Describe the new feature or enhancement

Currently, there is no dedicated way to store details about eye tracking related annotations. Several properties of saccades (e.g., peak velocity, start+end position, ...) and fixations (e.g., avg position) are useful information downstream. (This info can/will hopefully in future be generated by a [to be implemented] ET analysis module or [for now] be read from the Eyelink output file.)

Atm, annotations only allow to store onset, duration, description, (and orig_time) as they need to be generic. So extending Annotations with additional (eye tracking specific) fields is not an option.
For reference: EYE-EEG stores {'latency', 'duration', 'sac_vmax', 'sac_amplitude', 'sac_angle', 'epoch', 'sac_startpos_x', 'sac_startpos_y', 'sac_endpos_x', 'sac_endpos_y'} for saccades and {'latency', 'duration', 'fix_avgpos_x', 'fix_avgpos_y', 'epoch', 'sac_vmax', 'sac_amplitude', 'sac_angle', 'sac_startpos_x', 'sac_startpos_y', 'sac_endpos_x', 'sac_endpos_y'} for fixations (Code).

Describe your proposed implementation

[1] With @dominikwelke, @mscheltienne , @sappelhoff we discussed having a separate structure (probably a dict) that holds the according info and can be indexed with a unique identifier linked to a given annotation object. This identifier could be a hash formed by combining onset, duration, and description.

[2] Ideally, the information can still be linked once the annotations are transferred into events. I guess, the event_dict route could be extended accordingly but have not thought this through or discussed with others.

Describe possible alternatives

As discussed above, extending the Annotation class does not seem a viable alternative.

Alternatively, we can leave it to the user as the metrics can theoretically be calculated from the gaze data on the fly. This, however, seems to be tedious and non-trivial (esp., for parameters like peak velocity).

Additional context

@scott-huberty (and @larsoner ): do you have ideas/opinions regarding this? As you have implemented the bulk of the existing functionality regarding eye tracking data and probably already thought about similar questions. Thx!