Y’CBCR from studio RGB

decode to R’G’B’ in the range [0…219] from 8-bit BT.601 Y’CBCR, invert Equation 29.8:

The entries of 256 in this matrix indicate that the corresponding component can simply be added; there is no need for a multiplication operation. These transforms assume that the R’G’B’ components incorporate gamma correction, such as that specified by BT.709; see page 333.

Y’CBCR from computer RGB

In computing it is conventional to use 8-bit R’G’B’ components, with no headroom and no footroom: Black is at code 0 and white is at 255. To encode 8-bit BT.601 Y’CBCR from R’G’B’ in this range, scale the matrix of Equation 29.6 by 256⁄255:

To decode R’G’B’ in the range [0…255] from 8-bit

BT.601 Y’CBCR, use the transform of Equation 29.11:

BT.601 Y’CBCR uses the extremes of the coding range to handle signal overshoot and undershoot. Clipping is required when decoding to an R’G’B’ range that has no headroom or footroom.

“Full-swing” Y’CBCR

The Y’CBCR coding used in JPEG/JFIF stillframes in computing conventionally uses “full-swing” coding with no footroom and no headroom. Luma (Y’) is scaled to an excursion of 255 and represented in 8 bits: Black is at code 0 and white is at code 255. Obviously, luma codes 0 and 255 are not prohibited! Colour difference

0quantizer, emphasizing that pre-offset chroma code

To decode into R’G’B’ in the range [0…255] from fullrange 8-bit Y’CBCR, use the transform in Equation 29.13:

Y’UV, Y’IQ confusion

Ihave detailed Y’PBPR and Y’CBCR. These are both based on [B’-Y’, R’-Y’] components, but they have different

scale factors suitable for component analog and component digital interface, respectively.

Colour differences pairs [U, V] and [I, Q] are also based on B’-Y’ and R’-Y’, but have yet another set of scale factors. UV scaling – or IQ scaling and rotation – is appropriate only when the signals are destined for composite encoding, as in NTSC or PAL.

Unfortunately, the notation Y’UV – or worse, YUV – is sometimes loosely applied to any form of colour difference coding based on [B’-Y’, R’-Y’]. Do not be misled by video equipment having connectors labelled Y’UV or Y’, B’-Y’, R’-Y’, or these symbols without primes, or by JPEG being described as utilizing Y’UV coding. In fact

the analog connectors convey signals with Y’PBPR scaling, and the JPEG standard itself specifies what

Iwould denote 255601Y’CBCR.

When the term Y’UV (or YUV) is encountered in a computer graphics or image-processing context, usually BT.601 Y’CBCR is meant, but beware!

•Any image data supposedly coded to the original 1953 NTSC primaries is suspect, because it has been roughly four decades since any equipment using these primaries has been built.

•Generally no mention is made of the transfer function of the underlying R’G’B’ components, and no account is taken of the nonlinear formation of luma.

When the term Y’IQ (or YIQ) is encountered, beware!

•Image data supposedly coded in Y’IQ is suspect since no analog or digital interface for Y’IQ components has ever been standardized.

•NTSC encoders and decoders built since 1970 have been based upon Y’UV components, not Y’IQ. Contrary to much published information, Y’IQ components have not been used for “NTSC” for about 4 decades.