Chroma subsampling
Encyclopedia
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.
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 (4:4:4) R'G'B'. This reduction results in almost no visual difference as perceived by the viewer.
is less sensitive to the position and motion of color than luminance, bandwidth can be optimized by storing more luminance detail than color detail. At normal viewing distances, there is no perceptible loss incurred by sampling
the color detail at a lower rate. 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
).
Chroma subsampling deviates from color science in that the luma and chroma components are formed as a weighted sum of gamma-corrected
(tristimulus) R'G'B' components instead of linear (tristimulus) RGB components. As a result, luminance and color detail are not completely independent of one another. There is some "bleeding" of luminance and color information between the luma and chroma components. The error is greatest for highly-saturated colors and can be somewhat noticeable in between the magenta and green bars of a color bars test pattern (that has chroma subsampling applied). This engineering approximation (by reversing the order of operations between gamma correction and forming the weighted sum) allows color subsampling to be more easily implemented.
Original without color subsampling. 200% zoom.
Image after color subsampling (compressed with Sony Vegas DV codec, box filtering applied.)
An explanatory image of different chroma subsampling schemes can be seen at the following link: http://lea.hamradio.si/~s51kq/subsample.gif
(source: "Basics of Video": http://lea.hamradio.si/~s51kq/V-BAS.HTM) or in details in Chrominance Subsampling in Digital Images, by Douglas Kerr.
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).
components have the same sample rate. This scheme is sometimes used in high-end film scanners and cinematic postproduction. Two SDI
links (connections) are normally required to carry this bandwidth: Link A would carry a 4:2:2 signal, Link B a 0:2:2, when combined would make 4:4:4.
Many high-end digital video formats and interfaces use this scheme:
is similar, in using lower resolution for blue/yellow than red/green, which in turn has less resolution than luma.
format was not considered to be broadcast quality and was only acceptable for low-end and consumer applications. Currently, DV-based formats (some of which use 4:1:1 chroma subsampling) are 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 VCR's 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:
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.
The PAL and SECAM
color systems are especially well-suited to this kind of data reduction. 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 2X2. 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.
In the vertical dimension, both luma and chroma are sampled at the full HD sampling rate (1080 samples vertically).
colors can occur upon chroma reconstruction. Suppose the image consisted of alternating 1-pixel red and black lines. 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.
Filtering during subsampling can also cause colors to go out of gamut.
refers to an analog encoding scheme 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).
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/chrominance of video engineering differs from the chrominance of color science. The chroma/chrominance of video engineering is formed from weighted tristimulus components, not linear components. In video engineering practice, the terms chroma, chrominance, and saturation are often used interchangeably to refer to chrominance.
, 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.
Proper upsampling of chroma can require knowing whether the source is progressive or interlaced, information which is often not available to the upsampler.
Chroma subsampling causes problems for film makers trying to do keying with blue or green screening. The chroma interpolation along edges produces noticeable haloing artifacts.
Chrominance
Chrominance is the signal used in video systems to convey the color information of the picture, separately from the accompanying luma signal . Chrominance is usually represented as two color-difference components: U = B' − Y' and V = R' − Y'...
information
Information
Information in its most restricted technical sense is a message or collection of messages that consists of an ordered sequence of symbols, or it is the meaning that can be interpreted from such a message or collection of messages. Information can be recorded or transmitted. It can be recorded as...
than for luma
Luma (video)
In video, luma, sometimes called luminance, represents the brightness in an image . Luma is typically paired with chrominance. Luma represents the achromatic image without any color, while the chroma components represent the color information...
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
JPEG
In computing, JPEG . The degree of compression can be adjusted, allowing a selectable tradeoff between storage size and image quality. JPEG typically achieves 10:1 compression with little perceptible loss in image quality....
encoding.
Rationale
Because of storage and transmission limitations, there is always a desire to reduce (or compress) the signal. 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 lumaLuma (video)
In video, luma, sometimes called luminance, represents the brightness in an image . Luma is typically paired with chrominance. Luma represents the achromatic image without any color, while the chroma components represent the color information...
component (usually denoted Y'), than to the color difference components Cb and Cr. In compressed images, for example, the 4:2:2 Y'CbCr
YCbCr
YCbCr or Y′CbCr, sometimes written or , 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 component and CB and CR are the blue-difference and red-difference chroma components...
scheme requires two-thirds the bandwidth of (4:4:4) R'G'B'. This reduction results in almost no visual difference as perceived by the viewer.
How subsampling works
Because the human visual systemVisual system
The visual system is the part of the central nervous system which enables organisms to process visual detail, as well as enabling several non-image forming photoresponse functions. It interprets information from visible light to build a representation of the surrounding world...
is less sensitive to the position and motion of color than luminance, bandwidth can be optimized by storing more luminance detail than color detail. At normal viewing distances, there is no perceptible loss incurred by sampling
Sampling (signal processing)
In signal processing, sampling is the reduction of a continuous signal to a discrete signal. A common example is the conversion of a sound wave to a sequence of samples ....
the color detail at a lower rate. In video systems, this is achieved through the use of color difference components. The signal is divided into a luma
Luma (video)
In video, luma, sometimes called luminance, represents the brightness in an image . Luma is typically paired with chrominance. Luma represents the achromatic image without any color, while the chroma components represent the color information...
(Y') component and two color difference components (chroma
Chrominance
Chrominance is the signal used in video systems to convey the color information of the picture, separately from the accompanying luma signal . Chrominance is usually represented as two color-difference components: U = B' − Y' and V = R' − Y'...
).
Chroma subsampling deviates from color science in that the luma and chroma components are formed as a weighted sum of gamma-corrected
Gamma correction
Gamma correction, gamma nonlinearity, gamma encoding, or often simply gamma, is the name of a nonlinear operation used to code and decode luminance or tristimulus values in video or still image systems...
(tristimulus) R'G'B' components instead of linear (tristimulus) RGB components. As a result, luminance and color detail are not completely independent of one another. There is some "bleeding" of luminance and color information between the luma and chroma components. The error is greatest for highly-saturated colors and can be somewhat noticeable in between the magenta and green bars of a color bars test pattern (that has chroma subsampling applied). This engineering approximation (by reversing the order of operations between gamma correction and forming the weighted sum) allows color subsampling to be more easily implemented.
Original without color subsampling. 200% zoom.
Image after color subsampling (compressed with Sony Vegas DV codec, box filtering applied.)
Sampling systems and ratios
The subsampling scheme is commonly expressed as a three part ratio J:a:b (e.g. 4:2:2), although sometimes expressed as four parts (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 (additional) chrominance samples (Cr, Cb) in the second row of J pixels.
- AlphaAlpha compositingIn computer graphics, alpha compositing is the process of combining an image with a background to create the appearance of partial or full transparency. It is often useful to render image elements in separate passes, and then combine the resulting multiple 2D images into a single, final image in a...
: horizontal factor (relative to first digit). May be omitted if alpha component is not present, and is equal to J when present.
An explanatory image of different chroma subsampling schemes can be seen at the following link: http://lea.hamradio.si/~s51kq/subsample.gif
(source: "Basics of Video": http://lea.hamradio.si/~s51kq/V-BAS.HTM) or in details in Chrominance Subsampling in Digital Images, by Douglas Kerr.
| 4:1:1 | 4:2:0 | 4:2:2 | 4:4:4 | |||||||||||||||||
| Y'CrCb | ||||||||||||||||||||
| = | = | = | = | |||||||||||||||||
| Y' | ||||||||||||||||||||
| + | + | + | + | |||||||||||||||||
| 1 | 2 | 3 | 4 | J = 4 | 1 | 2 | 3 | 4 | J = 4 | 1 | 2 | 3 | 4 | J = 4 | 1 | 2 | 3 | 4 | J = 4 | |
| (Cr, Cb) | 1 | a = 1 | 1 | 2 | a = 2 | 1 | 2 | a = 2 | 1 | 2 | 3 | 4 | a = 4 | |||||||
| 1 | b = 1 | b = 0 | 1 | 2 | b = 2 | 1 | 2 | 3 | 4 | b = 4 | ||||||||||
| ¼ horizontal resolution, full vertical resolution |
½ horizontal resolution, ½ vertical resolution |
½ horizontal resolution, full vertical resolution |
full horizontal resolution, full vertical resolution |
|||||||||||||||||
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
Aliasing
In signal processing and related disciplines, aliasing refers to an effect that causes different signals to become indistinguishable when sampled...
.
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).
4:4:4 Y'CbCr
Each of the three Y'CbCrYCbCr
YCbCr or Y′CbCr, sometimes written or , 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 component and CB and CR are the blue-difference and red-difference chroma components...
components have the same sample rate. This scheme is sometimes used in high-end film scanners and cinematic postproduction. Two SDI
Serial Digital Interface
Serial digital interface is a family of video interfaces standardized by SMPTE. For example, ITU-R BT.656 and SMPTE 259M define digital video interfaces used for broadcast-grade video...
links (connections) are normally required to carry this bandwidth: Link A would carry a 4:2:2 signal, Link B a 0:2:2, when combined would make 4:4:4.
4:4:4 R'G'B' (no subsampling)
Note that "4:4:4" may instead be referring to R'G'B' color space, which implicitly does not have any chroma subsampling at all. 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 sample rate of luma: the horizontal chroma resolution is halved. This reduces the bandwidth of an uncompressed video signal by one-third with little to no visual difference.Many high-end digital video formats and interfaces use this scheme:
- AVC-Intra 100AVC-IntraAVC-Intra is a type of video coding developed by Panasonic that is fully compliant with the H.264/MPEG-4 AVC standard and additionally follows the SMPTE RP 2027-2007 recommended practice specification....
- Digital BetacamBetacamBetacam is family of half-inch professional videocassette products developed by Sony in 1982. In colloquial use, "Betacam" singly is often used to refer to a Betacam camcorder, a Betacam tape, a Betacam video recorder or the format itself....
- DVCPRO50 and DVCPRO HD
- Digital-SDigital-SD-9 or Digital S as it was originally known, is a professional digital video videocassette format created by JVC in 1995. It is a direct competitor to Digital Betacam. Its name was changed to D-9 in 1999 by the SMPTE...
- CCIR 601CCIR 601ITU-R Recommendation BT.601, more commonly known by the abbreviations Rec. 601 or BT.601 is a standard published in 1982 by International Telecommunication Union - Radiocommunications sector for encoding interlaced analog video signals in digital video form...
/ Serial Digital InterfaceSerial Digital InterfaceSerial digital interface is a family of video interfaces standardized by SMPTE. For example, ITU-R BT.656 and SMPTE 259M define digital video interfaces used for broadcast-grade video...
/ D1D1 (Sony)D-1 is an SMPTE digital recording video standard, introduced in 1986 through efforts by SMPTE engineering committees. It started as a Sony and Bosch - BTS product and was the first major professional digital video format.- Format :... - ProRes (HQ, 422, LT, and Proxy)ProRes 422ProRes 422 is a standard-definition and high-definition lossy video compression format developed by Apple Inc. for use in post production. It was introduced in 2007 with Final Cut Studio 2 and is comparable to Avid's DNxHD codec which has the same purpose and uses similar bit rates...
- XDCAM HD422XDCAMXDCAM is a a series of products for digital recording using random access solid-state memory media, introduced by Sony in 2003. Four different product lines — the XDCAM SD, XDCAM HD, XDCAM EX and XDCAM HD422 — differ in types of encoder used, frame size, container type and in...
- Canon MXF HD422
4:2:1
Although this mode is technically defined, very few software or hardware codecs use this sampling mode. Cb horizontal resolution is twice as low as one of Cr (and four times as low as one of Y). This exploits the fact that human eye has less spatial sensitivity to blue/yellow than to red/green. NTSCNTSC
NTSC, named for the National Television System Committee, is the analog television system that is used in most of North America, most of South America , Burma, South Korea, Taiwan, Japan, the Philippines, and some Pacific island nations and territories .Most countries using the NTSC standard, as...
is similar, in using lower resolution for blue/yellow than red/green, which in turn has less resolution than luma.
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 DVDV
DV is a format for the digital recording and playing back of digital video. The DV codec was launched in 1995 with joint efforts of leading producers of video camcorders....
format was not considered to be broadcast quality and was only acceptable for low-end and consumer applications. Currently, DV-based formats (some of which use 4:1:1 chroma subsampling) are used professionally in electronic news gathering and in playout servers. DV has also been sporadically used in feature films and in digital cinematography
Digital cinematography
Digital cinematography is the process of capturing motion pictures as digital images, rather than on film. Digital capture may occur on video tape, hard disks, flash memory, or other media which can record digital data. As digital technology has improved, this practice has become increasingly common...
.
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 VCR's 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 (NTSCNTSCNTSC, named for the National Television System Committee, is the analog television system that is used in most of North America, most of South America , Burma, South Korea, Taiwan, Japan, the Philippines, and some Pacific island nations and territories .Most countries using the NTSC standard, as...
and PALPALPAL, short for Phase Alternating Line, is an analogue television colour encoding system used in broadcast television systems in many countries. Other common analogue television systems are NTSC and SECAM. This page primarily discusses the PAL colour encoding system...
) - NTSC DVDVDV is a format for the digital recording and playing back of digital video. The DV codec was launched in 1995 with joint efforts of leading producers of video camcorders....
and DVCAM - D-7
4:2:0
Different variants of 4:2:0 chroma configurations are found in:- All ISOInternational Organization for StandardizationThe International Organization for Standardization , widely known as ISO, is an international standard-setting body composed of representatives from various national standards organizations. Founded on February 23, 1947, the organization promulgates worldwide proprietary, industrial and commercial...
/IECInternational Electrotechnical CommissionThe International Electrotechnical Commission is a non-profit, non-governmental international standards organization that prepares and publishes International Standards for all electrical, electronic and related technologies – collectively known as "electrotechnology"...
MPEG and ITU-TITU-TThe ITU Telecommunication Standardization Sector is one of the three sectors of the International Telecommunication Union ; it coordinates standards for telecommunications....
VCEGVCEGThe Video Coding Experts Group or Visual Coding Experts Group is the informal name of Question 6 of Working Party 3 of Study Group 16 of the ITU-T. Its abbreviated title is ITU-T Q.6/SG 16...
H.26x video coding standards, including H.262/MPEG-2 Part 2H.262/MPEG-2 Part 2H.262 or MPEG-2 Part 2 is a digital video compression and encoding standard developed and maintained jointly by ITU-T Video Coding Experts Group and ISO/IEC Moving Picture Experts Group . It is the second part of the ISO/IEC MPEG-2 standard...
implementations such as DVDDVDA DVD is an optical disc storage media format, invented and developed by Philips, Sony, Toshiba, and Panasonic in 1995. DVDs offer higher storage capacity than Compact Discs while having the same dimensions....
(although some profiles of MPEG-4 Part 2MPEG-4 Part 2MPEG-4 Part 2, MPEG-4 Visual is a video compression technology developed by MPEG. It belongs to the MPEG-4 ISO/IEC standards. It is a discrete cosine transform compression standard, similar to previous standards such as MPEG-1 and MPEG-2...
and H.264/MPEG-4 AVCH.264/MPEG-4 AVCH.264/MPEG-4 Part 10 or AVC is a standard for video compression, and is currently one of the most commonly used formats for the recording, compression, and distribution of high definition video...
allow higher-quality sampling schemes such as 4:4:4) - PAL DVDVDV is a format for the digital recording and playing back of digital video. The DV codec was launched in 1995 with joint efforts of leading producers of video camcorders....
and DVCAM - HDVHDVHDV is a format for recording of high-definition video on DV cassette tape. The format was originally developed by JVC and supported by Sony, Canon and Sharp...
- AVCHDAVCHDAVCHD is a file-based format for the digital recording and playback of high-definition video....
and AVC-Intra 50AVC-IntraAVC-Intra is a type of video coding developed by Panasonic that is fully compliant with the H.264/MPEG-4 AVC standard and additionally follows the SMPTE RP 2027-2007 recommended practice specification.... - Apple Intermediate CodecApple Intermediate CodecThe Apple Intermediate Codec abbreviated AIC is a video codec designed by Apple Inc. to be an intermediate format in an HDV workflow. It features high performance and quality, being less processor intensive to work with than other editing formats...
- most common JPEG/JFIFJPEGIn computing, JPEG . The degree of compression can be adjusted, allowing a selectable tradeoff between storage size and image quality. JPEG typically achieves 10:1 compression with little perceptible loss in image quality....
and MJPEGMJPEGIn multimedia, Motion JPEG is an informal name for a class of video formats where each video frame or interlaced field of a digital video sequence is separately compressed as a JPEG image...
implementations - VC-1VC-1VC-1 is the informal name of the SMPTE 421M video codec standard, which was initially developed as a proprietary video format by Microsoft before it was released as a formal SMPTE standard video format on April 3, 2006...
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.
- In MPEG-2, 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.
The PAL and SECAM
SECAM
SECAM, also written SÉCAM , is an analog color television system first used in France....
color systems are especially well-suited to this kind of data reduction. 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 2X2. 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 x 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.- 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).
Out-of-gamut colors
One of the artifacts that can occur with chroma subsampling is that out-of-gamutGamut
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 circumstance, such as within a given color space or by a...
colors can occur upon chroma reconstruction. Suppose the image consisted of alternating 1-pixel red and black lines. 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.
Filtering during subsampling can also cause colors to go out of gamut.
Terminology
The term Y'UVYUV
YUV is a color space typically used as part of a color image pipeline. It encodes a color image or video taking human perception into account, allowing reduced bandwidth for chrominance components, thereby typically enabling transmission errors or compression artifacts to be more efficiently...
refers to an analog encoding scheme 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).
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
International Commission on Illumination
The International Commission on Illumination is the international authority on light, illumination, color, and color spaces...
). 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
International Commission on Illumination
The International Commission on Illumination is the international authority on light, illumination, color, and color spaces...
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/chrominance of video engineering differs from the chrominance of color science. The chroma/chrominance of video engineering is formed from weighted tristimulus components, not linear components. In video engineering practice, the terms chroma, chrominance, and saturation are often used interchangeably to refer to chrominance.
History
Chroma subsampling was developed in the 1950s by Alda Bedford for the development of color television by RCARCA
RCA Corporation, founded as the Radio Corporation of America, was an American electronics company in existence from 1919 to 1986. The RCA trademark is currently owned by the French conglomerate Technicolor SA through RCA Trademark Management S.A., a company owned by Technicolor...
, which developed into the NTSC
NTSC
NTSC, named for the National Television System Committee, is the analog television system that is used in most of North America, most of South America , Burma, South Korea, Taiwan, Japan, the Philippines, and some Pacific island nations and territories .Most countries using the NTSC standard, as...
standard; luma-chroma separation was developed earlier, in 1938 by Georges Valensi
Georges Valensi
Georges Valensi was a French telecommunications engineer who, in 1938, invented and patented a method of transmitting color images so that they could be received on both color and black & white television sets....
.
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
YIQ
YIQ is the color space used by the NTSC color TV system, employed mainly in North and Central America, and Japan. It is currently in use only for low-power television stations, as full-power analog transmission was ended by the U.S. Federal Communications Commission on 12 June 2009...
color space, and is roughly analogous to 4:2:1 subsampling, in that it has decreasing resolution for luma, yellow/green, and red/blue.
Effectiveness
While chroma subsampling can easily reduce the size of an uncompressed image by 50% with minimal loss of quality, the final effect on the size of a compressed image is considerably less. This is because image compression algorithms also remove redundant chroma information. In fact, by applying something as rudimentary as chroma subsampling prior to compression, information is removed from the image that could be used by the compression algorithm to produce a higher quality result with no increase in size. For example, with wavelet compression methods, better results are obtained by dropping the highest frequency chroma layer inside the compression algorithm than by applying chroma subsampling prior to compression. This is because wavelet compression operates by repeatedly using wavelets as high and low pass filters to separate frequency bands in an image, and the wavelets do a better job than chroma subsampling does.Controversy
The details of chroma subsampling implementation cause considerable confusion. Is the upper leftmost chroma value stored, or the rightmost, or is it the average of all the chroma values? This must be exactly specified in standards and followed by all implementors. Incorrect implementations cause the chroma of an image to be offset from the luma. Repeated compression/decompression can cause the chroma to "travel" in one direction. Different standards may use different versions for example of "4:2:0" with respect to how the chroma value is determined, making one version of "4:2:0" incompatible with another version of "4:2:0".Proper upsampling of chroma can require knowing whether the source is progressive or interlaced, information which is often not available to the upsampler.
Chroma subsampling causes problems for film makers trying to do keying with blue or green screening. The chroma interpolation along edges produces noticeable haloing artifacts.
See also
- Color spaceColor spaceA color model is an abstract mathematical model describing the way colors can be represented as tuples of numbers, typically as three or four values or color components...
- SMPTE - Society of Motion Picture and Television Engineers
- Digital videoDigital videoDigital video is a type of digital recording system that works by using a digital rather than an analog video signal.The terms camera, video camera, and camcorder are used interchangeably in this article.- History :...
- HDTV
- YCbCrYCbCrYCbCr or Y′CbCr, sometimes written or , 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 component and CB and CR are the blue-difference and red-difference chroma components...
- YPbPrYPbPr' is a color space used in video electronics, in particular in reference to component video cables. is the analog version of the YCBCR color space; the two are numerically equivalent, but YPBPR is designed for use in analog systems whereas YCBCR is intended for digital video. cables are also...
- CCIR 601CCIR 601ITU-R Recommendation BT.601, more commonly known by the abbreviations Rec. 601 or BT.601 is a standard published in 1982 by International Telecommunication Union - Radiocommunications sector for encoding interlaced analog video signals in digital video form...
4:2:2 SDTV - YUVYUVYUV is a color space typically used as part of a color image pipeline. It encodes a color image or video taking human perception into account, allowing reduced bandwidth for chrominance components, thereby typically enabling transmission errors or compression artifacts to be more efficiently...
- ColorColorColor or colour is the visual perceptual property corresponding in humans to the categories called red, green, blue and others. Color derives from the spectrum of light interacting in the eye with the spectral sensitivities of the light receptors...
- color visionColor visionColor vision is the capacity of an organism or machine to distinguish objects based on the wavelengths of the light they reflect, emit, or transmit...
- Rod cellRod cellRod cells, or rods, are photoreceptor cells in the retina of the eye that can function in less intense light than can the other type of visual photoreceptor, cone cells. Named for their cylindrical shape, rods are concentrated at the outer edges of the retina and are used in peripheral vision. On...
- cone cells
- Rod cell
- Better pictorial explanation here http://www.nattress.com/Chroma_Investigation/chromasampling.htm