> As things get smaller a greater PPI is needed to render the same level of detail.
It really doesn't work that way. If your VR display is too low-resolution to convincingly render a virtual display 15 feet away, then it's too low-resolution to convincingly render the virtual display 15 inches away. Your physical display has the same resolution regardless of where the virtual screen is, and if it lacks sufficient resolution to render an object convincingly, it just cannot do it regardless of how distantly the object is rendered. To put it another way, your display has the ability to render a certain amount of detail, full stop. Where a virtual object is rendered is irrelevant.
Try this. Sit close to your monitor, close enough that text isn't crisp enough. Now zoom your display (Ctrl+'+' in most browsers), which is equivalent to bringing a virtual display closer. Note that the text still isn't crisp. If you could see the pixelation before, you can still see it. Moving the virtual display closer does nothing, because you aren't adding more resolution to the physical display.
> To put it another way, your display has the ability to render a certain amount of detail, full stop. Where a virtual object is rendered is irrelevant.
But we're talking about a virtual display displaying pre-existing content (e.g. video footage). We aren't talking about the object itself.
If it is one foot from you in virtual space, each pixel of video footage might be mapped 1:1 into each pixel of the VR headset's screen resolution. Therefore the loss of detail is 0%. Every pixel of video detail is transposed onto the display in the VR headset.
If the virtual object is further way (e.g. six feet) it takes up a smaller percentage of the VR headset's display, and therefore fewer physical pixels on the VR headset's display are used to render it. So now instead of 1:1 you might be 1:4, and you're seeing 25% of the video's original detail actually rendered on the VR headset's display.
> Try this. Sit close to your monitor, close enough that text isn't crisp enough. Now zoom your display (Ctrl+'+' in most browsers), which is equivalent to bringing a virtual display closer. Note that the text still isn't crisp. If you could see the pixelation before, you can still see it. Moving the virtual display closer does nothing, because you aren't adding more resolution to the physical display.
Your analogy is backwards and doesn't apply.
But sticking to that same analogy, go get 1080p resolution image. View it on a 1080p display. You're seeing 1:1 (100%) of the detail within the image (all of it). Now zoom out, and again, and again. Detail has been lost. First a little, then a lot, and as you go too far back substantial amounts have been. This is because a 1080p image on a 1080p display can only display 100% of the detail when it is viewed at 100%. As the image is zoomed (or further away in the VR headset's case) it is not possible to continue to consume all of the details as the pixels don't physically exist to display it.
So solve this problem you need a higher resolution than the source material, and it gets higher and higher the more you zoom. If you viewed a 1080p image on a 8640p display, you could zoom out significantly with absolutely no loss of detail, because the pixel density is such that it can still render every dot within the image.
So in order for a VR headset to be able to render stuff far away with no loss of detail the resolution of the headset has to be much higher than the source video. The further away the video gets, the higher the resolution. Again, that is if you want absolutely no loss of detail (in the real world, some loss of detail is acceptable).
In order to play back a 1080p video on a 1080p VR headset, the video either needs to be right in front of the viewer (at 1:1), you'll have to accept loss of detail when moving it further away, or you'll need a higher resolution on the VR headset than the source video (e.g. 4x more pixels on the headset than the source video, when it only takes up 1/4th of the VR headset's display space).
If the virtual display-video is only 1cm squared on each of the VR headset's displays, then that 1cm needs to have the same pixels as 1080p to display 1080p video with no loss of detail. It is just physics. As you move things further away in virtual reality they get rendered by fewer pixels on the real-world VR headset displays, and therefore loss of detail is unavoidable. You can only counteract this with higher PPI than the source video (or just accepting the loss of detail).
> If it is one foot from you in virtual space, each pixel of video footage might be mapped 1:1 into each pixel of the VR headset's screen resolution.
If you can actually map pixels 1:1, you aren't in a virtual space anyway. Accomplishing this 1:1 mapping means locking the view into a single position. If you allow me to look around, then as soon as I turn my head, the angle skews and you're not mapping 1:1 any more.
> So now instead of 1:1 you might be 1:4, and you're seeing 25% of the video's original detail actually rendered on the VR headset's display.
If 1:1 looks good, then 1:4 rendered 4 times as far away looks exactly as good. If you can't see the pixels at 1:1 then you can't see the lost detail at 1:4. If you could see the lost detail at 1:4, it would mean the 1:1 image looks pixelated. Again, your physical display doesn't care about how far away something is rendered. It has a fixed resolution, and if it's too low to be convincing, it's too low to be convincing at any rendered distance.
Go play an N64 game (or find some screenshots). You'll see that everything in the foreground is pretty rough looking. Everything in the background is equally rough looking, because rendered distance doesn't change the display resolution.
> Now zoom out, and again, and again. Detail has been lost. First a little, then a lot, and as you go too far back substantial amounts have been.
This doesn't matter. If it looks good at 1:1, then it will look good at 1:4. If it doesn't look good at 1:4, it didn't look good at 1:1. The lost detail is irrelevant.
> In order to play back a 1080p video on a 1080p VR headset...
I'm pretty sure you just can't do this convincingly, no matter the "zoom" level. If you rendered the video taking the entire display, then it would unpleasantly fill your entire vision and it wouldn't be VR and it would look terrible anyway.
> that 1cm needs to have the same pixels as 1080p to display 1080p video with no loss of detail
You're stuck on the idea that you need 1:1 pixels. You don't. You need a display with sufficient resolution to be convincing to your eyes. Once you have that, you can render your virtual 1080p image at any distance that would work in the real world and not care about how pixels are mapping.
It really doesn't work that way. If your VR display is too low-resolution to convincingly render a virtual display 15 feet away, then it's too low-resolution to convincingly render the virtual display 15 inches away. Your physical display has the same resolution regardless of where the virtual screen is, and if it lacks sufficient resolution to render an object convincingly, it just cannot do it regardless of how distantly the object is rendered. To put it another way, your display has the ability to render a certain amount of detail, full stop. Where a virtual object is rendered is irrelevant.
Try this. Sit close to your monitor, close enough that text isn't crisp enough. Now zoom your display (Ctrl+'+' in most browsers), which is equivalent to bringing a virtual display closer. Note that the text still isn't crisp. If you could see the pixelation before, you can still see it. Moving the virtual display closer does nothing, because you aren't adding more resolution to the physical display.