[x3d-public] LineSensor and avoiding uncontrolled dragging

[Brutzman, Donald (Don) (CIV)]

thanks for the in-depth look at these challenges.

yes, good specification preparation really helps a lot... this work all takes time, which is our only limited constraint.

On 3/27/2019 8:07 PM, Andreas Plesch wrote:
> In preparation for a well presented spec. comment, I developed a
> minimal but discerning example scene illustrating the problematic on
> edge view for 1D PlaneSensors:
> 
> https://gist.github.com/andreasplesch/33016fd0918178f5dfcf4632d477f4e4#file-planesensor1donedge-x3d

In the writeup there you say:
=============================
Drag the red cone
This scene illustrates the difficulty of using a PlaneSensor when the view is parallel to the tracking plane (light blue). Dragging works well in the default view normal to the tracking plane. Press PgUp to change the viewpoint to an on edge view. Dragging cannot work because the intersection of the bearing with tracking the plane becomes ill defined.
=============================

A not-uncommon 3D pathology when dragging along a direction that is nearly perpendicular to the screen is that translations can become wildly large, moving things behind the user's view, or inside /outside the view frustum clipping planes.  At which point, the selected geometry is not visible and the handle is lost if the drag sensor is released.

A good practice for implementers is "don't do that" but am not sure that is codified anywhere.  Not seeing any cautionary prose like that in the X3D specification.

Perhaps we can come up with a functional limitation on drag sensors that avoids this pathology?

Perhaps
a. Drag sensors must avoid operating when the tracking geometry becomes ill defined and does not permit positive user control.
b. Dragging past the boundaries of the view frustum is not allowed.
c. something else?

all the best, Don
-- 
Don Brutzman  Naval Postgraduate School, Code USW/Br       brutzman at nps.edu
Watkins 270,  MOVES Institute, Monterey CA 93943-5000 USA   +1.831.656.2149
X3D graphics, virtual worlds, navy robotics http://faculty.nps.edu/brutzman