Chroma subsampling

Chroma subsampling is the practice of encoding images by implementing less resolution for chroma information than for luma information, taking advantage of the human visual system's lower acuity for color differences than for luminance.
It is used in many video encoding schemes both analog and digital and also in JPEG encoding.
Rationale.
Digital signals are often compressed to reduce file size and save transmission time. Since the human visual system is much more sensitive to variations in brightness than color, a video system can be optimized by devoting more bandwidth to the luma component (usually denoted Y'), than to the color difference components Cb and Cr. In compressed images, for example, the 4:2:2 Y'CbCr scheme requires two-thirds the bandwidth of non-subsampled "4:4:4" R'G'B'. This reduction results in almost no visual difference as perceived by the viewer.
How subsampling works.
At normal viewing distances, there is no perceptible loss incurred by sampling the color detail at a lower rate, i.e. with a lower resolution. In video systems, this is achieved through the use of color difference components. The signal is divided into a luma (Y') component and two color difference components (chroma). A variety of filtering methods can be used to arrive at the resolution-reduced chroma values.
Luma (Y') is differentiated from luminance (Y) by the presence of "gamma correction" in its calculation, hence the prime symbol added here. A gamma-corrected signal has the advantage of emulating the logarithmic sensitivity of human vision, with more levels dedicated to the darker levels than the lighter ones. As a result, it is ubiquitously used in the source tristimulus signal, the R'G'B' input. Examples of such color spaces include sRGB, the TV Rec. 601, Rec. 709, and Rec. 2020; the concept is also generalized to "optical transfer functions" in Rec. 2020.
Sampling systems and ratios.
The subsampling scheme is commonly expressed as a three-part ratio "J":"a":"b" (e.g. 4:2:2) or four parts, if alpha channel is present (e.g. 4:2:2:4), that describe the number of luminance and chrominance samples in a conceptual region that is "J" pixels wide and 2 pixels high. The parts are (in their respective order):
This notation is not valid for all combinations and has exceptions, e.g. 4:1:0 (where the height of the region is not 2 pixels, but 4 pixels, so if 8 bits per component are used, the media would be 9 bits per pixel) and 4:2:1.
The mapping examples given are only theoretical and for illustration. Also note that the diagram does not indicate any chroma filtering, which should be applied to avoid aliasing.
To calculate required bandwidth factor relative to 4:4:4 (or 4:4:4:4), one needs to sum all the factors and divide the result by 12 (or 16, if alpha is present).
Types of sampling and subsampling.
4:4:4.
Each of the three Y'CbCr components has the same sample rate, thus there is no chroma subsampling. This scheme is sometimes used in high-end film scanners and cinematic post-production.
Note that "4:4:4" may instead be wrongly referring to R'G'B' color space, which implicitly also does not have any chroma subsampling (except in JPEG R'G'B' can be subsampled). Formats such as HDCAM SR can record 4:4:4 R'G'B' over dual-link HD-SDI.
4:2:2.
The two chroma components are sampled at half the horizontal sample rate of luma: the horizontal chroma resolution is halved. This reduces the bandwidth of an uncompressed video signal by one-third.
Many high-end digital video formats and interfaces use this scheme:
4:2:1.
This sampling mode is not expressible in J:a:b notation. "4:2:1" is an obsolete term from a previous notational scheme, and very few software or hardware codecs use it. Cb horizontal resolution is half that of Cr (and a quarter of the horizontal resolution of Y).
4:1:1.
In 4:1:1 chroma subsampling, the horizontal color resolution is quartered, and the bandwidth is halved compared to no chroma subsampling. Initially, 4:1:1 chroma subsampling of the DV format was not considered to be broadcast quality and was only acceptable for low-end and consumer applications. However, DV-based formats (some of which use 4:1:1 chroma subsampling) have been used professionally in electronic news gathering and in playout servers. DV has also been sporadically used in feature films and in digital cinematography.
In the NTSC system, if the luma is sampled at 13.5 MHz, then this means that the Cr and Cb signals will each be sampled at 3.375 MHz, which corresponds to a maximum Nyquist bandwidth of 1.6875 MHz, whereas traditional "high-end broadcast analog NTSC encoder" would have a Nyquist bandwidth of 1.5 MHz and 0.5 MHz for the I/Q channels. However, in most equipment, especially cheap TV sets and VHS/Betamax VCRs, the chroma channels have only the 0.5 MHz bandwidth for both Cr and Cb (or equivalently for I/Q). Thus the DV system actually provides a superior color bandwidth compared to the best composite analog specifications for NTSC, despite having only 1/4 of the chroma bandwidth of a "full" digital signal.
Formats that use 4:1:1 chroma subsampling include:
4:2:0.
In 4:2:0, the horizontal sampling is doubled compared to 4:1:1, but as the Cb and Cr channels are only sampled on each alternate line in this scheme, the vertical resolution is halved. The data rate is thus the same. This fits reasonably well with the PAL color encoding system, since this has only half the vertical chrominance resolution of NTSC. It would also fit extremely well with the SECAM color encoding system, since like that format, 4:2:0 only stores and transmits one color channel per line (the other channel being recovered from the previous line). However, little equipment has actually been produced that outputs a SECAM analogue video signal. In general, SECAM territories either have to use a PAL-capable display or a transcoder to convert the PAL signal to SECAM for display.
Different variants of 4:2:0 chroma configurations are found in:
Cb and Cr are each subsampled at a factor of 2 both horizontally and vertically.
There are three variants of 4:2:0 schemes, having different horizontal and vertical siting.
Most digital video formats corresponding to PAL use 4:2:0 chroma subsampling, with the exception of DVCPRO25, which uses 4:1:1 chroma subsampling. Both the 4:1:1 and 4:2:0 schemes halve the bandwidth compared to no chroma subsampling.
With interlaced material, 4:2:0 chroma subsampling can result in motion artifacts if it is implemented the same way as for progressive material. The luma samples are derived from separate time intervals, while the chroma samples would be derived from both time intervals. It is this difference that can result in motion artifacts. The MPEG-2 standard allows for an alternate interlaced sampling scheme, where 4:2:0 is applied to each field (not both fields at once). This solves the problem of motion artifacts, reduces the vertical chroma resolution by half, and can introduce comb-like artifacts in the image.
Original. This image shows a single field. The moving text has some motion blur applied to it.
4:2:0 progressive sampling applied to moving "interlaced" material. Note that the chroma leads and trails the moving text. This image shows a single field.
4:2:0 interlaced sampling applied to moving "interlaced" material. This image shows a single field.
In the 4:2:0 interlaced scheme, however, vertical resolution of the chroma is roughly halved, since the chroma samples effectively describe an area 2 samples wide by 4 samples tall instead of 2×2. As well, the spatial displacement between both fields can result in the appearance of comb-like chroma artifacts.
Original still image.
4:2:0 progressive sampling applied to a still image. Both fields are shown.
4:2:0 interlaced sampling applied to a still image. Both fields are shown.
If the interlaced material is to be de-interlaced, the comb-like chroma artifacts (from 4:2:0 interlaced sampling) can be removed by blurring the chroma vertically.
4:1:0.
This ratio is possible, and some codecs support it, but it is not widely used. This ratio uses half of the vertical and one-fourth the horizontal color resolutions, with only one-eighth of the bandwidth of the maximum color resolutions used. Uncompressed video in this format with 8-bit quantization uses 10 bytes for every macropixel (which is 4×2 pixels). It has the equivalent chrominance bandwidth of a PAL I signal decoded with a delay line decoder, and still very much superior to NTSC.
3:1:1.
Used by Sony in their HDCAM High Definition recorders (not HDCAM SR). In the horizontal dimension, luma is sampled horizontally at three quarters of the full HD sampling rate 1440 samples per row instead of 1920. Chroma is sampled at 480 samples per row, a third of the luma sampling rate.
In the vertical dimension, both luma and chroma are sampled at the full HD sampling rate (1080 samples vertically).
Artifacts.
Chroma subsampling suffers from two main types of artifacts, causing degradation more noticeable than intended where colors change abruptly.
Gamma error.
Gamma-corrected signals like Y'CbCr have in issue where chroma errors "bleed" into luma. In those signals, a low chroma actually makes a color appear less bright than one with equivalent luma. As a result, when a saturated color blends with an unsaturated or complementary color, a loss of luminance occurs at the border. This can be seen in the example between magenta and green. To arrive at a set of subsampled values that more closely resembles the original, it is necessary to undo the gamma correction, perform the calculation, and then step back into the gamma-corrected space. More efficient approximations are also possible, such as with a luma-weighted average or iteratively with lookup tables in WebP and sjpeg's "Sharp YUV" feature.
Out-of-gamut colors.
Another artifact that can occur with chroma subsampling is that out-of-gamut colors can occur upon chroma reconstruction. Suppose the image consisted of alternating 1-pixel red and black lines and the subsampling omitted the chroma for the black pixels. Chroma from the red pixels will be reconstructed onto the black pixels, causing the new pixels to have positive red and "negative" green and blue values. As displays cannot output negative light (negative light does not exist), these negative values will effectively be clipped, and the resulting luma value will be too high. Similar artifacts arise in the less artificial example of gradation near a fairly sharp red/black boundary.
Other types of filtering during subsampling can also cause colors to go out of gamut.
Terminology.
The term Y'UV refers to an analog TV encoding scheme (ITU-R Rec. BT.470) while Y'CbCr refers to a digital encoding scheme. One difference between the two is that the scale factors on the chroma components (U, V, Cb, and Cr) are different. However, the term YUV is often used erroneously to refer to Y'CbCr encoding. Hence, expressions like "4:2:2 YUV" always refer to 4:2:2 Y'CbCr, since there simply is no such thing as 4:x:x in analog encoding (such as YUV). Pixel formats used in Y'CbCr can be referred to as YUV too, for example yuv420p, yuvj420p and many others.
In a similar vein, the term luminance and the symbol Y are often used erroneously to refer to luma, which is denoted with the symbol Y'. Note that the "luma" (Y') of video engineering deviates from the "luminance" (Y) of color science (as defined by CIE). Luma is formed as the weighted sum of "gamma-corrected" (tristimulus) RGB components. Luminance is formed as a weighed sum of "linear" (tristimulus) RGB components.
In practice, the CIE symbol Y is often incorrectly used to denote luma. In 1993, SMPTE adopted Engineering Guideline EG 28, clarifying the two terms. Note that the prime symbol ' is used to indicate gamma correction.
Similarly, the chroma of video engineering differs from the chrominance of color science. The chroma of video engineering is formed from weighted tristimulus components (gamma corrected, OETF), not linear components. In video engineering practice, the terms "chroma", "chrominance", and "saturation" are often used interchangeably to refer to chrominance, but it is not a good practice, as ITU-T Rec H.273 says.
History.
Chroma subsampling was developed in the 1950s by Alda Bedford for the development of color television by RCA, which developed into the NTSC standard; luma–chroma separation was developed earlier, in 1938 by Georges Valensi. Through studies, he showed that the human eye has high resolution only for black and white, somewhat less for "mid-range" colors like yellows and greens, and much less for colors on the end of the spectrum, reds and blues. Using this knowledge allowed RCA to develop a system in which they discarded most of the blue signal after it comes from the camera, keeping most of the green and only some of the red; this is chroma subsampling in the YIQ color space and is roughly analogous to 4:2:1 subsampling, in that it has decreasing resolution for luma, yellow/green, and red/blue.