Stable Static Worlds

There were some gnarly physics edge cases that did not like my moving world shape colliders. It was time to finish these off once and for all. I decided the best solution would be to only ever interact with physics on the current world while having other worlds effectively pause until visited again. Then I'd freeze the one active world as a static object at the origin of Unity's world space coordinates. So large world surfaces no longer move and physics passes can be a lot simpler.

To freeze the current world the solar system has to simulate around a fixed point as if the fixed point were moving naturally within the solar system. Hmm.

I set about scribbling vectors, orbits, and rotations and came up with a simple set of rules. The current world is static and placed at the zero position with no rotation. The orbit of this static world is disabled, then each orbital parent up to the Sun have their positioning orbit ellipses replaced with inverse orbits where they derive their position from their designated child bodies orbit. This solves positioning the solar system.

Above you'll see as Mars (left side green circle that doesn't move) 'orbits' the sun (left side small yellow dot) via the sun orbiting Mars with Earth getting dragged along. Looks odd but to a Martian it's indistinguishable from Mars flying around in space. Almost.

Next comes rotation, all worlds can maintain their own rotation, but they also need to look to the static world and then orbit their own parent by the same amount the static world should have rotated by. Finally they need to also rotate about their axis by the same amount.

Whew, when all that's done by every object in the solar system but the static one it produces an identical effect as if the static body is rotating about it's own axis.

With all that everything looks like the place you're standing on is moving and spinning through space.

On the art side of things, someone suggested that black bars could look cool. So I took a crack at making a cinematic intro. I like how it interacts with my parallax system a bit.