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Rotating an object

Started by IceGod Apr 28, 2001 at 4:36 AM 3 replies 500+ views

Original Post

Apr 28, 2001 at 4:36 AM

Hi, Say I have an object that is centered on the screen and at z = 5.0, and I have my camera at the origin facing the object. When I run the program everything is cool, the object is centered perfectly on the screen. When I apply a rotation to the object, using the matrix created by D3DXMatrixRotationY and some small angle, the object ends up completely somewhere else. I don''t know where exactly because its off the screen. Eventually the object rotates back into view, but my question is, why doesn''t the object start rotating from where it is originally? This makes no sense to me =\ Its like applying a small rotation to the object warps it to some other place for no reason at all. My code is fine, is there some misunderstanding I have here? Thanks, Tom

uber_sandman

Apr 28, 2001 at 5:05 AM

You are rotating in world space ... to move into the objects local space add the reverse of the transform and then rotate, and then move back.

ie:

translate(-posx, -posy, -posz)

rotate(angle, axis)

translate(pox, posy, poz)

hope that helps

IceGod

Author

IceGod

Apr 28, 2001 at 5:34 PM

Thats just the thing, I actually want the object to rotate around world space

But it gets all funky, it starts rotating in some weird spot which it shouldn''t do. Like I said, when the object is centered on the screen and placed at z = 5.0, when I rotate it it starts off in some wierd area and eventually gets back to z=5.0, instead of starting the rotation from z=5.0.

Thanks,
Tom

IceGod

Author

IceGod

Apr 28, 2001 at 6:24 PM

I figured it out, I was messing with the SDK examples, and they use TimeGetTime() / 1500.0f for their rotation value a lot. Well it turns out that the angles this produces are pretty large, and thats why it was starting out in random places. So I set my angle to a static variable with a very small angle, and it works fine. Thanks for the answers though.

Tom

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