Information about Chroma Subsampling
Chroma subsampling is the practice of implementing more resolution for the (quantity representative of) luminance than the (quantity representative of) color. It is used in many video encoding schemes (both analog and digital) and also in JPEG encoding.
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.)
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.
Many high-end digital video formats and interfaces use this scheme:
Formats that use 4:1:1 chroma subsampling include:
There are three variants of 4:2:0 schemes, having different horizontal and vertical siting.
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.
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.[3][4]
In the vertical dimension, both luma and chroma are sampled at the full HD sampling rate (1080 samples vertically).
In a similar vein, the term luminance and symbol Y is often (erroneously) used to refer to luma, 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 color engineering practice, the terms chroma, chrominance, and saturation are often (and perhaps ambiguously!) used to refer to the same concept.
(See for formats and for codecs)
Why subsampling works
Because the human eye is less sensitive to 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.)
Sampling systems and ratios
The subsampling scheme is commonly expressed as a three part ratio (e.g. 4:2:2), although sometimes expressed as four parts (e.g. 4:2:2:4). The parts are (in their respective order):- Luma horizontal sampling reference (originally, as a multiple of 3.579 MHz in the NTSC television system)
- Cr horizontal factor (relative to first digit)
- Cb horizontal factor (relative to first digit), except when zero. Zero indicates that Cb horizontal factor is equal to second digit, and, in addition, both Cr and Cb are subsampled 2:1 vertically. Zero is chosen for the bandwidth calculation formula (see below) to remain correct.
- Alpha horizontal factor (relative to first digit). May be omitted if alpha component is not present.
)
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.
Why is this done?
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 Y' than the color difference components Cb and Cr. The 4:2:2 Y'CbCr scheme for example 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.Types of subsampling
8:4:4 Y'CbCr
Each of the two Chroma, Cb Cr, components have the same sample rate. The Luminance has twice the resolution as Chroma components. This scheme is sometimes used in high-end Film scanners, DataCine, telecine and color grading. In NTSC this would be about 10 MHz Luma and 5 MHz chroma resolution, (as compared to 4:4:4: in which all three would have 5 MHz resolution.) Two links (connections) are required to carry this bandwidth. These links are often referred to as Link A and Link B. Each link would carry a 4:2:2 signal, when combined these would make 8:4:4. A down sample converter could later convert 8:4:4 to 4:4:4 or 4:2:2.4:4:4 Y'CbCr
Each of the three Y'CbCr components have the same sample rate. This scheme is sometimes used in high-end film scanners and cinematic postproduction. Two 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, so horizontal chroma resolution is cut in half. This reduces the bandwidth of a video signal by one-third with little to no visual difference.Many high-end digital video formats and interfaces use this scheme:
- Digital Betacam
- DVCPRO50 and DVCPRO HD
- Digital-S
- CCIR 601 / Serial Digital Interface / D1
- ProRes 422
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 is less sensitive to blue color than to red.4:1:1
In 4:1:1 chroma subsampling, the horizontal color resolution is quartered. The bandwidth is halved compared to no chroma subsampling. In some professional circles, the 4:1:1 chroma subsampling of the DV format was initially not considered broadcast quality and only acceptable for low-end and consumer applications[1][2]. Currently, DV-based formats (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.Formats that use 4:1:1 chroma subsampling include:
4:2:0
This scheme is found in:- All versions of MPEG, including MPEG-2 implementations such as DVD (although some profiles of MPEG-4 allow higher-quality sampling schemes such as 4:4:4)
- PAL DV and DVCAM
- HDV
- most common JPEG/JFIF, H.261, and MJPEG implementations
- VC-1
There are three variants of 4:2:0 schemes, having different horizontal and vertical siting.
- In MPEG-2, Cb and Cr are cosited horizontally.
- 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 alternate line by line.
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.
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.[3][4]
4:1:0
This ratio is possible (indeed, some codecs do support it), but not widely used. It means half the vertical and quarter 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 (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 higher than VHS quality without having to take too large of a hit on bandwidth.
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).
Terminology
The term Y'UV 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 (erroneously) used to refer to Y'CbCr encoding. Hence, terms 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 symbol Y is often (erroneously) used to refer to luma, 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 color engineering practice, the terms chroma, chrominance, and saturation are often (and perhaps ambiguously!) used to refer to the same concept.
See Also
- Color space
- SMPTE - Society of Motion Picture and Television Engineers
- Digital video
- HDTV
- YCbCr
- YPbPr
- CCIR 601 4:2:2 SDTV
- YUV
- Color
- color vision
- Rod cell
- cone cells
References
- Poynton, Charles. "YUV and luminance considered harmful: A plea for precise terminology in video" http://www.poynton.com/papers/YUV_and_luminance_harmful.html
- Poynton, Charles. "Digital Video and HDTV: Algorithms and Interfaces." USA: Morgan Kaufmann Publishers, 2003.
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Color or colour[1] (see spelling differences) is the visual perceptual property corresponding in humans to the categories called red, yellow, blue, black, etc.
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JPEG
A photo of a flower compressed with successively more lossy compression ratios from left to right.
File extension:
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A photo of a flower compressed with successively more lossy compression ratios from left to right.
File extension:
.jpeg, .jpg, .jpe
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Eyes are organs of vision that detect light. Different kinds of light-sensitive organs are found in a variety of organisms. The simplest eyes do nothing but detect whether the surroundings are light or dark, while more complex eyes can distinguish shapes and colors.
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Color or colour[1] (see spelling differences) is the visual perceptual property corresponding in humans to the categories called red, yellow, blue, black, etc.
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Luminance is a photometric measure of the density of luminous intensity in a given direction. It describes the amount of light that passes through or is emitted from a particular area, and falls within a given solid angle.
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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 (a continuous-time signal) to a sequence of samples (a discrete-time signal).
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As applied to video signals, luma represents the brightness in an image (the "black and white" or achromatic portion of the image). Luma is typically paired with chroma. Luma represents the achromatic image without any color, while the chroma components represent the color
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Chrominance (chroma for short), is the signal used in many video systems to carry the color information of the picture separately from the accompanying luma signal.
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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.
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In graphics and visual effects, keying is an informal term for compositing two full frame images together, by discriminating the visual information into values of color and light.
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aliasing refers to an effect that causes different continuous signals to become indistinguishable (or aliases of one another) when sampled. It also refers to the distortion or artifact that results when a signal is sampled and reconstructed as an alias of the original
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RGB color model is an additive model in which red, green, and blue (often used in additive light models) are combined in various ways to reproduce other colors. The name of the model and the abbreviation ‘RGB’ come from the three primary colors, red, green, and blue and
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film scanner is a device made for scanning photographic film directly into a computer without the use of any intermediate printmaking. They provide several benefits over using a flatbed scanner to scan in a print of any size — the photographer has direct control over cropping
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Telecine (IPA pronunciation: [ˈtɛləˌsɪni] or [ˌtɛləˈsɪni]; [ˌtɛləˈsɪnə]; also [ˌtɛləˈsiːn].
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You can assist by [ editing it] now. A how-to guide is available, as is general .
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YCbCr or Y'CbCr is a family of color spaces used in video and digital photography systems. Y' is the luma component and Cb and Cr are the blue and red chroma components. The prime on the Y is to distinguish the luma from luminance.
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RGB color model is an additive model in which red, green, and blue (often used in additive light models) are combined in various ways to reproduce other colors. The name of the model and the abbreviation ‘RGB’ come from the three primary colors, red, green, and blue and
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Betacam is a family of half-inch professional videotape products developed by Sony from 1982 onwards. In casual use, "Betacam" singly is often used to refer to a Betacam camcorder, a Betacam tape, or a Betacam video recorder.
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Digital Video (DV) is a digital video format launched in 1994, and, in its smaller tape form factor MiniDV, has since become a standard for home and semiprofessional video production; it is sometimes used for professional purposes as well, such as filmmaking and electronic
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D-9 or Digital S as it was originally known, is a professional digital videotape 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.
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CCIR 601 is the old name of a standard published by the CCIR (now ITU-R) for encoding interlaced analogue video signals in digital form. The new name of the standard is ITU-R BT.601, but the old name is still in common use in informal contexts.
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D-1 stored uncompressed digitized component video, encoded at using the CCIR 601 raster format, along with PCM audio tracks as well as
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Digital Video (DV) is a digital video format launched in 1994, and, in its smaller tape form factor MiniDV, has since become a standard for home and semiprofessional video production; it is sometimes used for professional purposes as well, such as filmmaking and electronic
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Digital cinematography is the process of capturing motion pictures as digital images, rather than on film. Digital capture may occur on tape, hard disks, flash memory, or other media which can record digital data.
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