Two Keys to Optimal HDR TVs: Dynamic HDR Metadata and Tone Mapping

The Society for Information Display LA Chapter held its 14th annual One-Day Conference on February 3, 2017 at the Costa Mesa Country Club in Costa Mesa, California. At the conference, Gerard Catapano gave a presentation entitled “HDR, Today into Tomorrow.”

Catapano, formerly Associate Director of Electronics Testing at Consumer Reports and now Director of Quality Assurance at Samsung’s QA Lab in Pine Brook, New Jersey, introduced high dynamic range (HDR) as “the latest and most innovative technology that helps film studios deliver a better expression of details in shadows and highlights to the consumer.” He presented the Consumer Technology Association’s definition of an HDR-compatible display as one that has at least these four attributes:

• Includes at least one interface that supports HDR signaling as defined in CEA-861-F, as extended by CEA-861.3.
• Receives and processes static HDR metadata compliant with CEA-861.3 for uncompressed video.
• Receives and processes the HDR10 Media Profile from IP, HDMI, or other video delivery sources. Other media profiles may be supported in addition.
• Applies an appropriate electro-optical transfer function (EOTF) before rendering the image.

The HDR standard has been endorsed by a variety of organizations, include the Blu-ray Disc Association, MPEG, the UHD Alliance, and the ITU. Although HDR is currently a premium feature, Catapano predicted that it will become a basic feature of TVs over all screen sizes and display technologies.

Samsung TV supports only the HDR10 media profile because it is an open standard that does not require licensing fees and, as a result, permits customization with

The CIE 1976 LAB Color Space clearly indicates the range of colors available for each level of luminance.

in the profile. Since use of at least HDR10 is required by the CTA definition of an HDR-compatible display, it will be supported by all major manufacturers.  Catapano noted that at NAB 2016, the major encoder manufacturers were offering 4K HDR as an option, and the major mastering and editing tool sets were implementing it.

The CTA definition only requires HDR sets to support static HDR metadata: metadata that is constant throughout the entire film or video. But much more can be done with dynamic HDR metadata, which changes scene by scene.  SMPTE ST.2094-40 provides for the use of dynamic metadata for tone mapping with HDR10.  In a subsequent conversation, Mindoo Chun, an engineer at the QA Lab, told me the dynamic metadata and tone-mapping technology codified in ST.2094-40 was developed by Samsung and a made available to SMPTE.

Tone mapping is a key technology in HDR TVs, Catapano said. It is a color-volume transform that renders incoming HDR content for a display having a dynamic range that is smaller than that for which the contents were coded. With static metadata, the only way to compress the scene with the greatest color volume so it fits into the set’s color volume is to over-compress the much larger number of less demanding scenes.

With dynamic metadata, each scene can be optimally compressed, with that result that many scenes will not require color-volume compression at all. Catapano observed that Samsung HDR TVs for the 2017 model year “are ready for ST.2094-40.”

An ICC color profile based on CIELAB. As with most real devices, this one can reproduce only a portion of the colors defined in the CIELAB color space.

Let’s say a little more about “color volume.”  The most common way of looking at color gamut is still with the 85-year-old CIE 1931 color diagram, which compresses the luminance (“brightness”) Z-axis so that the color space is pressed into a plane.  With the limited luminance capabilities displays have had until recently, that was a simple and (perhaps) adequate approach.  But with high dynamic range, you lose a lot of information that way.  Over the years many three-dimensional color spaces have been developed, with the CIELAB color space being the most common.  Now you can think of each value of luminance as having a two-dimensional color gamut associated with it, and the entire color volume is the stack of these two-dimensional gamuts running from black to white.  The gamut decreases at low and high luminance values, and one of the things you want to do in HDR  set is to have a relatively large gamut at high luminance levels so bright colors do not wash out.

Tone mapping maps the colors in the program material’s color volume to the smaller color volume of a less capable TV set while providing the best possible picture.  From Samsung’s point of view, it’s very convenient that the OLED TVs of arch-rival LG inherently have a smaller color volume than HDR LCD sets because they have a substantially smaller maximum luminance.  There is much more to say about that, but we’ll save it for another time.

Now, when you go into Costco or Fry’s to buy your next TV set, you can ask the sales associate whether the set supports SMPTE ST.2094-40. I look forward to hearing how that conversation goes.
Ken Werner is Principal of Nutmeg Consultants, specializing in the display industry, manufacturing, technology, and applications, including mobile devices and television. He consults for attorneys, investment analysts, and companies re-positioning themselves within the display industry or using displays in their products. He is the 2017 winner of the Society for Informational Display’s Lewis and Beatrice Winner Award.

You can reach him at kwerner@nutmegconsultants.com.