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

[Akarsh Simha]

On Mon, Nov 14, 2011 at 09:27:43AM +0300, Alexey Khudyakov wrote:
> > There is no answer in Meeus, he only presents simplified versions, but
> > precise enough for practical purposes. Precession has the main component
> > a rotation in the ecliptic plane, but also a much smaller component of
> > tilting the ecliptic plane. Nutation is a little more complicated, it
> > has oscillatory terms.
> >
> In the end it's just a sphere rotation so it could be represented as quaternion
> and it'll be same for all objects. It's surely non-trivial task to calculate it.
> 
> 
> > The final question is the map drawing - do you need a final transform to
> > spherical coordinates, or you can modify the drawing algorithm to use
> > directly 3D Cartesian coordinates? On the plus side, it still avoids a
> > lot of direct/inverse trig calls in the intermediate steps.
> >
> No. Most of projections are easier to compute from Cartesian coordinates.
> Only equirectangular projection require trigonometry calls (asin, acos).
> 
> asin/acos are slower than sin/cos, I beleive they solve equation, but we may
> take advantage of the fact that we don't need of full 15 digit precision as long
> as deviation of projections from true location are less than say 0.1
> pixel it's OK

What exactly do you mean by Cartesian Coordinates? All objects are
stuck to the celestial sphere (KStars doesn't know distances to most
objects, unfortunately), so you could certainly use Cartesian
Coordinates, with unit norm. But isn't spherical coordinates simpler
here? Also, we typically would want to be able to show the user
Altitude and Azimuth of an object.

Regards
Akarsh