LineSegments2: Check bounding box and bounding sphere when raycasting

[gkjohnson]

Related issue: --

Description

Improve LineSegments2 raycasting performance by checking against the bounding sphere and bounding box including a margin computed from the closest distance to the bounds and the screen space line width. Tested by locally adding raycasting to the "webgl_lines_fat" example and zooming inside the bounds as well out far while checking that intersections are returned for both thin (1px) and thick (50px) lines.

	50px width	1px width
box margin	~17.6 units	~0.37 units
box screenshot
sphere margin	~23.6 units	~0.47 units
sphere screenshot

@WestLangley your eyes on the logic would be appreciated here 😁

[WestLangley]

@mrdoob Garrett is maintaining the fat-lines raycast code, so I would trust him on this. TBH, I have not yet studied it.

//

Also, I think a separate test case would be nice. Thing is, the fat lines example was selected to be pretty, not typical. In the existing example, the line segments are very short, and consist of mostly end-caps. Relatively-longer line segments in a raycast test would make the most sense to me. For example:

const divisions = points.length - 1;

[gkjohnson]

Well from my testing it all works as expected. I tried it in a more practical case in a project with a lot of LineSegments2 objects and I saw performance improvement from around 40-50ms down to 10ms when raycasting so it's definitely an improvement, as well. It all seems reasonable and ready to merge to me but a bit of a review if possible is always nice.

Also, I think a separate test case would be nice. Thing is, the fat lines example was selected to be pretty, not typical. In the existing example, the line segments are very short, and consist of mostly end-caps. Relatively-longer line segments in a raycast test would make the most sense to me. For example:

I'm happy to add another example to test raycasting in a subsequent PR if @mrdoob okay with that. In my opinion short line segments aren't necessarily an uncommon case -- I use these frequently to render arcs or circles as well as longer lines. I think a raycasting example with both types of segments would be good.

[mrdoob]

Thanks!

[gkjohnson]

@mrdoob are you interested in another example that demonstrates raycasting for LineSegments2 for testing or should I hold off on that.

[mrdoob]

Hmm, I think so yes! 👍