Hi @vwgx3, you are quite correct that the concept of a fixation is the maintaining of the visual gaze on a single location, however there is not necessarily an intention not to deviate… it should be seen as something natural that occurs when viewing certain stimuli. For example, an interesting logo that catches your eyes for a few seconds would likely be determined as a fixation.
However the actual *calculation* of what constitutes a fixation based upon raw gaze data is non-trivial and something that should be considered before moving forward. Put in simple terms; input would be the inherent jittery raw gaze data which is then fed into a fixation filter algorithm which would ideally produce a series of stable fixation points with a corresponding time duration.
Accordingly, the specific algorithm you use is of importance so It would be most useful if you could explain how you are calculating fixations along with which SDK API you are currently using with the EyeX.
For your interest there are a number of free fixation filters which perhaps you can translate for your purposes:
https://se.mathworks.com/matlabcentral/fileexchange/56236-djangraw-gazevistoolbox
https://github.com/davebraze/FDBeye/wiki/Researcher-Contributed-Eye-Tracking-Tools
https://www.tobiipro.com/about/partners-resellers/app-market/fixation-detector/
I also worry about you using the raw positions over time and differentiating.. I assume to determine saccadic velocity? This is not a valid technique I am afraid… one needs to get the distance between two properly established fixation points and divide by the time difference to obtain saccadic velocity.
You are quite right that the variable sample rate will be causing issues should you need exact velocities, and indeed the EyeX is not designed for detailed analysis but simple interaction purposes, so that is something else to bear in mind. I found for you an article published by a third party research group concerning the EyeX which may be of use to you that discusses the consequences of the variation @ https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5429393/
In any event, please be aware that any analysis of data undertaken using the EyeX and associated SDK’s (Unity, Core SDK, etc) in expressly prohibited without the purchase of a special analytical licence. More information regarding this can be found @ https://analyticaluse.tobii.com/
If you could kindly describe your intentions with the EyeX that would be helpful.