What is pupil swim?

[phoenixdigital]

Can anyone explain what pupil swim is in detail?

I've seen people talk about it on internet forums but when asked nobody has really been able to explain what it is.

There are a lot of vague attempts all over the internet but nothing really talks about

• what it is exactly?
• specifically what physical processes cause it?
• what are its effects on vision in VR?
• why is it bad?
  
• Is there an example I can try to "force" these effects on  Rift/Vive?
  
• what are the technical reasons why the Rift apparently has worse pupil swim than the Vive?
• How will eye tracking resolve it?
  

As an engineer not understanding how or why something works drives me mental.

I've seen explanations ranging from "the way an image distorts as you look at different parts of the lens" or "the way an image distorts as you turn your head in VR"

Can anyone clarify what it is?

[Roaster]

I think it's basically where the optical axis of the source lens does not line up with the eye's lens.  Moving the rift up and down and tilting the angle to achieve focus makes optical alignment unlikely. The vive has a more fixed alignment.

[brantlew]

Without getting into too much technical detail - "pupil swim" is a nick-name for the variation in lens distortion that you experience as your eye rotates off the center axis and across the lens.  Even with perfect distortion correction at the center viewing position the user will still experience some distortion (as motion) when moving the eye around and especially toward the edge of the lens because the distortion field at the edge is different than the field in the center.  This will be true of every lens to varying degrees based on the lens design and the distortion correction implementation.

[phoenixdigital]

Thanks for the info.

So would I be right in the assumption then that the apparent larger issue of pupil swim with Rift lenses is a tradeoff for putting more pixels in the centre of the FOV. The more uniform the lens the less pupil swim which is apparent.

Is there a way to "see" it in action as I own both headsets and would love to actually see what this "pupil swim" looks like?

I'm not scared of technical detail either if you felt like dropping a knowledge bomb on everyone 🙂

[PatimPatam]

I think everything mentioned so far in this thread is correct and related, but let me try to connect the different pieces a bit more..

As Brantlew said "pupil swim" is kind of "the way an image distorts as you look at different parts of the lens", but this is actually the same as "the way an image distorts as you turn your head in VR" IF you are focusing at one part of the VR scene, because your eyes will automatically and unconsciously turn to compensate for your neck movement, as they are "locked" looking at a specific feature.

I also have both HMDs, and i don't think the Rift has worse pupil swim than the Vive because of an inferior software distortion or because of worse lens properties (i personally prefer the Rift's lenses much more).

Having said that, the Rift CAN have worse pupil swim than the Vive because of the simple fact that, as Roaster mentioned, it's much easier to have a sub-optimal optical alignment. The fact that you can tilt the Rift up and down makes the Rift more comfortable, it can better adapt to different face shapes, but this also makes optical alignment more unlikely than with the Vive, which leads to worse pupil swim.

You can test this very easily; for example, in Oculus home look at your right (by turning your body) and focus on the vertical lines that you can see near the red robot. Now move your head left and right while focusing at a vertical line, it should stay straight. Now slightly tilt the Rift up and do the same thing again, you will probably notice that the vertical lines don't stay straight, they actually bend as you turn your head.

So basically, with the Rift if you are lucky and have a face shape that lets you tilt the Rift all the way "down" then you can have a good alignment and small swim; if you are unlucky (or don't set it right) then you will experience much worse distortion than with the Vive.

I know it's all a matter of tradeoffs in VR, but personally i think Oculus made a mistake by allowing to tilt the HMD up and down. At best optical misalignment and pupil swim makes everything look less solid and breaks presence, at worst it can really mess you up and cause severe headache or nausea.

Eye tracking could improve pupil swim by changing the software distortion on the fly, depending on what part of the lens you are looking at. At the same time it could also detect that you don't have a correct optical alignment and give you a warning for instance.

Anyway at least that's the way i understand it, but i could be wrong.. Hope this helps!

[danknugz]

wouldnt tilting the rift up or down result i  the same kind of distortion, because in both cases your eyes are centered on either the higher part of the sweet spot (if tilted down) or the lower (if tilted up)? makes no sense, i thought the optimal position was to have your eyes centered in the sweet spot as much as possible.

[PatimPatam]

@danknugz Well just try the test i mentioned while keeping your eyes centered in the sweet spot.. can't see any difference?

[danknugz]

i will try

[brantlew]

One of the most important issues is vertical alignment.  It is really crucial that you get your eyes vertically centered on the lens - even if it's not the optimal comfort fit.  I coach people all the time on this.  New users of the Rift will tend to place it too high, resting on the cheek bone which causes a lot of swim.  Wearing it low tends to place it in best optical alignment.  The best way to test is to wear it really high then yaw your head side to side.  Observe the distortion "swivel" and then push the headset down until that effect is minimized.

Another factor is eye-relief.  The Rift is designed for about 15mm eye-relief.  Some people like a really relaxed fit which helps your face breath but may put you at 25mm relief or more which will create a good deal more distortion and also some chromatic abberation.

[Anonymous]

Are you all completely stupid..?

A pupil swim is what happens when schoolkids go to the swimming pool for PE.

You crazy fools.