HDR is Highly Dynamic
By Tom Fiske
Judging from the papers presented at Display Week this year, no one technology or company has a lock on high dynamic range (HDR) displays, the HDR ecosystem, or even on how to describe HDR performance. The main technology contenders are OLED displays, dual-modulation LCDs, and dual-modulated projection light-valve systems. The ecosystem includes Dolby Vision and ACES encoding and delivery schemes, among others. The measurement protocols and standards for HDR are still in development.
OLED displays are still fairly new (and therefore expensive), but deliver a superb HDR experience. The black levels are amazing and almost unmeasurably low. The peak luminance can be up to several hundred cd/m2, but is limited by average picture level (APL). The dual-modulation techniques for LCDs require a directly illuminated local dimming backlight to achieve enough backlight “resolution” for good HDR performance. Depending on implementation, halo artifacts can be a problem. A second LCD panel (with a normal backlight) can also be used as the modulation source. Projection systems need an additional light valve to render HDR images. All this is to say that HDR will not be inexpensive to realize. The display devices are new; the workflow is not quite settled; and the pipeline, from acquisition to processing and content delivery, is bigger and more complicated than those for standard dynamic range (SDR) content.
A few things of note around HDR at Display Week. LG Display showed off a beautiful (and thin – 6mm!) “wallpaper” 77-in. UHD OLED display at its booth on the exhibit floor. In fact, this display won SID’s Display of the Year award. It provides a truly amazing HDR image (black levels to die for and highlights up to 800 cd/m2 at 10% APL). It will also retail at around $28,000, so it won’t be for everyone. Authors from the University of Florida and AU Optronics delivered the paper, “High-Dynamic-Range LCD with Pixel-Level Local Dimming,” which describes a system with a TN LCD with no color filter paired with an FFS LCD to give 1,000,000:1 CR and good angular performance (see schematic below). There are still some challenges to address, but this approach could be promising if parallax issues can be avoided.
Source: University of Florida and AU Optronics
Dolby authors delivered an invited paper, “Prediction of Overall HDR Quality by Using Perceptually Transformed Display Measurements.” They provided a good case for an HDR quality metric that is well supported with human factors data. A couple of noteworthy papers were given in the session titled Visual Quality of HDR Displays, including, “Visual Quality of Global Dimming Backlight with High Contrast Liquid Crystal Panel for High Dynamic Range Displays,” which showed that a system with a global dimming backlight with a high CR LCD is preferable to a local dimming backlight with a moderate CR LCD. The paper “Reproducing High-Dynamic-Range Contents Adaptively Based on Display Specifications” showed that an SDR display can reasonably render HDR content with an appropriate gamma and color mapping algorithm and a dynamic range of 0.1 to 640 cd/m2.
So, no matter what your favorite flavor of HDR or how you want to describe it, it looks like there’s still opportunity to make a contribution.