[x3d-public] LineSensor

[Andreas Plesch]

I rethought the LineSensor idea and instead opted to add an internal
line sensor mode to PlaneSensor. The idea is that if the x or y
components of both minPosition and maxPosition are 0, PlaneSensor acts
as a LineSensor and is free to use a better intersection plane than
the default XY plane which can become completely unusable in the case
when it is viewed on edge.

In the implementation used in the fiddle below, the intersection plane
normal in line sensor mode is computed as the cross product of the
line axis (x or y axis) with the cross product of the line axis and
the view direction when dragging starts:

https://jsfiddle.net/andreasplesch/mzg9dqyc/111/

It seems to work well and allows programming of a translation widget
in X3D without scripts as shown. On edge view of the PlaneSensor XY
plane still works because the XY plane is not actually used.

This approach is more spec friendly as it does not require a new node
and just allows the defacto use as LineSensor to become robust.
However, since it requires use of another plane than the XY plane, it
may be necessary to add some language. Perhaps:

"This technique provides a way to implement a line sensor that maps
dragging motion into a translation in one dimension. In this case, the
tracking plane can be any plane that contains the one unconstrained
axis (X or Y). A browser is therefore free to choose a tracking plane
suitable to robustly determine intersections, in particular when the
XY plane is unusable. In this case, the generated trackpoint_changed
value becomes implementation dependent."

I would prefer this over a LineSensor node, and it was pretty
straightforward to add to the x3dom PlaneSensor implementation.

I think it would be possible to check for other, equal values than 0
and then use the same approach but I am not sure if it would be
useful.

If there is feedback, especially on the suggested prose, I will glad
it to add it to a potential standard comment.

For example, one could tighten the spec. by giving a formula for the
orientation of a suitable tracking plane in the one dimensional case
but that may be overreach.

Cheers, -Andreas

On Thu, Mar 21, 2019 at 6:06 PM Andreas Plesch <andreasplesch at gmail.com> wrote:
>
> I now understand the need for LineSensor and may give it a try for
> x3dom. I know freewrl has it. It may make sense to replicate it.
>
> My initial idea would be to use the x axis of the local coordinate
> system as the Line orientation. Or maybe the y axis since X3D
> geometries tend to align along y (cylinder, extrusion).
>
> Then one can construct an intersection plane which includes the x axis
> and another direction at a high angle with the viewing direction to
> get a clean intersection for the inital point and subsequent points
> during dragging.
>
> Is that how freewrl does it ? I think I saw that it may use an
> additional field for the line orientation but it may be best to avoid
> it.
>
> Andreas
>
> --
> Andreas Plesch
> Waltham, MA 02453



-- 
Andreas Plesch
Waltham, MA 02453