compute reference plane on the fly for draggable cube example

[NicolasRannou]

Do not rely on pre-comupted plane for dragging cubes because if we zoom out too much, the reference plane used for dragging gets too small and dragging stops working.

http://threejs.org/examples/#webgl_interactive_draggablecubes

See #8727

[mrdoob]

@WestLangley looks good?

[WestLangley]

You need to make use of the optionalTarget to avoid instantiating Vector3's unnecessarily.

[WestLangley]

@NicolasRannou The use of a theoretical plane instead of a geometric one is a good idea. : - )

The cubes "jump" when you start to drag them. Compare with the smooth action of the current version.

I will not be able to check the math in at timely fashion. /ping @Mugen87 for some help with the rewrite?

[NicolasRannou]

Thanks for the feed back, I'll address the issues and come back to you!

[NicolasRannou]

here you go!

For the math, I described the logic there:
https://github.com/FNNDSC/ami/blob/master/src/core/core.intersections.js#L263-L289
(I am currently writing a JS library to bring medical imaging to the web thanks to THREEJS and its shaders materials [1] [2] - the example page will look familiar... :) )

cc: @WestLangley @mrdoob

[1](Volume Reslicing)[http://fnndsc.github.io/ami/#geometries_slice]
[2](Volume Rendering)[http://fnndsc.github.io/ami/#vr_singlepass_raycasting]

[Mugen87]

What about Ray .intersectsPlane. Isn't it better to have just one method for intersection test of two entities?

@NicolasRannou Why don't you do this in your code?

raycaster.ray.intersectsPlane( plane );

In this case, you don't need Plane . intersectRay.

[NicolasRannou]

wow I missed this thanks @Mugen87 !

[NicolasRannou]

I create a new pull request to replace this one:
#8758