Improve VR Resolution Using Subpixel Rendering without Color Filters in Periphery

Visual Field of the Naked Eye, section 3.1 of Howlett, SID 1992.

Wide angle virtual reality (VR) displays need to wrap around up to 280° horizontally and  120° veritically to cover the human visual field¹;  within that area, eye motion can swivel the fovea over a circular area approximately 90° horizontally and vertically. That’s a lot of screen real estate! For comparison, THX recommends sitting just 6.5 feet (2m) from your new 65-inch HDTV but that still only fills 40° of view.

VR head mounted displays (HMDs) typically use lenses to magnify the screen so that it fills up more of our visual field. At high magnification though, the pixels get very large and image quality fades. A standard way to improve quality works with our eyes’ natural limits; the farther things are from the center of vision, the less clearly we see them.  Fisheye lens magnifiers work well with this limitation; they stretch the center part of the image less where we see better and magnify the outside areas more where we don’t see as well.

Another way to get more quality out of the image is to work with another limit of our peripheral vision.  Outside of the foveal sweep (aka the 90° direct field where we see sharply) our eyes do not perceive color.  Our brains keep track of color for us in the periphery, but our eyes don’t actually see that color; it’s literally all in our head. Wide angle VR display systems, therefore, can use sub-pixel rendering without applying a low-pass color filter; chromatic aliasing will not be visible for pixel triads in the periphery; this triples horizontal resolution outside of the foveal area for standard LCD stripe arrays.

Sub-pixel rendering without color filtering outside of the direct field can be used either to push more pixels into the direct field or to extend the peripheral field.  HMDs using “large expanse, extra perspective” (LEEP) optics, for example, can increase the fisheye magnification on the edges of the display and further improve the projected visual field. The constant k can be increased from the typical LEEP value of 0.18 to even higher levels of magnification.

LEEP optical compression. r = F (Θ – .18 Θ³) ≈ F sin Θ

If the HMD designer has input into the LCD design, additional resolution improvements can be incorporated by specifying a unique LCD color filter arrangement.  In the direct field portion of the LCD viewable by the fovea, individual sub-pixels should be colored in a Bayer pattern that better  mimics the physiology of the human eye.  Outside of the direct field, the LCD color filters should be eliminated altogether to improve the range of brightness and simplify sub-pixel rendering.

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