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A better color matching between HD and UHD Content

October 23, 2014

Lars Borg, principal scientist at Adobe, said that creating narrow versus wide color gamut is not a simple task. "Narrow and wide are the only parameters I'm interested in," he said, referring to a chart showing HD, UHD1 and UHD2 images. "My quest would be to see if any existing HD to SD method can be leveraged for converting HD colors to wide-gamut UHD colors. Color scientists know all of this, but this is for those who are not color scientists."

He showed some slides underlining how gamuts do make a difference for HD (narrow) and UHD (wide)images. "If I have the same color in the wide gamut, it's mathematically the same grade but gives different results," he said. "This is because we're facing two different gamut sizes." The difference increases with saturation and gamut size; there's no difference for neutrals.

A common HD-SD method fails for UHD. He showed converting through scene colorimetry, simulating a UHD camera that uses 709 transfer functions. "This is a nice round tripping, very logical," he said. "But it's a slight mismatch for UHD." Another way is to convert through display colorimetry , using 1886 display transfer curves...and it's an exact match. "This is the right way of doing it, but it's not supported by standards," he said. "But I've found other ways to do things." He referred to standards in revision: EOTF shall be 1886, with display colorimetry out; OETF shall  be 709, with camera colorimetry in. "This applies to system gamma yet again," he said. "It fails already in HD to SD. You can't use this for a conversion method." Display colorimetry method, using only 1886 curves, provides accurate conversion.

He looked at TV standards. Current TV standards forces use of scene colorimetry in transcoding. Revisions won't help the problem.  "What we need is OETF for camera, with 709 optional for gray response, and OETF for transcoding with 1886," he said. With regard to video codecs, AVC/H.264 and HEVC/H.252 have no option to indicate the 1886 encoding curve; users will be forced to use the 709 curve. "It would be nice to add a 1886 option," said Borg. The conclusion? Color-accurate transcoding to Wide UHD is do-able using the 1886 standard. But the standards are wrong.

Tag(s): HD , UHD , SD

Debra Kaufman

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