Live From SMPTE: Got Color? Next-Gen Challenges of More-Accurate Viewing Experience Kick Off Day Two
One of the opportunities provided by the move to UHD is the ability to deliver more-realistic colors and imagery to viewers, but there is still plenty of work to be done to ensure that the promise becomes reality. Dr. Kenichiro Masaoka, of NHK’s Science and Technology Research Labs, discussed one possible solution at the SMPTE Technical Conference and Exhibition in Los Angeles today.
The first step in the process took place in 2012 when the International Telecommunication Union (ITU) made Recommendation BT.2020 (also known as Rec. 2020) public. Rec. 2020 defines a bit depth of either 10 or 12 bits per sample and is supported by H.264 as well as HEVC, manufacturers involved in 4K, and the 4K Blu-ray Disc format.
“But the proper implementation and color management of 2020 is urgently needed,” said Masaoka.
There is a two-way challenge with respect to next-generation colorimetry. First, there is the need to convert HD material conforming to the Rec. 709 standard to Rec. 2020. In that case, Masaoka and NHK propose the use of a 3×3 linear matrix. And, for converting from Rec. 2020 to Rec. 709, the recommendation is to use a high-quality gamut-mapping algorithm that NHK is currently developing.
The expanded color-gamut process begins, of course, with acquisition. Masaoka pointed out that cameras that conform to Rec. 2020 offer improved colorimetry because the linear matrix is closer so that ideal spatial sensitivity can offer less-noisy images and greater color accuracy and saturation.
“At an open house at NHK,” he explained, “we were able to show the reproduced images and the real objects and then switch between Rec. 2020 and 709. Viewers could easily distinguish between the two standards.”
Besides the Rec. 2020 cameras, there is a need for displays that are Rec. 2020-compatible. Masaoka said that QD-LED displays hold the most promise for accuracy, providing a next-generation backlight source for wide-gamut–capable LCDs. A Sharp 8K LCD with 120 Hz that could display 85% of the wide-gamut Rec. 2020 spec was on display at a recent industry trade show in Japan and walked away with an award from the Japanese government.
“It’s important to design displays with non-monochromatic light sources,” he added.
Lars Borg, Adobe principal scientist, laid out the benefits of UHD with respect to color gamut.
“UHD has 40% wider color gamut so there is more room for saturated colors,” he pointed out, “but the dynamic range is the same as HD.”
Borg addressed the ability to have HD and UHD gamut match. The most accurate way to do that, he said, is to make sure monitors conform to BT.1886, a response curve that he says can provide accurate color conversion between HD and UHD.
“It offers an exact match and is the right way of doing it, but BT.1886 is not supported in the standards,” he explained. “But using only BT.18886 curves provides accurate conversion.”
Video codecs, like H.264 and HEVC, for example, use the Rec. 709 curve, which has narrower color gamut.
“Adding BT.1886 to table E4,” Borg said, “would solve some of the confusion.”