How do you set "ground level" in a skybox?

[Tad Carlucci]

1) It is not a bug. It's a combination of a round-off error and the fact that KittyCatS (and virtually noone else) switch the prim from physical to non-physical.

2) The size of the prim does not matter. It could happen to all objects, including Avatars. The reason it does not happen often is so very, very few objects switch from being non-physical to physical.

Basically, it works like this: the cat decides it want to move. So it becomes physical and begins to move. As a physical object, moving along the ground, a skybox floor prim, or a mesh physics face, it cannot go through, so it moves along the upper surface. But, at some point the cat decides to stop moving and becomes non-physical (which, in the days before mesh, was the only way to reduce physics-phase lag). When the cat becomes non-physical, it's 'root' is sitting just above the floor. But 'just above' is an approximation. The value wiggles a bit. Sometimes it's a teeny-tiny bit above, and all is well. But sometimes it's a teeny-tiny bit below the top surface of the floor. As a non-physical object, this does not matter .. gravity does not effect it.

But, at some point, the cat will decide to move again. When it does, if it's a teeny-tiny bit below the top surface of the floor, it is 'inside' the floor prim. Whenever ANY physical object (including your Avatar .. and, yes, you can test it) is 'inside' another, the two become 'temporarily phantom' but only with respect to each other. That means the cat (still physical with respect to gravity) does not even 'see' the floor prim while it's 'inside' the prim. Gravity takes over. The cat (or you, or the prim you're testing with) falls through the floor. Once it falls below the floor, the two are no longer 'temporarily phantom' to each other. But, of course, the floor is now above the cat.

On interesting effect is to take a cat HIGH up (like 4000 meters), home it, and set a 3 meter range. Then remove the floor prim. You'll notice, as the cat walks, it spirals downward. This is because it's physical and gravity is pulling it down. When the cat stops walking, it stops falling and floats in air for a few moments. Then it begins to walk again and continues its downward spiral. Eventually, it reaches the ground and stop falling, circling and pausing, on the ground. Eventually, the cat sleeps. At that point it ZOOMS up to where you homed it, and remains there, sleeping, floating in air, until it wakes and, once again, spirals slowly down to the ground.