MarchingCubes: Migrate to Mesh.

[Mugen87]

Related issue: -

Description

MarchingCubes is now derived from Mesh.

In order to retain the functionality which was previously based on ImmediateRenderObject, it is now necessary to specify the maximum expected triangle/poly count when creating an instance of MarchingCubes.

The logic that updates the geometry buffers is now implemented in onBeforeRender(). If it detects too small buffers, it notifies the user with a warning.

[Mugen87]

https://raw.githack.com/Mugen87/three.js/ddf8c983b37b1b06c7371d091e0911a9c633ef4f/examples/webgl_marchingcubes.html
https://raw.githack.com/Mugen87/three.js/ddf8c983b37b1b06c7371d091e0911a9c633ef4f/examples/webxr_vr_sculpt.html

[Mugen87]

Instead of resetting the vertices like before, I was trying to use setDrawRange() to limit the rendered data, However, this introduces strange flickering (it seems the number of rendered triangles varies from frame to frame). Hence, I've sticked to the previous approach for now.

[mrdoob]

Yay! This code was always so hard to deal with.

[Mugen87]

It seems the new approach is at least as fast as the previous approach. I was not able to detect a performance regression.

[mrdoob]

Thanks!

[gkjohnson]

Is it no longer possible to "bake" the geometry to a static BufferGeometry any more? I just upgraded one of my bvh demos and saw this changed. In terms of performance it's significantly faster if you don't have a dynamic marching cubes geometry to generate a static geometry to render.

I think it might be nice if the marching cubes logic were more separated from a "Mesh" instance and could serve more as a utility that something like a mesh class uses. Maybe I'll look into that at some point.