[R] 3d polar coordinates in rgl function view3d()
Duncan Murdoch
murdoch.duncan at gmail.com
Tue Oct 9 02:56:43 CEST 2012
On 12-10-08 7:12 PM, Peter Rossi wrote:
> Dear all:
>
> I'm enjoying using rgl and I want to set a specific viewpoint.
>
> I understand that view3d() will do so by setting the 3d polar
> coordinate angles, theta and phi.
That's incorrect, see below.
> In standard polar coordinates, theta is the angle away from the X axis
> in the X-Y plane and phi (inclination angle) is the angle between the
> Z axis and the radius vector
>
> It appears that when you use view3d() theta is defined as the angle
> away from the x axis in the X-Z plane and phi is the angle between the
> Y axis and the radius vector.
Those descriptions are both incomplete. Rotations in 3D don't commute,
so you need to give them in order: e.g. "first rotate by x about y,
then rotate by z about w."
I don't remember what theta and phi correspond to in those terms, but if
you want to get fine control, set the userMatrix member of par3d()
explicitly.
>
> When I use view3d(theta=0, phi=15), my image is flipped on the side
> with the Y axis pointing upward and the Z axis pointing toward me.
> This seems contrary to the standard definition of polar coordinates.
>
> I have read the rgl help files but these do not define theta or phi.
> I also looked at documentation on the rgl site to no avail.
You could also read the source code -- it's open source.
> I wonder if someone out there could enlighten me s to what I've got
> wrong or how to translate properly between rgl definition and standard
> polar coordinates.
You can't, it's not polar coordinates. It's a sequence of rotations.
Duncan Murdoch
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