HTC Vive vs Oculus Rift

7 May 2016

I've been using the Vive and the Rift for a while now and wanted to write down my thoughts on the differences between the two. As of right now, the Vive has motion controllers and the Rift does not. This is the largest difference but that should change once Oculus releases Touch in the second half of the year. Since I do not have access to the touch controllers, I can only compare the headsets and the tracking.

Resolution

Resolution is a very important aspect of an HMD. Both the Rift and the Vive have the same resolution. They both have two displays (one per eye) with a resolution of 1080x1200 per eye coming to a total of 2160x1200. Each of them runs at 90hz. While the resolutions are the same, they are used in different ways in each device. This all comes down to the actual screen sizes and the lenses.

The Rift screen is a couple millimeters smaller than the Vive screen. To me this makes a big difference. A different screen size means a different pixel density. The Rift screens have a slightly higher pixel density. This is important when you consider that the lenses of the HMD zoom in on the screen to make it fill up a significant portion of your field of view. The denser the pixels are, the less space there will be between them when blown up by the lenses. This space between the lenses is called the Screen Door Effect (SDE). SDE essentially makes everything look like you are seeing it through a screen door. This is not as much of a problem with the first generation headsets compared to early developer kits. SDE is there but it's not as easy to notice. It is though, just slightly better on the Rift.

The Rift ends up with a clearer display that makes it easier to read text in game. It is only just slightly better so it's not a dealbreaker with the Vive. If you have both and quickly A/B test them with the same material, you will be able to tell. Other than that, it's not really a problem.

Field of View

Field of View (FOV) is something that can be completely subjective and in the long run, doesn't really matter. I'm not going to list what each FOV is because I can't get a straight answer. There are blog posts out there about it and everyone seems to differ on how to measure it. Some people place a camera lens directly on the HMD lens and measure it that way. That is disingenuous because no one uses an HMD with their eye touching the lens. Having your eye at different distances changes the FOV. The rift has a set FOV because there is no eye distance adjustment. The Vive has a changing FOV because you can set the lens distance with the knobs on the side.

If you do not wear glasses, you can get the Vive FOV fairly big as you can get close to the lenses. I wear glasses so the distance from the lenses I have set gives me a very slightly larger FOV than the Rift. FOV is not everything though. A bigger FOV doesn't mean much if it is the same resolution. The resolution of the rift put into a smaller FOV than the Vive gives it a clarity advantage.

IPD adjustment

The lenses are a certain distance apart. This is to account for your Interpupillary Distance (IPD). Your IPD is just the distance between your pupils when you are focused to infinity. Everyone has a different IPD so there are adjustments in either headset. The Rift comes with an in headset tool you can access that helps set your IPD. It has a set of green lines on screen and you move the IPD slider back and forth until the lines focus for you. It also lists your IPD in mm on screen for reference. The Vive is not so easy. There is a knob to turn but no on screen guide. You need to measure your IPD yourself (with a ruler in the mirror) or grab it from your glasses prescription paper. All the Vive does is tell you what the IPD that matches up with the current knob setting.

The Rift and Vive also have different IPD ranges they support. The Rift goes a bit narrower and the Vive goes a bit wider. Unless you fall somewhere way outside the norm, either headset should be able to support your IPD.

God Rays

A big issue that affects both headsets is crepuscular rays, also known as god rays. They are most easily noticable on a dark screen with a white shape floating somewhere in the distance. As you move your head around, you will notice rays coming off the edges of the white shape. Due to the different types of lenses, they affect the Rift and Vive differently.

The Rift has hybrid fresnel lenses. The different elements of the fresnel lens blend together so there aren't any visible lines like a normal fresnel lens. This setup tends to let the god rays overpower what's on the screen because they create a big blur of light coming from the bright object. In a dark scene with bright objects, the god rays are pretty bad and very distracting. This is not normally a problem in a scene where most things are the same brightness.

The Vive has fresnel lenses. There are clear lines between each element of the lens. God rays in the Vive are only visible at the actual lines between the elements. If the lens is made up of a bunch of concentric circles, the god rays will light up sections of these circles. Another issue is that the Vive lenses seem to reproduce a mirrored image on the lens of whatever object is on screen. So if there is a big white triangle on screen, you will also see a ghostly, upside down version of the triangle that's bouncing off the lens.

Weight

The weight of the headset is an important consideration. If the headset is too heavy or if the weight distribution is wrong, it can ruin your immersion. In this matter, the Rift gets it right and the Vive gets it slightly wrong. The Rift is very light and balanced. When wearing it, the Rift is barely perceptible. It feels like a slight pressure all around the head but doesn't ruin immersion. Swinging your head around leaves the rift firmly in place and it never feels like it is going to come off. The Vive on the other hand is a bit heavy. It is not so heavy that it is terrible, but it is heavy enough that it constantly reminds you that you are wearing a headset. All of the weight of the vive is in the actual headset which means all of the weight is on the front of your head. Unless you have it properly adjusted, the Vive will move around slightly which doesn't give confidence that it will stay on at all times.

Fitment

Headset fitment is what keeps the headset on your head. Each device has three straps that affix with velcro to tighten the fit of the headset. For each device, you undo all three straps. Place the headset on your head and make sure the back of the strap cradles the back of your head. Then you tighten the top strap so the device is held up so it aligns with your eyes. Then you tighten the side straps at the same time make sure the headset is snug against your face.

This is easiest on the rift. Just follow the previous instructions and you're ready to go. Not so much with the Vive. The cable placement on the vive is questionable as the three cables sit right on top of the top adjustment strap. The Rift runs its single cable next to the left strap. When fitting the Vive, you have to pull the cables up to loosen them so you can access the top strap, adjust the strap and then pull the cables back down. Not the easiest thing to do.

The straps on the Rift are solid and curve up to go around your ears. The actual velcro bits stretch out to allow you to pull the straps out to put it on. The straps on the Vive are just elastic. When you have the back of the strap assembly positioned properly to cradle your head, the side straps end up sitting on top of your ears. For me, constant pressure on my ears leads to them hurting after a long period of time. The strap setup for the Vive is definitely one of its weakest aspects.

Audio

Each headset has gone with its own audio solution, each with its own pros and cons.

The Rift comes with attached headphones. The headphones are removable and have a proprietary connector. If you don't want to mess around with audio, this is the easiest solution of the two. The headphones snap outward to allow you to easily put the headset on and then you can snap them back in. They rotate, slide up and down and pivot so you can get them on your ears just right. They are on ear and not over the ear so external sounds leak in. Whether this is good or bad is up to you. Some people prefer not hearing anything side from whatever is in their game. Other people would like to be able to hear if something is happening in the room that needs their attention. If you want to use your own headphones with the Rift, you will have to remove the installed headphones and either attach some others via the connector (not a thing yet) or use your own headphones and run another cable back to your computer.

The Vive comes with detachable earbuds. There is an audio cable comes out the back of the headset that you can plug headphones into. You can use the included earbuds which are of questionable quality and have a short cable or you can use your own headphones and deal with the length of the cord in whatever way you can. I've opted to just live with the included earbuds.

Positional Tracking

Headset tracking seems to be a weird voodoo that people don't understand (or don't bother learning). The Rift does outside-in tracking and the Vive does inside-out tracking. I'll get into that in a second but it should be noted that both of these are the secondary tracking method for each device. Both the Rift and Vive use Inertial Measurement Units (IMUs) as the primary tracking method. These are very accurate over a short period of time but will start to drift over a long period of time. The secondary tracking method of each headset is used to correct IMU drift.

The rift uses outside-in tracking which means the device is covered in LEDs and there is a camera that reads these LEDs. Each LED sends out a coded ID over time to say which LED it is. The camera syncs up with the refresh of the LEDs and grabs LED IDs over a certain number of frames. Every LED has its own ID number so the camera only needs to see 3 LEDs and it will know which 3 they are and where they are on the headset to get an exact location and orientation of the entire headset. One issue that can pop up with this is when the headset is so far away that a movement of an LED is less than one pixel on the camera's sensor. So a significant movement of the headset does not translate to a movement on the camera. It doesn't seem like it is possible to get to this issue with the length cables are able to be at the moment. Another issue is the refresh rate of the camera. The camera has to read enough frames in order to get the IDs of the LEDs it can see. This means a siginificant portion of each second is spent waiting for the IDs to come through. This is the easiest of the two to set up. You simply place a camera (or two) on your desk, hit center and you're good to go.

The Vive uses inside-out tracking which means the device is covered in sensors and there are two base stations that shoot out infrared (IR) light. Each basestation (or Lighthouse as the standard calls it) has an array of IR LEDs and two motors spinning perpendicular to each other with IR lasers attached. The LED array flashes to tell the headset to expect the lasers. One the flash is over, one of the motors sweeps its laser followed by the other motor. This gives vertical as well as horizontal location. The headset does its calculations based on info from all its sensors and reports its location back to the computer. With this method, you have to do a detailed room setup. If you happen to bump one of the basestations, you have to rerun the room setup. This method is definitely more accurate but more of a pain to get set up.

Forward Facing Camera

The Vive has a camera on the front of the headset that allows you to see what is going on in the real world. This is probably something the Rift should have come with. It is very helpful when you need to look at something in your room or if you are done and need to know where things are to be able to take the headset off and put everything down.

Conclusion

Both headsets are very good. Like I said earlier, the Vive has the advantage of having motion controls which are very important for immersion. Using a gamepad is ok, but it's just a gamepad. Using motion controllers allows you to actually reach into the world and interact with things. Once Touch is released, I will be able to compare it with the vive controllers.

Comparing headset to headset though, I have to give it to the Rift but just barely. The Rift is just an overall more polished device. It is more comfortable to use and I think that is very important considering the specs are so similar with the two.

If you are able to get a headset right now and need to choose, Vive is the choice. Motion controls makes up for any other lacking feature. If you are getting a headset in the future, I think Rift will end up being the better device once Touch comes out. I can't guarantee anything as I haven't used Touch. If it is in parity with the vive controllers, the quality of the Rift headset will help it beat out the Vive.

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