[R-sig-Geo] STIDF - endTime, STI -> Track

[Chris English]

Hi,

I am wondering about the role of endTime in STIDF objects.  I am examining eye tracking data (previously cleaned of blinks) in relation to 
presented stimuli that is for some subjects an optical illusion and for others not.  I want to examine where they were looking and when.

My process is to make a STIDF from the eye tracking data case and a STSDF of the stimuli that was presented where and for how long,
convert the STIDF to a Track then do some 'over' analysis.

If I build my endTime for the STIDF using the delta() function on N samples, I think I get something like N-1 endTimes, or every sample
is an endTime so N = N.

If instead I am thinking of endTime(s) as an interval during which there is a cross hair and some tangential stimulus on the screen and
endTime is when a subject responds in some manner I can't build an STDIF due to the following test:

> eye_5v1_stidf <- STIDF(eye_5v1_sp, eye_5v1_time, eye_5v1_data,
+ eye_5v1_endTime)
Error: nrow(object at time) == length(object at endTime) is not TRUE
> nrow(eye_5v1_time)
[1] 4724
> length(eye_5v1_endTime)
[1] 63
> 

Indeed, it is not true. But what information do I have in endTime other than my sensor sampling rate adjusted for blinks?  What I hoped to
achieve was to compare the spacetime aspects of the Track data through time periods consistent with the time periods in the STSDF.
Perhaps 'over' takes care of this for me and I don't have to attend if I just accept that endTime in the case of the STIDF is the end of each
sample.

The eye tracking data I am examining is fairly simple: x, y, cumulative sum of samples in ms, duration between samples; from which
an STI can be constructed.  Not much more data than where the eyes were when.  It would seem that there would be a lot of simple sensor
data of this sort so I wonder if Track can relax its requirement of STIDF to allow STI. 

Thank you in advance for your time and consideration.

Cheers,

Chris