I've implemented--but not committed--a change that allows the triaxial ellipsoids to be specified for ssc objects. It's similar to what's in use now for stars: a property called SemiAxes is used to set the dimensions of the ellipsoid. It may be used either with or without Radius. In either case, the radius and semi-axes are multiplied to give the final size of the object. If no radius is specified, the default value is one. Thus for Mimas, you'd write:
SemiAxes [ 209.1 196.2 191.4 ]
The big difference is that with stars, the default radius is not one
but some value calculated from the luminosity and temperature.
Another difference is that the y and z axes are swapped. With the introduction of reference frames in 1.5.0, I've made some efforts to make Celestia conform to a more standard astronomical coordinate system, with primary rotation axis = +z. I propose that the SemiAxes property for stars be changed as well. It's not a very widely use feature, so the number of affected add-ons should be very small. The only change required in the standard Celestia files is for Altair in nearstars.stc. Add-on creators: has anyone made extensive use of ellipsoidal stars? How disruptive would it be to swap the y and z axes?
--Chris
Triaxial ellipsoids
Re: Triaxial ellipsoids
chris wrote:Add-on creators: has anyone made extensive use of ellipsoidal stars? How disruptive would it be to swap the y and z axes?
I'll have to check. I know I already used that property in some few cases only. If you change the property, I could certainly modify easily the few addons I created which are using the SemiAxes.
"Well! I've often seen a cat without a grin", thought Alice; "but a grin without a cat! It's the most curious thing I ever saw in all my life!"
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Topic authorchris
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ajtribick wrote:Further question: what happens if both Oblateness and SemiAxes are defined?
Oblateness is ignored if SemiAxes is defined. It's just as easy to multiply everything together, so that the z-axis is radius*semiaxes_z*(1-oblateness). That's one less special case, but otherwise it doesn't make much sense.
--Chris