some serious reading up on Linear Algebra. The more you know the easier questions like this will be to solve yourself :)

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