sde – vui – nclc – nclx

Video has been standardized with various colour encodings over the decades. Attempts have been made to describe video encodings in metadata.

The first such attempt was the sequence display extension of MPEG‑2, which provided three enumerations: colour primaries, transfer characteristics, and matrix coefficients. Colour encodings following that model have been introduced into MPEG‑4 Part 10 (AVC/H.264) and H.265 (HEVC), and have come to be described as vectors of three small integers. The following combinations are common:

1‑1‑1: BT.709 (e.g., HD).

5‑1‑6: 576i SD.

6‑1‑6: 480i SD.

Apple incorporated the metadata into the “colr” parameter atom of QuickTime (as three 16‑bit integer enumerated values contained in the “nclc” parameter). The scheme was first documented in an Appple Developer note, Uncompressed Y’CbCr Video in QuickTime Files, (also known as Letters from the Ice Floe [sic] Dispatch number 19, dated 1999‑12‑14). That document reveals “nclc” to be a mnemonic for “nonconstant luminance coding.” The current definitive reference from Apple seems to be the QuickTime File Format Specification (dated 2011‑07‑13). Further information is contained in Apple Technical Note TN2227, Video Color Management in AV Foundation and QTKit. The QuickTime file format was adapted to become the ISO Base Media File Format, but with the color characteristic atom changed to “nclx” and the data augmented by an additional field that indicates whether the colour data is encoded at studio-swing (as is standard in BT.601 and BT.709) of “full-swing” (as is the custom for so-called sYCC). There is virtually no professional content encoded with full-swing Y’C_BC_R. In professional video, there is no need to clip luma to 16..235 or chroma to 16..240, though professional material is often mastered such that no sample values in the footroom or headroom regions are presented to the encoder.

In H.264, metadata is contained in the video usability information (VUI), an optional structure within a sequence parameter set.

The metadata elements have been expanded over the years, and now include log100 and log316 transfer functions, Adobe RGB 1998 primaries, and BT.2020 primaries.

All of these schemes fail more or less seriously for various reasons documented in the Metadata chapter of my book Digital Video and HD Algorithms and Interfaces, Second Edition (Burlington, Mass.: Elsevier/Morgan Kaufmann: 2012).