[x3d-public] dynamic cycleInterval changes

[Don Brutzman]

[continuing dialog...]

On 11/23/2016 10:59 AM, Leonard Daly wrote:
> Don,
>
> I disagree with your concluding comment "Best place to fix this is in TimeSensor algorithm/code itself." The specification states (as you quote) "An active TimeSensor node ignores set_cycleInterval ... events". The spec needs to change for the TimeSensor to receive, process, and act of an incoming event that changes cycleInterval.
>
> Perhaps a better solution is to change the spec to allow TimeSensor to accept and process cycleInterval changes.

Yes that was the entire point of the specification comment: to change TimeSensor specification prose (and corresponding algorithm) to allow dynamic cycleInterval changes.

So perhaps there is some basis for agreement here after all?

Am looking at the specification comment description, hopefully it is clear.  Roy if appropriate please improve when entering in Mantis, as you think best.

	"Issue: TimeSensor cycleInterval needs to be modifiable when running"

> The question that would need to be resolved is what happens to the current cycle.

Yes that is the important next question if cycleInterval is dynamically modified for a running TimeSensor.

>  I can see three options
>
> 1) The TimeSensor would reset (stop and start from the beginning). This works for looping sensors, but may cause problems on a single pass sensor, or one that must have a specific number of cycles before stopping.
>
> 2) The TimeSensor would jump to the same fractional time. For example if the TimeSensor is .4 of the way through a cycle with cycleInterval of 10 (i.e. 4 seconds in), and the cycleInterval was changed to 20, the fractional time would still be .4, but it would take 12 seconds to complete the remaining interval. This is probably the most benign option. It does lead to sudden velocity changes in any animating object.
>
> 3) The TimeSensor would jump to the same real time. For example if the TimeSensor is .4 of the way through a cycle with cycleInterval of 10 (i.e. 4 seconds in), and the cycleInterval was changed to 20, the fractional time would become .2 taking the full 20 seconds to complete this cycle. This options leads to sudden position changes in any animating object.
>
> There can be other variants where the change happens smoothly, but it may not be worth trying to standardize all possibilities.
>
> Leonard Daly

My thinking was that, other than accepting a set_cycleInterval event, we should try to avoid any other algorithmic changes unless shown to be absolutely necessary.  Specification complexity can be confusing to authors (witness the current cycleInterval problem) and inconsistently implemented if not careful.

Suggested approach is to consider typical animation examples, use those to determine if anything must be changed in spec, implement and evaluate.

I suspect that your option (2) is most likely closest to a least-interference update in the algorithm.

This minimalist approach seems workable if cycleInterval is dynamically reduced (or lengthened) significantly.  Of interest is that subsequent set_fraction events make sense as well.

Of course an author retains full control whatever is decided here...  They can always stop/modify/restart a TimeSensor clock, but have to be willing to do that across multiple event loops in order to achieve deterministic response (just like the currently existing situation).

Thanks for scrutiny of this relatively small X3D change which appears to have important potential improvements for HTML5/DOM and VR external interoperability.


>> Comment on 19775-1: Abstract X3D Definitions - V3.3
>> 8.4.1 TimeSensor
>> http://www.web3d.org/documents/specifications/19775-1/V3.3/Part01/components/time.html#TimeSensor
>>
>> -----------------
>> Issue: TimeSensor cycleInterval needs to be modifiable when running
>>
>> It is not feasible to modify TimeSensor cycleInterval without major
>> machinations and likely failure.
>>
>> Spec sayeth:
>>
>> 8.4.1 TimeSensor
>> "An active TimeSensor node ignores set_cycleInterval and set_startTime
>> events. An active TimeSensor node also ignores set_stopTime events for
>> set_stopTime less than or equal to startTime."
>>
>> Email discussion illustrates difficulties with modifying cycleInterval
>> without error.
>> Subject: dynamic cycleInterval changes
>> http://web3d.org/pipermail/x3d-public_web3d.org/2016-November/005566.html
>>
>> Issues such as event-arrival nondeterminism, usability and HTML5/DOM
>> interoperability are all interrelated on this topic.
>>
>> Best place to fix this is in TimeSensor algorithm/code itself.  Necessary
>> change is to make TimeSensor cycleInterval modifiable when running in order
>> to avoid two-phase commits and a host of other difficulties.
>>
>> -----------------
>>
>> Submitted on Wednesday, 2016,  November 23 - 8:42am
>> by brutzman (brutzman )
>> IP: 162.225.68.164
>>
>> See: http://www.web3d.org/node/1694/submission/1072

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