Google and LG’s recent tease of a breakthrough 1443-pixel per inch (PPI) screen for virtual reality headsets raised plenty of questions, and a new research paper (spotted by Road to VR) answers many of them ahead of the screen’s debut today at Display Week. The upshot: Incredibly high-resolution screens are coming, but they will depend on software tricks to replace pixelization with less objectionable blurring.
According to Google and LG, the new 4.3-inch OLED display boasts an 18-megapixel resolution with a refresh rate of up to 120Hz and is designed to be used in left-right pairs, with one for each eye. The supported field of view is 120 degrees by 96 degrees per eye, versus a 160 by 150 maximum. Notably, placing over 18 million pixels in such a small area almost entirely eliminates the eye’s ability to see the black grid of gaps between pixels that’s commonly known as the “screen door effect.”
While the screen’s per eye pixel count of 4800 by 3840 is staggering in comparison with current-generation VR headsets, it actually falls well short of what researchers say is the upper bound of human vision: 9600 by 9000. That would eclipse the Google/LG screen’s 1443 pixels per inch to hit 2183 pixels per inch — roughly 10 times the pixel density of Apple’s current “Retina display” laptops.
Not surprisingly, engineering a screen to these specs is challenging, as everything from lighting to subpixel design becomes harder to implement on such a tiny scale. Google and LG chose a white OLED with color filters, a technology used in OLED TVs that can scale down to micro displays. The companies also built a custom screen driver for the left-right pairs, supporting an incredible total resolution of 7680 by 4800.
There’s one particularly huge problem with placing such a high-definition screen in a VR headset: No consumer hardware, and particularly no standalone mobile VR chip, has the ability to push enough pixels to fill the displays even 30 times a second, to say nothing of the 120Hz maximum refresh rate.
For now, Google and LG intend to fill the screens with unsharp pixels. Foveated rendering will use eye tracking to create very small sharp areas in the centers of the screens — the researchers propose sharper 640 by 640 boxes within a 1920 by 1280 total image size. They will then upscale the total image to each screen’s native 4800 by 3840 pixel count, updating the sharp area to match the user’s eye movements while rendering the rest of the image at a lower resolution. If necessary, developers can set up low-, medium-, and high-detail regions to make the differences less obvious to the user.
Another compromise is in the screens’ actual refresh rate. Though the screens support 120Hz, researchers explained that they “settled on foveated transport pixel counts near 1280 × 1920/75Hz to fit within [the mobile GPU’s] bandwidth limitation.” A 75Hz refresh rate is arguably the bare minimum for VR applications, and sits notably below the 90Hz used by most VR headsets these days.
The solutions above are both practical and somewhat disappointing, at least in concept. While they will enable developers to virtually eliminate the screen door effect, they will fill screens with soft pixels that are unlikely to make great use of the high-resolution displays — even the small high-detail regions will be upscaled rather than using the native screen resolution. This will work as a stopgap until mobile and desktop chips catch up with display technology but will not make ideal use of the screens.
Google and LG will present the new screen technology today at Display Week, starting at 11:10 a.m. Pacific Time, though the product’s commercial availability remains a question mark. Taiwanese developer INT will show a 2228 PPI VR-ready display at the same event. We will update this article after the Google/LG announcement if additional details become available.
— Digital Blocks Inc. (@DigitalBlocksIP) May 22, 2018
Update at 12:17 p.m. Pacific: New photos from the Display Week show floor place the Google/LG innovation in perspective. The image above shows the 1443ppi screen next to a current-generation 538ppi display, each viewed through a VR lens. LG’s new screen offers around three times the pixel density of a good current VR display.
The photo above by UploadVR shows what the two displays look like when viewed through the lenses, as captured by the camera of an iPhone 8. On the left is the 1443ppi screen, where the individual city and town names on maps are legible, while on the right side, the smaller names are generally too fuzzy to be read. Note that in actual use, visuals displayed on the 1443ppi screen would have a central sharp zone and blurrier perimeter, versus the 538ppi display, which would likely be pixelated all across the frame.