Add note about zero-scale meshes.

[emackey]

Following conversation from #1314 (comment) and subsequent comments, I'd like to add an implementation note encouraging optimizations for zero-scale meshes.

Zero-scale meshes can be used to animate visibility of a mesh, and can enable use-cases such as fluid simulation where the computed mesh changes shape and changes number of vertices over a set of known timesteps.

/cc @donmccurdy @javagl

[lexaknyazev]

I suppose it's just a hint for developers who may skip such an optimization in their scene graph implementations, right?

[scurest]

Only applies to unskinned meshes.

[lexaknyazev]

Transforms of a skinned mesh are controlled by the joints' transforms that could contain zero scale as well. However, the results may be unstable when only a part of the mesh is affected.

[emackey]

Is it enough to just change this to "un-skinned meshes," as I have just pushed here?

[lexaknyazev]

Is it enough to just change this to "un-skinned meshes," as I have just pushed here?

I'd say it's a bit imprecise because applying zero-scale to all joint nodes (or just their root) would lead to the same effect for skinned meshes.

[emackey]

How about this:

When the scale is zero on all three axes (by node transform or by animated scale), implementations are free to optimize away rendering of the node's mesh, and all of the node's children's meshes. This provides a mechanism to animate visibility. For skinned meshes, this optimization would only be applicable if all joints in the skin were scaled to zero simultaneously.

[emackey]

Or this?

When the scale is zero on all three axes (by node transform or by animated scale), implementations are free to optimize away rendering of the node's mesh, and all of the node's children's meshes. This provides a mechanism to animate visibility. Skinned meshes must not use this optimization unless all of the joints in the skin are scaled to zero simultaneously.

[lexaknyazev]

unless all of the joints in the skin are scaled to zero simultaneously

LGTM, assuming that this includes a case when the joints get a zero-scale transform from a root node.

[emackey]

Yes, that's the intention.