What Do We Need to Know About HMD?

HMD: its visual features and problems.

In this article, we decided to touch upon an integral part of the characteristics and problems, associated with the visual component of various HMDs.

We will reveal the basic concepts that will help expand the knowledge for those who want to purchase a headset, but do not know how to properly navigate through its specifications.

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What is FoV?

FoV stands for the field of view. This parameter indicates the width of the field of our peripheral vision. It is known that humans have a slightly more than 210-degree forward-facing horizontal arc of the visual field.

In VR, the field of view is one of the key ingredients that influence the level of immersion. There are two types of FoV that form human vision.

Pimax 8K	Odyssey	Acer WMR	HTC Pro	HTC Vive	Oculus Rift	Oculus Go	PSVR
200° diagonal 120° vertical	110°	100°	110°	110° diagonal ~110° vertical	110° diagonal ~90° horizontal ~90° vertical	110°	100°	FoV

What is an IPD and how to measure it? PPI and PPD

Interpupillary distance (IPD) is the actual distance between the centers of the eyes’ pupils. It is measured in millimeters.

On the average, the range of IPD in the vast majority of adults is 51-77 mm, and the minimum IPD in children is 40 mm.

If you want to have a clear picture that doesn’t cause discomfort, you should fine-tune the IPD in VR HMD.

If your IPD is significantly above or below the average, then it is obligatory to test the HMDs that do not provide customization options before purchasing.

Interpupillary distance:

When you look into your own pupils, it straightens your eyes to converge to infinity, and this forms perpendicular lines.
Since the lines are perpendicular, they cross the ruler in the same interval as the distance between the pupils.
As a result of this measurement, you see the distance between your pupils.

Odyssey	Acer WMR	HTC Pro	HTC Vive	Oculus Rift	Oculus Go	PSVR
Yes	Software IPD	Yes	IPD (60.8-74.6 mm), lens-to-eye distance ("eye-relief" adjustment)	IPD (58-72 mm), lens-to-eye distance (adjustable with optional glasses spacer)	Not	Software IPD, lens-to-eye distance	Adjustable for IPD

Pixels per inch, PPI

where

d_p – diagonal resolution in pixels

w_p – width resolution in pixels

h_p – height resolution in pixels

d_i – diagonal size in inches (this is the number advertised as the size of the display)

! A smaller number of PPI means larger SDE.

Pixels per degree, PPD

where

d – the viewing distance

r – the display resolution in pixels per unit length.

! PPD: Diagonal FoV the Oculus Rift and HTC Vive have different stereo overlap %'s. Oculus Rift per eye FoV is ~79v x ~89h while HTC Vive is ~110v x ~100h have different shape due to lens design.

Reddit: Also the lens design of the Oculus Rift puts more pixels in the center of the FoV (higher PPD) whereas the Vive has a more uniform distribution across the FoV.

Pimax 8K	Odyssey	Acer WMR	HTC Pro	HTC Vive	Oculus Rift
14	14	14	14	11	13	PPD (around)
530	615	705	615	450	450	PPI (around)

! Vive/Rift has a PPI around 450 while the PSVR has a PPI of 386. Despite this difference, the SDE on the PSVR is much less noticeable than on both PC VR HMDs. This is due to the fact that PSVR has a different subpixel arrangement in the display (RGB stripe in PSVR with potentially a diffuser, PenTile display in Vive/Rift).

What can eye-tracking technology do?

It makes a decent Depth of Field (DOF) effect to reduce VR motion sickness because one of the reasons for VR motion sickness is that the whole image is in focus, whereas in real life, we have a very small area in focus.
It performs selective rendering (optimization of the system performance). Eye-tracking can have a huge impact on performance with the help of a method known as “foveated rendering.”
It improves the quality of social interaction.
It studies the human attention and consumer market.

! 7invensun, one of the latest companies accepted into HTC’s Vive X accelerator program, is launching an eye-tracking add-on made especially for the Vive headset.

Problems: SDE, Mura, and Aliasing. An off-axis optical system

The screen door effect (SDE) means that there is the space between each individual pixel (or subpixels) in the form of fine black lines.

! This effect actually isn’t related to resolution, or even to pixel density. It is related to the pixel fill factor.

! The lens magnification exacerbates SDE.

The Mura effect is connected with the unevenness, caused by poor color and brightness consistency from one pixel to another. Basically, you see a grainy layer on top of the image.

! The MURA effect is especially noticeable in dark scenes.

Aliasing is a gradation of inclined lines of the image. For eliminating this problem, different smoothing algorithms are used.

! Anti-Aliasing Algorithms: FXAA, SMAA, MSAA, SSAA, TXAA

An off-axis optical system is an optical system in which the optical axis of the aperture is not coincident with the mechanical center of the aperture.

! When a user’s IPD substantially differs from the lens inter-axial distance (IAD), looking 'straight ahead' in their visual field produces an image off-axis from the lens.

The focal length is the distance between the eyes and the display. Some headsets feature the function of the focal distance adjustment, which is especially important for the visually impaired if it is impossible to wear glasses under a headset.

Chromatic aberration

Chromatic aberration is an issue that arises from the inability of a lens to focus all colors in the same place. This happens because the focal length depends on a property called refraction, which is the bending of a wave when it enters a medium where its speed is different. Since blue and red light have different indexes of refraction, their focal length is also slightly different.

Reddit: Chromatic Aberration will occur any time you look away from the center axis of the lens or any time the eye is not aligned to the center axis of the lens. The further off-axis, larger the relative color shift.

Sweet spot

Sweet spot (SS) is the point where the lenses are at their sharpest.

Acer WMR	HTC	HTC Pro	Oculus Rift	Oculus Go	Odyssey	PSVR
Small	Small	Small	Large	Large	Medium	Large	SS

Subpixels

The screen resolution is usually quoted as width × height, and it is measured in pixels. However, in the layout and color method, the subpixels in one pixel may differ, that affects the image quality.

Traditional RGB: Sony PSVR, Pimax 5k (stripe arrangement), Pimax 8k (diamond arrangement).

PenTile RGBG: HTC, HTC Vive Pro, Oculus Rift, Samsung Odyssey.

Acer WMR	Pimax 8k	HTC	HTC Pro	Oculus Rift	Oculus Go	Odyssey	PSVR
RGB	RGB	RGBG	RGBG	RGBG	RGB	RGBG	RGB	Subpixel
1440×1440	1920×2160	1200×1080	1440×1600	1200×1080	1280×1440	1440×1600	960×1080	Resolution (per eye)

Conclusion

The understanding of the visual characteristics of the HMD allows protecting the beginner from the rash choice of the device. Also, you can use VR comparison engine according to this list of VR HMDs variety. In this article, we have learned the following:

What FoV is, which indicators have monocular (114◦) and binocular (200◦-220◦) FoV in humans. Acer WMR has the lowest FoV = 95◦, whereas in Pimax 8K FoV reaches 200◦. Such HMDs as Odyssey, HTC, Oculus have FoV = 110◦.
What an IPD is, how to measure it, and the PPI and PPD formulas. Note that if your IPD is significantly above or below the average (from 51 to 77 mm in adults), then it is a must to test the HMDs that do not provide customization options before purchasing. Such HMDs as Acer WMR and Oculus Go are not adjustable for IPD.
What the eye-tracking technology is needed for. Visually impaired people can use the eye-tracking device with built-in lenses with the right diopters that will correct the vision (aGlass by 7invensun for HTC).
The problems of the visual nature the HMD user can encounter: SDE, Mura and Aliasing, an off-axis optical system.
What the focal length is and what a sweet spot is; HTC and Acer WMR have a small SS, then Oculus has a big SS.
What the chromatic aberration is.
The differences between the sub-pixel matrices in different HMDs (RGBG and RGB subpixel matrix).