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Lens flare (God rays) the technicalities

Anonymous
Not applicable
It would appear from reviews and user feedback that, to a certain extent,  "God rays" are an issue with the Rift and if also problematic for the Vive then possibly more so.  I'm curious as to why this is.  During development it was presumably noticed and one assumes noticeable, especially given that the Oculus logo seems to be particularly prone to this effect.  So what differences between the Rift and the Vive make the problem better or worse ?  Where the unique lenses in the Rift wasted effort, what design choices/trade offs came into play when making judgement calls between simple lenses, as in the Vive versus custom lenses in the Rift ?


14 REPLIES 14

Chassit
Heroic Explorer
As far as I understand, it's inevitable due to the use of Fresnel lenses.

Fresnel lenses rely on a stepped surface to redirect the light, like this:

yx4nr97f5g2e.jpg

The positive side is that you can have great zoom at almost point-blank range, without having to using a traditional lens about the thickness of a beer bottle bottom. For HMDs this is sort of necessary, or else two slabs of glass will add a lot of weight.

The negative trade-off is that light rays don't always follow this route. Sometimes when they hit the lens, they scatter a little sideways, causing the "god ray".

There's no fundamental difference between Vive and Rift in this regard. What I heard is that the density of ripples on the Vive lens is lower, so god ray is not as much obvious. But probably as the down side of this, screen door effect tends to be worse on Vive.

Zoomie
Expert Trustee

My Rift arrived yesterday and I've only had about an hour of testing so far.  The difference in focus and sharpness blows the DK2 out of the water.  Whereas the DK2 felt like it had a 20 degree sweet spot, the CV1 feels like the entire field of view is in focus.  Minor movement of the headset doesn't lose the sweet spot like it did with the DK2.  In my limited trial my CV1 felt like it was very loose, yet I still didn't lose focus or clarity at any point.  I was even able to look down at the floor without the unit pulling away from my face.  This may seem like a small difference, but it opens up options like mini-golf that would have been unplayable on the DK2 unless you adjusted your HMD constantly.

The god-rays are there.  However, they present more like light bloom when you look directly at a bright source.  If anything, it feels like the image is washed out when staring at a bright object.  I know some people say they may vary in intensity from Rift to Rift, so maybe I just got a good one?

I didn't come up with this, but the best description I've heard is that the Fresnel god rays act similarly to how your eyes see bright light after you've been in a pool with chlorine.  Normal vision is unaffected, but bright lights get a halo or glare to them.

In my VERY limited experience so far, the trade-off is absolutely a good one.  The overall experience is much sharper across the entire field of vision.  Yes, glare exists and I can see how it's annoying for some people, but for me it's still a massive step forward. 

I'm curious whether the majority of people complaining about god rays are previous owners of a DK1/DK2, or are brand new HMD owners.  As a previous DK2 owner, the upgrade in overall experience more than made up for the minor annoyance.

Hopefully Gen 2 will solve the god rays, will give us larger FOV, and will give us headsets that don't smell like airplane glue when you first get them.

Any sufficiently advanced technology is indistinguishable from magic. - Arthur C Clarke

blanes
Rising Star
Very nice exposition & explanation there of what is going on in God-Ray land, thanks   🙂  Oh and airplane glue ... mmm I always liked that as a kid actually !

Sarlin
Adventurer
The difference is due to the difference in the density of the Fresnel groves.  My assumption is that more gives (Rifts) give more opportunities for rays to refract errantly to create the issue.  But having more groves should provide a larger focus sweet spot, from what I have read. So it's a trade off that Oculus made to have more groves which could cause "God Rays" in some cases vs having a larger "sweet spot".  Sounds like it was a good call so far.

RobHermans
Superstar
Yes, really good @Chassit & @Zoomie 🙂 I feel all the better prepared for when my Rift arrives to be able to understand what I'll be seeing. Thanks, guys.

Anonymous
Not applicable

Chassit said:

As far as I understand, it's inevitable due to the use of Fresnel lenses.

Fresnel lenses rely on a stepped surface to redirect the light, like this:

yx4nr97f5g2e.jpg

The positive side is that you can have great zoom at almost point-blank range, without having to using a traditional lens about the thickness of a beer bottle bottom. For HMDs this is sort of necessary, or else two slabs of glass will add a lot of weight.

The negative trade-off is that light rays don't always follow this route. Sometimes when they hit the lens, they scatter a little sideways, causing the "god ray".

There's no fundamental difference between Vive and Rift in this regard. What I heard is that the density of ripples on the Vive lens is lower, so god ray is not as much obvious. But probably as the down side of this, screen door effect tends to be worse on Vive.


So, if I understand correctly, the Rifts lenses, with more fresnel grooves/rings provides a sharper image across the view but has the down side of refracting/smearing contrasting light, that is the trade off ?  Does the greater number of fresnel ridges/grooves really nullify the screen door, I would have thought a sharper image would have made it worse ?

Does the shape of the lenses or is it the number of ridges that contribute to the larger sweet spot for the Rift ?

Anonymous
Not applicable
I suppose one of the benefits of the new, custom, lenses with it's larger sweet spot will be utterly lost on the vast majority of new Rift users, because not having transitioned from the development kits, the perceived advantages are not fully understood or appreciated, all they will see is the trade off, downside.

Sarlin
Adventurer



So, if I understand correctly, the Rifts lenses, with more fresnel grooves/rings provides a sharper image across the view but has the down side of refracting/smearing contrasting light, that is the trade off ?  


Yes, exactly!




  Does the greater number of fresnel ridges/grooves really nullify the screen door, I would have thought a sharper image would have made it worse ?


It doesn't nullify but it makes it less apparent since the lines are less visible.  Even on the Vive you don't hear much complaint of SDE.




Does the shape of the lenses or is it the number of ridges that contribute to the larger sweet spot for the Rift ?


The number of ridges contributes to the larger sweet spot.  The lens for the most part is round so that light is refracted correctly into your eyes.


crim3
Expert Protege
How can a Fresnel lens contribute to reduce the SDE? To me is just a mean to achieve a certain focal length with less material than a conventional lens. If they managed to reduce so much the SDE it's been finding a good balance between FOV and the resolution of the displays.
Just curious.