[Kstars-devel] Fwd: Plans for threading in KStars

[Daniel-Marian Baboiu]

On Wed, 2011-11-09 at 06:59 +0300, Aleksey Khudyakov wrote:

> I think there is no need to work with Cartesian coordinates except when
> doing projections. Point on sphere could be represented as rotation which
> moves "north pole" (0,0,1) to the desired point on sphere.

A rotation from north pole to desired point on sphere (using  quaternions
or something else) still involves calculating sincos for both latitude and
longitude (the quaternion requires sin and cos of half-angle of
rotation). 

> There are infinitely many rotations which move one point on sphere to
> another one. This is where we get additional degeneracy. One way to deal
> with it is to require z=0 in addition to |q|=1. I'm not sure how easy is to
> implement such constraint.

Different rotations from one point to another have effect only on
objects of finite size; they will be brought with the center at the same
place, but rotated with respect tot each other. Obviously, not an issue
for single points. To rotate north pole to an arbitrary location, the
easiest way is to rotate to correct latitude (E-W axis), then around
polar axis to correct latitude. These can be combined (quaternion
multiplication), but still require two sincos calls.