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anchelito

Coordinate Systems, rotations and default values.

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hey everyone, there are some questions that i was unable to answer myself and i hope you can answer. 1.) default-direction: when i do have a rotation-vector to describe the rotation of my object with three angles and every angle is 0, in which direction am i looking??? 2.) is the direction different depending on whether i am using a left-handed or right-handed coordinate-system?? 3.) as far as i know rotation is normally clockwise. again: is this different depending on the coordinate-system being left-handed or right-handed? thanks a lot for the help! best regards

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1. Assuming you're talking about the OpenGL and DirectX rendering pipelines, projection is performed along the z-axis. And so the 'default' orientation will look along the z-axis.

2. Potentially, yes. Handedness is often described as being analogous to the direction in which the z-axis points, and so a straight switch in handedness of the coordinate system will invert this default direction. However, the projection operation can account for this if necessary, and so this may well be invariant between differently handed pipelines. Perhaps somebody who is fluent in both DirectX and OpenGL could tell us which is accepted as standard.

3. Rotation isn't normally anything, but if I had to pick, I'd say that anticlockwise is more common, for the simple reason that it leads to fewer negative parameters throughout geometry. But as in (2), a switch in coordinate handedness will lead to rotations going the 'wrong' way, simply as it can be viewed as a reflection in the xy-plane. However, many more factors come into play and so we couldn't say for sure.

3D geometry is littered with these parity idiosyncrasies and sometimes it can be a real chore making sure you have everything correct in the set {handedness, vector-major layout, row vs. column notation, y or z 'up', clockwise or anticlockwise rotations}.

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1. The direction of an unrotated vector is completely up to you. I generally use the X axis. In OpenGL, an unrotated camera looks in the -Z direction. In DirectX, an unrotated camera looks in the +Z direction. These might just be defaults.

2. No. It depends on the convention. See #1.

3. A positive rotation around an axis pointing towards you is counter-clockwise in a right-handed system and clockwise in a left-handed system.

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