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Almalence Digital Lens Improves VR Image Quality Even on Limited Resolution Displays

Almalence Digital Lens Improves VR Image Quality Even on Limited Resolution Displays

Almalence Digital Lens for VR (DLVR) is a dynamic optical aberration correction algorithm designed to compensate for the optical hardware limitations inherent in all lens types used in VR and AR displays. Such limitations result in optical smear, chromatic aberrations, pupil swim, and other effects harming the visual experience in VR/AR. The resolution of projectors used in most current head-mounted displays already exceeds the lens system's resolving power (or MTF). The optics have become the bottleneck limiting the details that can be delivered to the user's eyes. Some suggest using a sharpening algorithm to visually compensate for the lost resolution like Meta did with the Quest Pro; however, not only does sharpening not actually add any details, but it can also produce ugly artifacts. Almalence DLVR is the ideal solution to the problem of delivering higher image quality and more detail to the eye than the optics alone can provide. This article explains our recent discovery that even devices equipped with lower resolution displays, like that used in the Meta Quest Pro, also benefit from the DLVR compensations to deliver a clearer, sharper image, although subtler than when the display is of higher resolution.

The Meta Quest Pro

As VR display designs generationally progress along an improvement trajectory, the pancake lens optical design (also called folded or polarized catadioptric optics) is becoming the optics of choice.   Such a design allows HMDs to be more compact with more uniform sharpness across the entire field of view and eliminates the need for users to rotate their heads awkwardly to see the details out of the center of the picture. God's rays are also gone. 

In its Quest Pro design, Meta also chose to take advantage of the pancake lens but, unfortunately, used a low-resolution display (1800x1920 pixels per eye); thus, the MTF gains from the lens are muted. However, even with the display resolution falling behind the optics capability, optical aberrations correction technology, DLVR, improves overall image quality. The improvement is not as dramatic as it is for higher-resolution displays (see, for example, tests of Varjo <link> and HP G2 <link>); however, it's visible and cannot be achieved with other image pre-processing methods.

Measurement Results

Here you can see the difference DLVR brings with this particular choice of optical design and display resolution:

Typical example scene (from the 'Lab' game). The circle shows the fovea area.
QP-01-asis.jpg, QP-01-DL.jpg

Link Sharpening

In its tethered mode, when connected to a PC, the Quest Pro has built-in 'Link Sharpening,' a feature meant to visually compensate the optics deficiencies by delivering sharper images and enabled by default.

It produces a result somewhat similar to the above, although with pronounced color fringes:

QP-01-LS.jpg, QP-01-DL.jpg

With the high-quality optics with the MTF high enough not to limit the quality of the image projected by the low display resolution, it can be argued that simple sharpening works quite well most of the time. However, upon further investigation, issues do crop up. Let's take a look at the limits and flaws of Link Sharpening and compare it to optical aberrations correction technology, DLVR.

When a relatively low-resolution display is used, over-sharpening can be tolerated as it is masked by other imperfections, e.g., the screen-door effect. But there are cases where this over-sharpening is really objectionable, like human skin texture, as seen below:

Left: Original image. Right: As it looks in Quest Pro, texture loss.
Left: Quest Pro with Link Sharpening, texture corrupted.
Right: with Almalence Digital Lens, texture preserved.

The issues caused by sharpening can be clearly demonstrated by using a fine checkerboard and line pattern. This pattern provides the highest-frequency high-contrast content.

Original checkerboard patch

The fine structure of the pattern is not fully resolvable by the display due to texture re-sampling used to correct lens geometry distortion. This creates high-frequency peaks and valleys of light intensity when such a pattern is displayed in an HMD. If sharpening is applied to the checkerboard patch pattern, it becomes sprinkled with light intensity spike artifacts.

QP-patch-asis.png, QP-patch-LS.png

This means similar artifacts will also appear on any strong slanted edge or high-contrast fine textures in regular scenes. For comparison, the Digital Lens produces no artifacts:

QP-patch-LS.png, QP-patch-DL.png

So, why can't simple edge enhancement techniques like scene pre-sharpening be adjusted to avoid these artifacts? Many types of optical aberrations cause smear and blur (spherical aberration, coma, and astigmatism, to name a few). Sharpening techniques are not designed to match the optical aberrations in a precise way. Stronger sharpening does not do a better job; on the contrary, it results in overshoot and ringing.

And, of course, as optical aberrations depend on the eye position, a technique utilizing eye-tracking (such as DLVR) is required to eliminate them.

Lateral Chromatic Aberration example:

Full chart, the circle shows the fovea area.
QP-lcachart-asis.png, QP-lcachart-LS.png

Compare to the dynamic aberration correction by DLVR:

QP-lcachart-asis.png, QP-lcachart-DL.png

Conclusion

Even for a combination of a low-resolution projector and high-performance optics, a dynamic aberration correction technique can improve visual image quality. On the contrary, sharpening techniques result in artifacts and visually corrupted picture. The example images provided throughout this article were captured at the ideal position of the eye centered at the optical axis and the eye pupil following the gaze direction. The effects get even more pronounced as the eye moves further off the alignment sweet spot. Sharpening cannot possibly negate stronger, asymmetric, and wavelength-dependent optical smear, which appears in this case.

Furthermore, the capability of sharpening techniques to improve image quality in higher-resolution displays will be prohibitively low compared to dynamic optical aberration correction techniques such as Almalence DLVR, which address the crucial limiting factor.

Additional information: the images in this article were captured with the 87-ppd camera within Almalence Human Eye Simulator[link]...