Which HMD for flight simulator

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mechamania
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Which HMD for flight simulator

Post by mechamania »

Hi

We are developing a new kind of motion simulator for the entertainment market.
It has a 6DOF motion platform. Initially we planned to use a graphics system with 3 monitors.
But we think we could create an incredible immersive experience using a HMD.

I'm not sure we can find a HMD that is suitable for this.

The HMD should have the following features:
- High FOV (for good immersive experience).
- Quick put on/put off (a ride is only 15 minutes, so no time for difficult adjustments).
- Low weight and weight close to head (user can have a rough ride, and should not hurt his neck for instance).
- No big stuff behind the head (user is sitting in a race car like seat, back of head touches the seat)
- Effordable (There is no budget for a $10.000 HMD, the ride fee will get to high).
- Ruggedized (should have long life even when people do not threat it well)
And preferable:
- High resolution (people are used to high res gaming at home, a very low resolution in the ride would probably disapoint them).
- Minimal motion artifacts (for good immersive experience).

We also need good head tracking and low lag. But first we would like to select a suitable HMD.

What are my possibilities:

A: Professional HMD
A profesional HMD would be to expensive. Especially the high FOV types. I have not seen high FOV types below $10.000.

B: Consumer HMD
Consumer HMD like the Sony HMZ-T2 have very low FOV. Also they are not suitable for rugged conditions, although we could ruggedize them.

C: Oculus Rift
Seems a nice choice, but I'm concerned about its weight. Especially because there is relatively much weight far from the head.
It also has to be ruggedized.

D: Develop a new HMD
Would be a lot of work. Maybe an Oculus Clone using white screens and pico projectors? this to keep weight closer to the head.

E: Wait for new developments
I think waiting is never a good think to do. Life is to short for that.

Did I forget some important features?
Are there other solutions?
What is your experience in this field?

I hope you guys can help me make this simulator more immersive!
Ryuuken24
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Re: Which HMD for flight simulator

Post by Ryuuken24 »

Hey. I know of a company called Sensics, they produce very high quality HMDs. Just google search. Best of luck!
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Re: Which HMD for flight simulator

Post by FingerFlinger »

In fact, one of the Sensics guys has a blog. He has a series on HMD failure modes that you might useful, given that you're looking at the entertainment market.

Why HMDs Sometimes Break

EDIT: As you probably already know, Sensics products are going to blow that $10000 figure out of the water. But whatever platform you end up choosing, you'll need to so some serious work in order to ruggedize the case and cabling for commercial use.
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Re: Which HMD for flight simulator

Post by mechamania »

Yes, I'v seen the Sensics stuff. A high FOV will cost much more then $10.000.
Most are also not easy to put on. And heavy.

Unfortunately not suitable for my project.
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Re: Which HMD for flight simulator

Post by JustJoshin »

mechamania wrote:Hi

The HMD should have the following features:
- High FOV (for good immersive experience).
- Quick put on/put off (a ride is only 15 minutes, so no time for difficult adjustments).
- Low weight and weight close to head (user can have a rough ride, and should not hurt his neck for instance).
- No big stuff behind the head (user is sitting in a race car like seat, back of head touches the seat)
- Effordable (There is no budget for a $10.000 HMD, the ride fee will get to high).
- Ruggedized (should have long life even when people do not threat it well)
And preferable:
- High resolution (people are used to high res gaming at home, a very low resolution in the ride would probably disapoint them).
- Minimal motion artifacts (for good immersive experience).

We also need good head tracking and low lag. But first we would like to select a suitable HMD.
I think it's reasonable in this case to think about *why* the oculus rift exists? Palmer didn't set out to create a company and bring VR to the masses, at least not initially. He created the oculus rift because there just wasn't anything else out there on the market that was affordable and provided the level of immersion and experiences he wanted. Given this, you probably know that you aren't going to find something off the shelf within your budget that ticks all those boxes. If it did, you wouldn't have half the oculus rift forum wanting to mod their dev kits with a higher resolution panel :lol:

The rift does a very good job at providing a high FOV, low latency and responsive head tracking solution and when combined, creates an very immersive experience despite the low resolution of the dev kit. The dev kit though is just that...a dev kit. It wasn't / isn't intended for the consumer market let alone commercial market. It does have the advantage of a low price point though so starting from that you may be able to ruggedize it as you say. I don't think the weight of it would be too much of a concern, I can happily play half life 2 for a couple of hours. If you are only talking about 15 minute intervals than that should be fine. It is easy to slip on over the head but the real question I am wondering is...

You want something that is quick to put on and take off and get the next person through. You really need to calibrate the IPD for each person using it to ensure they have the best experience they can to prevent them from getting sick. Additionally, the dev kit takes a bit of playing around with the eye cups to work out what works best depending upon your level of short/near sigtedness. Regardless of the hardware, unless you have 20/20 vision I would imagine the calibration side of things could be a bit of an issue.

Lastly, as I alluded to earlier, if you read around the forums there are various people looking into the possibility of modding their dev kit with a higher resolution panel so this may also be an option. Considering the resolution is essentially halved per eye though even with a panel upgrade your resolution will still be lower than that of a comparable monitor.

What you have described is what we are all waiting for though :D It will just be a bit more of a wait.


Edit: If you just can't get around some of the various HMD problems maybe it would be worth going back to the idea of a large multi-monitor system in combination with something like TrackIR?
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Re: Which HMD for flight simulator

Post by mechamania »

The quick putting on/off bothers me too.
What things need to be adjusted when putting it on?
On the Oculus you can not adjust the pupil distance, can you? Do you have to adjust focus?

Weight, or actually weight distribution is an issue. If the HMD is not good balanced I might get problems to get it
certified for its use. People my hurt them self when it is not balanced. Note that they can have a rough flight in the
simulator.
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Re: Which HMD for flight simulator

Post by android78 »

I think that you should investigate printing a custom enclosure for a rift. The weight distribution of the rift isn't much of a problem because it is essentially distributed across a large part of your face.
For the entertainment market, where users are likely to be a lot more rough then home users, you would want something a lot more rugged and easier to put on/take off. Printing a custom enclosure, I think that a solid plastic support for top/sides of the head would be the way to go, and adding a flat counterweight at the back (I know you said no big stuff behind the head, but race drivers all wear helmets, right?) since you will need more plastic (hence weight) on the front to keep it nice and rugged.
While the product failed, I think that the enclosure for the Trimersion (http://www.youtube.com/watch?v=4yqVChaRBsQ) is similar to what you would want, but with weight at the back and slightly more headroom and sprung padding to allow for all head sizes. Alternatively, to make it cooler, you could make it like a fighter pilot helmet (http://www.youtube.com/watch?v=RRGSTECgW_w).

EDIT: regarding the Sony HMZ,; my friend recently purchased the HMZ-T2 and gave me a try, and can confirm that without any modification this is totally unsuitable. The design is a lot more front heavy then the rift and because of the single support point at the front, it makes it very difficult to keep in the right place. If you are creating a new enclosure, then you could make it work, but you can't just use it as is.
The display in the HMZ-T2 is amazingly crisp and clear though and it's really nice having blacks that look black. It is much better for movie viewing then I think it ever could be for gaming (or anything that requires even the smallest amount of head movement).
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Re: Which HMD for flight simulator

Post by mechamania »

Thanks
Yes, I agree that a helmet type of HMD would be best. I will give it a try and design my own.

I have not received the Oculus yet, but what kind of adjustment would be needed for the helmet with Oculus?
Off course it should be adjustable to different form and sizes of heads.
But what kind of adjustments are needed?
- left/right movement (of the Oculus screen)?
- forward/backward movement (of the Oculus screen)? For focusing?
- up/down movement (of the Oculus screen)?
- Lens positions for pupil distance?
- Others?
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Re: Which HMD for flight simulator

Post by JustJoshin »

With regards to adjustments per user I was referring to the fact that the oculus comes with three different lenses (A, B and C respectively) and the lense most appropriate to each user will depend on their level of nearsightedness. Add to this that IPD adjustment is done via software calibration. Not really something you are going to want to have to put users through.
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Re: Which HMD for flight simulator

Post by android78 »

There are some tricky trade-offs with regards to adjustments. I would err on the side of less adjustments, even if it means that the experience won't be optimal.
The points to think about are:
FOCUS - The rift has the three eye-cups to accommodate for those who wear glasses. I would not consider this for your product and would stick to the standard lens. Being a glasses wearer myself, I find it a lot more comfortable for my eyes to use my prescription and just have the rifts lenses further away. Given that the standard lenses focus at a far point, the distance from the lens doesn't make a lot of difference to the focus, only FOV.

FOV - Following on from the above, assuming that you are creating a custom shell, you could allow space so that a users glasses can fit. The problem with this is that the further the rift lenses are from your eyes, the lower the FOV (and less of the screen you will see), so you'll have to decide if you want to limit your potential customers, or have max FOV. You could always use larger custom lenses if you wanted to get adventurous.

POSITION, optical centres, etc - The Rift optics are quite high and viewing through the the edge of the optics is not a very comfortable experience. That being said, I think that having IPD adjustments is likely to cause more headaches then it's worth, which is probably why the rift doesn't have any. What you do need is some way to make sure the position is reasonably well maintained, so fixing it horizontally, but with a ratchet mechanism for a small vertical adjustment within a helmet design would be my preference. You should be able to get a few degrees of adjustment left/right for a helmet design anyway, so I would avoid making it more complicated then is needed by adding left right movement within the shell.
Thinking, once again, of those with glasses, if you could have the video part as a snap down visor this would allow people to put it on, then put their glasses on before completing enclosure (like what's needed for putting on motorbike helmet).

Other then this, the resolution of the rift developer kit is probably not high enough to create a huge following, so may be worth either creating your own high-res version or consider using the current rift for prototype and wait for the consumer version to be released before finalizing and launching your product.
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Re: Which HMD for flight simulator

Post by mechamania »

android78 wrote:You could always use larger custom lenses if you wanted to get adventurous.
Do you mean to compensate for different IPD?
And why is this adventurous?
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Re: Which HMD for flight simulator

Post by mechamania »

I'm thinking of designing a special helmet on which a HMD (like Oculus) can be clicked.
People could prepare for the ride by putting on their helmet and clicking a dummy HMD to it (with some fixed pictures) and do the adjustments.
Then, when the ride starts, they click the real HMD on the helmet and everything should be ok.

The question is how can we make the helmet such that it will stay in place stably enough. So that the dummy can be clicked of and the real HMD can
be clicked on whitout moving the helmet. Also the helmet should not move during the ride. I'm affraid the helmet will always move (a little).

And what adjustment are needed. I understand from previous post that a horizontal and vertical adjust are needed.
IPD adjust not really needed, but this would improve picture quality. Right?

About focus: I prefer when people wearing their own glasses. But smaller FOV would be a big disadvantage. Could I compensate for this using bigger lenses?
I could have a few special HMD units with different lenses for people who want to try. And if this does not work, they can use the standard but with smaller
FOV. But I do not really like this solution.
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Re: Which HMD for flight simulator

Post by brantlew »

I don't think there is really even a competition here. If your goal is inexpensive immersion, the Rift is by far your best option. You sound like you have never tried a Rift, so I think you will be very surprised how much the immersive feeling outweighs the need for high resolution displays. But higher res displays are coming relatively soon anyways. So with the consumer Rift specs it will probably best even the most expensive HMD's.
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Re: Which HMD for flight simulator

Post by mechamania »

brantlew wrote:You sound like you have never tried a Rift.
I orderred a dev kit. But it still did not arrive. I can't wait trying it!

I'm a little concerned about the motion artifacts like motion blur and judder.
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Re: Which HMD for flight simulator

Post by xa4 »

You mentioned a 6DOF motion platform ... I'm thinking about a similar setup, but don't know yet what's the best solution for handling undesired platform movements captured by the Rift tracker. For now I see two solutions : using position feedback from the platform or use a second tracker attached to the platform to recalculate the correct head orientation (with as less latency as possible)
What are your thoughts on this ?
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Re: Which HMD for flight simulator

Post by mechamania »

xa4 wrote:You mentioned a 6DOF motion platform ... I'm thinking about a similar setup, but don't know yet what's the best solution for handling undesired platform movements captured by the Rift tracker. For now I see two solutions : using position feedback from the platform or use a second tracker attached to the platform to recalculate the correct head orientation (with as less latency as possible)
What are your thoughts on this ?
I want to use some positional tracker. So that I have the position relative to the cabin. Using a second tracker on the cabin and calculate the relative position seems possible. But I'm afraid this will lead to less accuracy.

Are you going to adapt the Rift for your needs? If so, what are you going to change?
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Re: Which HMD for flight simulator

Post by android78 »

@mechamania - have you tried contacting Hannibalj2? He's doing an amazing job with creating an ultra-wide HMD:
http://www.mtbs3d.com/phpBB/viewtopic.php?f=26&t=18335

This looks like it would be appropriate to slot into a helmet design.

Regarding the tracking. You'll have to be a bit careful with this. I'm not sure what flight sim program you are using, or if it is a custom one, so it's hard to know the issues you might face. One thing they have in the rift development kit is a kind of fudged position adjustment based on the rotation data that kind of simulates the moving forward of the head when looking down (pivot is the neck, hence below the eyes), so this probably won't be of any use since the motion of the platform will cause havoc. Inertial sensors would be problematic in general, since you'll have to subtract the platform motion from the sensor data, likely to cause a lot of drift.
You're probably better to use some fixed high-speed cameras within the platform with a track-ir type of setup. Depending on your budget, something like http://www.naturalpoint.com/optitrack/p ... depth.html would be good (250fps would be great!), but there is always wiimote hacking as an option if you're trying to do things on the cheap. I've read that wiimote is 100fps, but there is the problem of bluetooth speed.
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Re: Which HMD for flight simulator

Post by xa4 »

@mechamania
I haven't really thought about modding the rift. I don't have my rift yet but I've tried it once during a presentation meetup and I don't see any real problems when it's going to be used by a lot of people. Maybe there are some specific regulations for using a HMD in the entertainment market for which the (consumer) Rift will need to be adjusted for and maybe you could say there are some hygienic issues, but I think you should wait until the consumer version will be presented and see what's possible.

The consumer version will also have positional tracking. Are you thinking about using positional tracking only for position and orientation, whitout the IMU/MARG sensor ? In other words deactivating the IMU/MARG sensor in the rift and only use a positional tracking system with a reference/base station attached to the motion platform to get relative position versus the platform and also head orientation ? I don't know what kind of positional tracking system the consumer rift will have, but it's possible it will work that way.

@android78
The Optitrac system is interesting, but not cheap, although a 6DOF platform isn't cheap either. A 6DOF platform can be build for around $6000-$10000 (DIY) which means a system like Optitrac would be a 20-30% premium. I still think a second IMU/MARG sensor is a better approach. With the right filters and todays hardware you should be able to eliminate drift with little latency. Or maybe the rift's implementation of positional tracking will do the job ?
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Re: Which HMD for flight simulator

Post by android78 »

xa4 wrote: I still think a second IMU/MARG sensor is a better approach. With the right filters and todays hardware you should be able to eliminate drift with little latency. Or maybe the rift's implementation of positional tracking will do the job ?
You could be right, however, I have a feeling that to get the absolute position as well as rotation, you're going to have a tough time with two IMUs. It's hard enough to get a position estimation when the user in in a static environment based on IMU/MARG and adding constant shifting reference with a motion platform will cause constant drift that will need to be corrected somehow. This is why I think that optical tracking is probably the way to go. Even the trackir has 120FPS, so the latency shouldn't be that much worse then the rift tracker, and gives you absolute position.
Given that there is a fixed structure for the motion platform anyway, mounting the camera would be easy, and having a few IR diodes on the HMD or helmet shouldn't be too much of a problem.
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Re: Which HMD for flight simulator

Post by FingerFlinger »

I have to agree with android78 on this one. An optical tracking system is really the simplest solution. With the optical system, you can just directly measure the HMD pose. With just IMU/MARGs for position, you need to solve an open problem in sensor fusion, in addition to non-trivial error mitigation.
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Re: Which HMD for flight simulator

Post by xa4 »

Ok, an optical system is definitely more straightforward. I'm just worried about the price for a system like Optitrack.

But the idea was not to use IMU/MARG sensors for position, but to find a solution for the orientation movements the build-in Rift tracker will pick up due to platform movements. I'm looking for a cheap and low latency solution and will look into the possibilities with trackir.

Is there somebody who has tried a race/flight sim with the Rift on an 6DOF platform yet ?
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Re: Which HMD for flight simulator

Post by mechamania »

xa4 wrote: Is there somebody who has tried a race/flight sim with the Rift on an 6DOF platform yet ?
It may be interesting to start a new topic about this.
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Re: Which HMD for flight simulator

Post by mechamania »

Finally got my Oculus Rift Dev kit! So I had my first experience with it.
It is really impressive how immersive this is. Even with the low resolution.

But (there is always a but) the low resolution and screen door effect bothers me.
Because of the screen door effect I have the tendency to focus on it. This destroys the 3D effect.
Also when moving hour head, the complete picture get blurred but the screen door effect is (off course) sharply visible. This makes me
focusing on the screen door again.
The low resolution also results in flickering pixels which are very visible.

Adjusting the HMD was surprisingly easy. Adjusting the IPD seems to work good.
The HMD is not very heavy. But when quickly moving your head, the HMD moves a lot relative to our head. So probably not suitable for
inside a 6 dof cabin. But I think this is very difficult to prevent because of the weight distribution.
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Re: Which HMD for flight simulator

Post by xa4 »

mechamania wrote:
xa4 wrote: Is there somebody who has tried a race/flight sim with the Rift on an 6DOF platform yet ?
It may be interesting to start a new topic about this.
I will start a new topic about 6DOF motion platforms in "Physical VR Tools"
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Re: Which HMD for flight simulator

Post by FingerFlinger »

xa4 wrote:Ok, an optical system is definitely more straightforward. I'm just worried about the price for a system like Optitrack.

But the idea was not to use IMU/MARG sensors for position, but to find a solution for the orientation movements the build-in Rift tracker will pick up due to platform movements. I'm looking for a cheap and low latency solution and will look into the possibilities with trackir.

Is there somebody who has tried a race/flight sim with the Rift on an 6DOF platform yet ?
Oh, I misunderstood, my apologies. I think that this is an achievable goal, and if combined with an optical system, could produce good results. The orientation latency from an IMU will likely be much lower than that of an optical system.
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Re: Which HMD for flight simulator

Post by FingerFlinger »

By the way, have you considered a physical tracking system? If the user will be fully enclosed in the sim, you might be able to hang a couple of retractable wires from the ceiling and measure the HMD's distance to each with rotary encoders.

Dead simple and cheap, but I'm not sure about robustness to mechanical failures.

EDIT: And extremely low-latency
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Re: Which HMD for flight simulator

Post by xa4 »

@fingerflinger
Hehe, I was already convinced the idea of using a second IMU/MARG sensor would be a bad idea. Now you say IMU/MARG sensors will have much less latency then an optical tracking system ... is it correct to say there is a one on one relation between FPS (optical) and Hz (IMU/MARG) ? How do you compare latency due to quaternion calculations with filters (not to mention the drift problem) with IMU/MARG sensors and latency due to calculations (I presume no quaternion calculations, but what kind of calculations ? also drift ?) with optical systems ?

Thx for the suggestion of retractable wires and rotary encoders. Could you point me to a site for this, I'm not sure what you mean.

By the way, I started a new topic about this subject in "Physical VR Tools"
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Re: Which HMD for flight simulator

Post by FingerFlinger »

There is roughly a 1:1 relation between camera frame-rate and IMU polling frequency. There's calculation overhead for both, but it's highly variable, depending on your approach. The YEI sensor, for example, measures 250 samples per second, and I believe that includes some on-board DSP to fuse the sensors' data. The PS3 Eye camera can record 180 frames per second, with significant additional latency from the computation time. You can buy faster cameras, but that begets more data to process.

A rotary encoder is vaguely like a potentiometer; it will output a distinct value for each angular position. So imagine having a wire coiled around the post of the pot. Also imagine that there is a spring that will allow the wire to unwind, but will automatically retract the wire so that it recoils around the pot. If you pull on the wire, it will unwind and rotate the pot, changing the output value. If you let go of the wire, it will retract, and the pot's output will return to it's original value. So, with this setup, you can measure the distance of the end of the wire to the rotary encoder. With 3 such devices attached to the HMD, you can resolve the position with simple trigonometry.

I don't really know of any useful websites, but if you search for rotary encoders, I think you will probably figure out what I mean.
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Re: Which HMD for flight simulator

Post by xa4 »

ok, thx man, I will look into it...
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Re: Which HMD for flight simulator

Post by Fredz »

Maybe something like Gametrak :
http://en.wikipedia.org/wiki/Gametrak

http://www.youtube.com/watch?v=nObCJFLvqrQ

It can be found for quite cheap on Amazon or ebay and some versions of it can be used on PC.

There is a thread about it here : http://www.mtbs3d.com/phpBB/viewtopic.php?f=138&t=15717

As for the PS Eye camera, the total end-to-end latency has been measured at 68 ms (±3 ms) by the author of the PS Move API for tracking the glowing orb of the PS Move : http://thp.io/2012/thesis/
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Re: Which HMD for flight simulator

Post by FingerFlinger »

Wow, 68ms! I guess that a significant portion of that must be due to the Bluetooth latency.
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Re: Which HMD for flight simulator

Post by Fredz »

I think it's only the latency of the camera, the PS Move is only used for its glowing orb in this case, ie. a simple independent LED. I don't know if the latency of the PS Move sensors has been estimated.
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Re: Which HMD for flight simulator

Post by FingerFlinger »

The link that you posted specifically talks about sensor fusion, so the IMU values must be communicated via Bluetooth... It wouldn't be 6dof otherwise.

Furthermore, the camera runs at 60Hz nominally, with one frame of latency, for 16.6ms. The vision processing will certainly have some overhead, but it's just blob tracking with sub-pixel edge detection of the orb. I think it would be pretty horrendous for that to take 58ms on a Core 2, which is the test platform that he describes.
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Re: Which HMD for flight simulator

Post by Fredz »

FingerFlinger wrote:The link that you posted specifically talks about sensor fusion, so the IMU values must be communicated via Bluetooth... It wouldn't be 6dof otherwise.
The thesis is about sensor fusion but in the link I gave the author spoke specifically about the end-to-end latency of the PS Eye camera :

"The total end-to-end system latency is 68 ms (±3 ms) on a 1.40 GHz Linux host with the PS Eye camera."
FingerFlinger wrote:Furthermore, the camera runs at 60Hz nominally, with one frame of latency, for 16.6ms. The vision processing will certainly have some overhead, but it's just blob tracking with sub-pixel edge detection of the orb. I think it would be pretty horrendous for that to take 58ms on a Core 2, which is the test platform that he describes.
The thesis gives more details about the measurements, the camera runs at 50 FPS under Linux and switching the controller off and grabbing the image takes 63 ms (± 4 ms) on Linux.

So I guess the added latency from the algorithm is roughly only 5ms (68 - 63). The camera latency can't be induced from its capture frequency, there may be additional processing taking place in the camera itself, in the USB communication, and in the camera driver on the CPU before a frame is available to the end-user.

That's also the case for other sensors, like the Ascension Flock of Birds for example which despite having a 144 Hz update rate has a 22.61 ms end-to-end latency. See : http://www.cs.unc.edu/techreports/93-033.pdf
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Re: Which HMD for flight simulator

Post by FingerFlinger »

The Omnivision datasheet specifies a single frame of latency, which is why I calculated it that way.

Huh, just read the relevant part of the thesis, and you're definitely correct about 68ms (for frame capture). I think his wording could have been more clear.

I should take some better measurements of my setup then, because I have significantly less than 60ms of latency after capturing from 2 cameras in parallel, doing optical flow on both and rendering the result through the Source engine. It's more like sub-40ms, and is dominated by computation time. His latency has got to be due to the drivers.
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Re: Which HMD for flight simulator

Post by Fredz »

Good to know you could attain 40 ms latency, are you using Windows with the CL Eye driver ?
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Re: Which HMD for flight simulator

Post by FingerFlinger »

Yep.
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Re: Which HMD for flight simulator

Post by KingZadeek »

@machamania - you might be interested in these 98" 3D Lightweight HMDs https://www.youtube.com/watch?v=HchwB4Dts9o
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Re: Which HMD for flight simulator

Post by PalmerTech »

KingZadeek wrote:@machamania - you might be interested in these 98" 3D Lightweight HMDs https://www.youtube.com/watch?v=HchwB4Dts9o
Oh man, you again?

http://www.mtbs3d.com/phpBB/viewtopic.p ... it=martian
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Re: Which HMD for flight simulator

Post by MSat »

I don't know how detailed the cockpit is expected to be, but if the user's hands are expected to move away from the throttle and stick, then immersion is going to break unless you can track those too.

You might be better off from a usability standpoint to stick with a multi-monitor or projection onto a curved surface for all elements outside the cockpit. Since the distances in flight sims tend to be quite large you can also forgo stereo projection and head tracking (though you can still do that too with an optical tracking system and a pair of shutter glasses).
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