Chroma subsampling
   HOME

TheInfoList



OR:

Chroma subsampling is the practice of encoding images by implementing less resolution for chroma
information Information is an Abstraction, abstract concept that refers to that which has the power to Communication, inform. At the most fundamental level information pertains to the Interpretation (logic), interpretation of that which may be sensed. ...
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 and still image encoding schemesboth analog and digitalincluding in
JPEG JPEG ( ) is a commonly used method of lossy compression for digital images, particularly for those images produced by digital photography. The degree of compression can be adjusted, allowing a selectable tradeoff between storage size and image ...
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 Filter, filtering or filters may refer to: Science and technology Computing * Filter (higher-order function), in functional programming * Filter (software), a computer program to process a data stream * Filter (video), a software component t ...
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 Gamma correction or gamma is a nonlinear operation used to encode and decode Relative luminance, luminance or CIE 1931 color space#Tristimulus values, tristimulus values in video or still image systems. Gamma correction is, in the simplest cases, ...
'' 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 sRGB is a standard RGB color space, RGB (red, green, blue) color space that Hewlett-Packard, HP and Microsoft created cooperatively in 1996 to use on monitors, printers, and the World Wide Web. It was subsequently standardized by the Internation ...
, the TV Rec. 601, Rec. 709, and Rec. 2020; the concept is also generalized to ''optical transfer functions'' in Rec. 2020.Poynton, Charles
"YUV and ''luminance'' considered harmful: A plea for precise terminology in video"
''Why 4K video looks better on a 1080p screen''
– The Daily Note (with graphics explaining chroma subsampling.


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): * ''J'': horizontal sampling reference (width of the conceptual region). Usually, 4. * ''a'': number of chrominance samples (Cr, Cb) in the first row of ''J'' pixels. * ''b'': number of changes of chrominance samples (Cr, Cb) between first and second row of ''J'' pixels. Note that ''b'' has to be either zero or equal to ''a'' (except in rare irregular cases like 4:4:1 and 4:2:1, which do not follow this convention). * ''
Alpha Alpha (uppercase , lowercase ; grc, ἄλφα, ', or ell, άλφα, álfa) is the first Letter (alphabet), letter of the Greek alphabet. In the system of Greek numerals, it has a value of one. Alpha is derived from the Phoenician alphabet, P ...
'': horizontal factor (relative to first digit). May be omitted if alpha component is not present, and is equal to ''J'' when present. 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 In signal processing and related disciplines, aliasing is an effect that causes different signals to become indistinguishable (or ''aliases'' of one another) when sampling (signal processing), sampled. It also often refers to the distortion or ...
. 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, which means for 8 bit per component without
alpha Alpha (uppercase , lowercase ; grc, ἄλφα, ', or ell, άλφα, álfa) is the first Letter (alphabet), letter of the Greek alphabet. In the system of Greek numerals, it has a value of one. Alpha is derived from the Phoenician alphabet, P ...
(24 bit per pixel) only 16 bits are enough, as in NV16. Many high-end digital video formats and interfaces use this scheme: * AVC-Intra 100 * Digital Betacam * Betacam SX * DVCPRO50 and
DVCPRO HD DV refers to a family of codecs and videotape, tape formats used for storing digital video, launched in 1995 by a consortium of camcorder, video camera manufacturers led by Sony and Panasonic. In the late 1990s and early 2000s, DV was strongly ...
* Digital-S * CCIR 601 / Serial Digital Interface / D1 * ProRes (HQ, 422, LT, and Proxy) * XDCAM HD422 * Canon MXF HD422


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
480i 480i is the video mode used for standard-definition television, standard-definition digital television in the Caribbean, Japan, South Korea, Taiwan, Philippines, Laos, Western Sahara, and most of the Americas (with the exception of Argentina, Par ...
"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 Betamax (also known as Beta, as in its logo) is a consumer-level analog recording and Videocassette#Cassette formats, cassette format of magnetic tape for video, commonly known as a video cassette recorder. It was developed by Sony and was rele ...
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: *
DVCPRO DV refers to a family of codecs and videotape, tape formats used for storing digital video, launched in 1995 by a consortium of camcorder, video camera manufacturers led by Sony and Panasonic. In the late 1990s and early 2000s, DV was strongly ...
(
NTSC The first American standard for analog television broadcast was developed by National Television System Committee (NTSC)National Television System Committee (1951–1953), Report and Reports of Panel No. 11, 11-A, 12–19, with Some supplementa ...
and
PAL Phase Alternating Line (PAL) is a colour encoding system for analogue television. It was one of three major analogue colour television standards, the others being NTSC and SECAM. In most countries it was broadcast at 625 lines, 50 fields (25 ...
) *
480i 480i is the video mode used for standard-definition television, standard-definition digital television in the Caribbean, Japan, South Korea, Taiwan, Philippines, Laos, Western Sahara, and most of the Americas (with the exception of Argentina, Par ...
"NTSC" DV and DVCAM * D-7


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 Phase Alternating Line (PAL) is a colour encoding system for analogue television. It was one of three major analogue colour television standards, the others being NTSC and SECAM. In most countries it was broadcast at 625 lines, 50 fields (25 ...
color encoding system, since this has only half the vertical chrominance resolution of
NTSC The first American standard for analog television broadcast was developed by National Television System Committee (NTSC)National Television System Committee (1951–1953), Report and Reports of Panel No. 11, 11-A, 12–19, with Some supplementa ...
. It would also fit extremely well with the
SECAM SECAM, also written SÉCAM (, ''Séquentiel de couleur à mémoire'', French for ''color sequential with memory''), is an analog television, analog color television system that was used in France, some parts of Europe and Africa, and Russia ...
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 Transcoding is the direct digital-to-digital conversion of one data compression, encoding to another, such as for video data files, audio files (e.g., MP3, WAV), or character encoding (e.g., UTF-8, ISO/IEC 8859). This is usually done in cases wh ...
to convert the PAL signal to SECAM for display. Different variants of 4:2:0 chroma configurations are found in: * All ISO/ IEC
MPEG The Moving Picture Experts Group (MPEG) is an alliance of working groups established jointly by International Organization for Standardization, ISO and International Electrotechnical Commission, IEC that sets standards for media coding, includ ...
and
ITU-T The ITU Telecommunication Standardization Sector (ITU-T) is one of the three sectors (divisions or units) of the International Telecommunication Union (ITU). It is responsible for coordinating standards for telecommunications and Information Commu ...
VCEG H.26x video coding standards including H.262/MPEG-2 Part 2 implementations (although some profiles of
MPEG-4 Part 2 MPEG-4 Part 2, MPEG-4 Visual (formally International Organization for Standardization, ISO/International Electrotechnical Commission, IEC 14496-2) is a video compression format developed by the Moving Picture Experts Group (MPEG). It belongs to the ...
and H.264/MPEG-4 AVC allow higher-quality sampling schemes such as 4:4:4) *
DVD-Video DVD-Video is a consumer video format used to store digital video on DVD discs. DVD-Video was the dominant consumer home video format in Asia, North America, Europe, and Australia in the 2000s until it was supplanted by the high-definition Blu-r ...
and
Blu-ray Disc The Blu-ray Disc (BD), often known simply as Blu-ray, is a Digital media, digital optical disc data storage format. It was invented and developed in 2005 and released on June 20, 2006 worldwide. It is designed to supersede the DVD format, and c ...
. *
576i 576i is a standard-definition television, standard-definition digital video mode, originally used for digitizing analog television in most countries of the world where the utility frequency for electric power distribution is 50 Hz. Because ...
"PAL" DV and DVCAM *
HDV HDV is a format for recording of high-definition video on DV cassette Videotape, tape. The format was originally developed by JVC and supported by Sony Corporation, Sony, Canon Inc., Canon, and Sharp Corporation, Sharp. The four companies formed ...
*
AVCHD AVCHD (Advanced Video Coding High Definition) is a file-based format for the digital recording and playback of high-definition video. It is H.264 and Dolby AC-3 packaged into the MPEG transport stream, with a set of constraints designed around the ...
and AVC-Intra 50 * Apple Intermediate Codec * Most common JPEG/JFIF and MJPEG implementations *
VC-1 SMPTE 421, informally known as VC-1, is a video coding format. Most of it was initially developed as Microsoft Microsoft Corporation is an American multinational corporation, multinational technology company, technology corporation produ ...
*
WebP WebP is an image file format developed by Google intended as a replacement for JPEG, Portable Network Graphics, PNG, and GIF file formats. It supports both lossy compression, lossy and Lossless compression, lossless compression, as well as Comp ...
* YJK, a proprietary
color space A color space is a specific organization of colors. In combination with color profiling supported by various physical devices, it supports reproducible representations of colorwhether such representation entails an analog signal, analog or a Dig ...
implemented by the
Yamaha V9958 The Yamaha V9958 is a Video Display Processor used in the MSX2+ and MSX turbo R series of home computers, as the successor to the Yamaha V9938 used in the MSX2. The main new features are three graphical YJK modes with up to 19268 colors and hori ...
graphic chip on MSX2+ computers. Cb and Cr are each subsampled at a factor of 2 both horizontally and vertically. Most digital video formats corresponding to 576i "PAL" use 4:2:0 chroma subsampling.


Sampling positions

There are three variants of 4:2:0 schemes, having different horizontal and vertical sampling siting. * In MPEG-2, MPEG-4 and AVC Cb and Cr are cosited horizontally. Cb and Cr are sited between pixels in the vertical direction (sited interstitially). * In JPEG/JFIF, H.261, and MPEG-1, Cb and Cr are sited interstitially, halfway between alternate luma samples. * In 4:2:0 DV, Cb and Cr are co-sited in the horizontal direction. In the vertical direction, they are co-sited on alternating lines. That is also what is used in HEVC in BT.2020 and BT.2100 content (in particular on Blu-rays). Also called top-left.


= Interlace and Progressive

= 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 A codec is a device or computer program A computer program is a sequence or set of instructions in a programming language for a computer to Execution (computing), execute. Computer programs are one component of software, which also includes so ...
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) or 10 bit for every pixel. It has the equivalent chrominance bandwidth of a PAL I signal decoded with a delay line decoder, and still very much superior to NTSC. Some video codecs may operate at 4:1:0.5 or 4:1:0.25 as an option, so as to allow similar to VHS quality.


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 an 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 WebP is an image file format developed by Google intended as a replacement for JPEG, Portable Network Graphics, PNG, and GIF file formats. It supports both lossy compression, lossy and Lossless compression, lossless compression, as well as Comp ...
and sjpeg's "Sharp YUV" feature.


Out-of-gamut colors

Another artifact that can occur with chroma subsampling is that out-of-
gamut In color reproduction, including computer graphics and photography, the gamut, or color gamut , is a certain ''complete subset'' of colors. The most common usage refers to the subset of colors which can be accurately represented in a given circ ...
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 The Society of Motion Picture and Television Engineers (SMPTE) (, rarely ), founded in 1916 as the Society of Motion Picture Engineers or SMPE, is a global professional association A professional association (also called a professional b ...
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 The RCA Corporation was a major American electronics company, which was founded as the Radio Corporation of America in 1919. It was initially a patent pool, patent trust owned by General Electric (GE), Westinghouse Electric Corporation, Westin ...
, which developed into the
NTSC The first American standard for analog television broadcast was developed by National Television System Committee (NTSC)National Television System Committee (1951–1953), Report and Reports of Panel No. 11, 11-A, 12–19, with Some supplementa ...
standard; luma–chroma separation was developed earlier, in 1938 by
Georges Valensi M. Georges Valensi (1889–1980) was a French telecommunications engineer who, in 1938, invented and patented a method of transmitting color images via luma (video), luma and chrominance so that they could be received on both color and black & white ...
. 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. 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.


See also

*
Color space A color space is a specific organization of colors. In combination with color profiling supported by various physical devices, it supports reproducible representations of colorwhether such representation entails an analog signal, analog or a Dig ...
*
Multiple sub-Nyquist sampling encoding MUSE (Multiple sub-Nyquist Sampling Encoding), commercially known as Hi-Vision (a contraction of HIgh-definition teleVISION) was a Japan Japan ( ja, 日本, or , and formally , ''Nihonkoku'') is an island country in East Asia. It is situ ...
*
SMPTE The Society of Motion Picture and Television Engineers (SMPTE) (, rarely ), founded in 1916 as the Society of Motion Picture Engineers or SMPE, is a global professional association A professional association (also called a professional b ...
- Society of Motion Picture and Television Engineers *
Digital video Digital video is an electronic representation of moving visual images (video) in the form of encoded digital data. This is in contrast to analog video, which represents moving visual images in the form of analog signals. Digital video comprises ...
*
High-definition television High-definition television (HD or HDTV) describes a television system which provides a substantially higher image resolution than the previous generation of technologies. The term has been used since 1936; in more recent times, it refers to the ...
*
YCbCr YCbCr, Y′CbCr, or Y Pb/Cb Pr/Cr, also written as YCBCR or Y′CBCR, is a family of color spaces used as a part of the color image pipeline in video and digital photography systems. Y′ is the Luma (video), luma component and CB and CR are t ...
*
YPbPr YPbPr or Y'PbPr, also written as , is a color space used in video electronics, in particular in reference to component video cables. YPBPR is Gamma correction, gamma corrected YCbCr, YCBCR color space (it is not analog YUV that was used for analo ...
* Rec. 601 4:2:2
SDTV Standard-definition television (SDTV, SD, often shortened to standard definition) is a television system which uses a resolution that is not considered to be either high-definition television, high or enhanced-definition television, enhanced ...
* YUV * YJK *
Color Color (American English) or colour (British English) is the visual perception, visual perceptual Physical property, property deriving from the spectrum of light interacting with the photoreceptor cells of the eyes. Color categories and physica ...
*
Color vision Color vision, a feature of visual perception, is an ability to perceive differences between light composed of different wavelengths (i.e., different spectral power distributions) independently of light intensity. Color perception is a part of ...
**
Rod cell Rod cells are photoreceptor cells in the retina of the eye that can function in lower light better than the other type of visual photoreceptor, cone cells. Rods are usually found concentrated at the outer edges of the retina and are used in per ...
**
Cone cell Cone cells, or cones, are photoreceptor cells in the retinas of vertebrate eyes including the human eye. They respond differently to light of different Visible spectrum, wavelengths, and the combination of their responses is responsible for col ...


References

* Poynton, Charles. "Digital Video and HDTV: Algorithms and Interfaces". U.S.: Morgan Kaufmann Publishers, 2003. *


External links


Chroma Sampling: An Investigation
{{DEFAULTSORT:Chroma Subsampling Film and video technology Image compression