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sftrabbit
Up and Coming
July 5, 2013

Does an incorrect IPD cause divergent parallax?

I've seen suggestions that if you have the IPD set to greater than the viewer's actual IPD, it can cause their eyes to uncomfortably diverge (so each eye looks outwards, away from the other). However, as far as I understand, the IPD only changes the distance between the virtual cameras, and making it match the viewer's actual IPD only helps to make the scene look as they would see it. Once the projection is done, the result is aligned with the centre of the lenses. If the IPD were too great, it should only make the viewer feel as though their eyes are further apart than normal (maybe a strange sensation, but not divergence).

I expect that divergence would only occur if the resulting projection were not offset to the lens centres correctly. That is, if the images on the display are centred further out than the lens centres, there will be divergence.

Is this right?

    16 replies

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    Nekto2
    Inspiring
    May 29, 2014
    "tomf" wrote:
    All that is changing is the virtual camera separation - the ICD. As this changes, the 65mm-sized cube will not be distorted, but it will appear larger or smaller as the ICD changes.


    It's too simple to think it will appear larger or smaller. You should compare it's front and back size. Both will be larger or smaller, but multiplication will be different.

    First let's have real example: IPD = 65, ICD = 65, cube size = 65.
    Camera on the left side can't see left side of the cube. Same for right camera and right side.
    Cube in video will be a cube and user also can't see sides. So he think this is a cube with side 65mm.

    Next we will have user with IPD = 70. If he looks at 65mm cube in real life he will see sides of cube.
    But if you show him the video (with ICD=65) user can't see sides! So he will think it is not a cube, but a pyramid.
    Horizontal and vertical size of front side still will appear to be 65mm and virtual distance from eyes to cube will be the same. But back side will not be 65mm any more.
    ;)

    If you want to scale cube to virtual 70mm (so it will appear larger) you should change both 1) zoom of video, 2) L/R separation (and it will change depth of cube).

    Let's make it simple:
    If you want to scale a cube so it's side will appear in video twice as large (and no geometry distortion in depth) you should scale everything:
    1) ICD should be divided by 2
    2) distance should be divided by 2

    So you will be like a Lilliput (like your IPD changed to 65/2 in real world).

    And the next thing is how to test it. User can't change his IPD :) But he could change ICD settings in rift config. So he will have IPD/ICD mismatch in demo game very similar to what will be with video (fixed ICD) if it's ICD not the same as users IPD.
    tomf
    Participant
    June 3, 2014
    No, you just scale the ICD. Also, change the scale on the head motion in the same way, or you get "giraffe sickness".
    Tom Forsyth - OculusVR coder
    Nekto2
    Inspiring
    June 5, 2014
    Why you think so?
    Example image: http://img-6.photosight.ru/93b/4567414_large.jpg
    Top image has ICD 65mm, bottom is 32.5mm

    Don't you think that sizes are the same, but depth is different?
    2EyeGuy
    Up and Coming
    July 5, 2014
    So, I am uncomfortably diverging the whole time (IPD 6.2)? And children can't safely use the Rift?
    Why don't you adjust the distortion?
    Currently working on Dolphin VR Follow along: https://github.com/CarlKenner/dolphin/commits/VR-Hydra Latest Version: viewtopic.php?f=42&t=11241&start=1020#p249426
    Jedi2016
    Up and Coming
    July 7, 2014
    Maybe I'm missing something here, but the distortion would change, wouldn't it? Otherwise, how does the Rift, with fixed lenses, accommodate users with larger or smaller IPD? Would it not have to slightly alter the barrel distortion so that the images, when viewed through the lenses from an out-of-center position, still look correct? I'm not even thinking about rendering so much as the viewpoint through the physical lenses.. coming at it from a different angle would yield a different distortion, that's plain math there.

    That's also good news to know about opticians measuring IPD.. I have an appointment next month (likely before I get my Rift) and I plan to ask them to take a measurement while I'm there, so I should probably mention what I'm using it for.
    brantlew
    Participant
    July 7, 2014
    Collimated lenses cause the radial center of distortion to be projected straight into your pupil regardless of the lateral position of your eye. There is some asymmetrical warping around the edges at extreme IPD's that is not being accounted for currently. But the important message to take away is this - different IPDs do NOT cause users to diverge or converge their eyes. Everybody is looking directly at the same center pixel regardless of IPD.

    However, distortion does change a good bit with different eye-reliefs. The Oculus SDK does alter distortion based on how far your eyes are from the lenses (assuming you have calibrated).

    Additionally FOV is dependent on both eye-relief and IPD and the SDK will alter the shape of the rendered fov based on your eye position.
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