Chroma subsampling
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In digital image processing, chroma subsampling is the use of lower resolution for the colour (chroma) information in an image than for the brightness (intensity or luma) information. It is used when an analog component video or YUV signal is digitally sampled.
Because the human eye is less sensitive to colour than intensity, the chroma components of an image need not be as well defined as the luma component, so many video systems sample the colour difference channels at a lower definition (i.e., sample frequency) than the brightness. This reduces the overall bandwidth of the video signal without apparent loss of picture quality. The missing values will be interpolated or repeated from the preceding sample for that channel.
The subsampling in a video system is usually expressed as a three part ratio. The three terms of the ratio are: the number of brightness ("luminance" "luma" or Y) samples, followed by the number of samples of the two colour ("chroma") components: U/Cb then V/Cr, for each complete sample area. For quality comparison, only the ratio between those values is important, so 4:4:4 could easily be called 1:1:1; however, traditionally the value for brightness is always 4, with the rest of the values scaled accordingly.
Sometimes, four part relations are written, like 4:2:2:4. In these cases, the fourth number means the sampling frequency ratio of a key channel.In virtually all cases, that number will be 4, since high quality is very desirable in keying applications.
NOTE: The mapping examples given are only theoretical and for illustration. The bitstreams of real-life implementations will probably differ.
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4:4:4
Each of the three channels has the same sample rate, so each single pixel in the resulting image gets three full words (usually 8 or 10 bits long) of information, resulting in 3 bytes per pixel for 8-bit quantization when not using compression.
Mapping:
The bitstream
Y0 U0 V0 Y1 U1 V1 Y2 U2 V2 Y3 U3 V3
will map to the following four pixels:
[Y0 U0 V0] [Y1 U1 V1] [Y2 U2 V2] [Y3 U3 V3]
This is the best color sampling ratio (it yields a perfect representation of each pixel's color), and is used as an intermediate format in high-end film scanners and cinematic postproduction. Note that 4:4:4 may (and often indeed does, as in some modes of HDCAM SR) also mean that the three values are all color values in the RGB color space, which must always be sampled at the same frequency.
4:2:2
Each of the two color-difference channels has half the sample rate of the brightness channel, so horizontal color resolution is only half that of 4:4:4. For uncompressed video and 8-bit quantization, each macropixel of two neighbouring pixels uses 4 bytes of memory.
Mapping:
The bitstream
Y0 U0 Y1 V1 Y2 U2 Y3 V3
will map to the following four pixels:
[Y0 U0 V1] [Y1 U0 V1] [Y2 U2 V3] [Y3 U2 V3]
This is still a very good quality, and most higher-end digital video formats use this ratio:
4:1:1
In 4:1:1 chroma subsampling, the horizontal color resolution is quartered. This is still acceptable for lower-end and consumer applications. Uncompressed video in this format with 8-bit quantization uses 6 bytes for every macropixel (4 pixels in a row).
Mapping:
The bitstream
Y0 U0 Y1 Y2 V2 Y3
will map to the following four pixels:
[Y0 U0 V2] [Y1 U0 V2] [Y2 U0 V2] [Y3 U0 V2]
Formats that use 4:1:1 chroma subsampling include:
4:2:0
4:2:0 does not mean that there is no V or Cr information stored at all, it means that in each line, only one color difference channel is stored with half the horizontal resolution. The channel which is stored flips each line, so the ratio is 4:2:0 for one line, 4:0:2 in the next, then 4:2:0 again, and so on. This leads to half the horizontal as well as half the vertical resolution, giving a quarter of the color resolution overall. The PAL and SECAM color systems are especially well-suited to this kind of data reduction. Uncompressed video in this format with 8-bit quantization uses 6 bytes for every macropixel (2×2 pixels).
Mapping:
The bitstream
Yo0 Uo0 Yo1 Yo2 Uo2 Yo3
Ye0 Ve0 Ye1 Ye2 Ve2 Ye3
will map to the following two lines of four pixels each:
[Yo0 Uo0 Ve0] [Yo1 Uo0 Ve0] [Yo2 Uo2 Ve2] [Yo3 Uo2 Ve2]
[Ye0 Uo0 Ve0] [Ye1 Uo0 Ve0] [Ye2 Uo2 Ve2] [Ye3 Uo2 Ve2]
The quality of this method is very close to 4:1:1, and it is used in the following formats:
- DVD and other Main Profile MPEG-2 implementations
- PAL DV and DVCAM
- most common JPEG and MJPEG implementations
4:1:0
This ratio is possible (indeed, some codecs do support it), but not widely used, since its color fidelity is even below that of VHS. 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).
Mapping:
The bitstream
Yo0 Uo0 Yo1 Yo2 Yo3
Ye0 Ve0 Ye1 Ye2 Ye3
will map to the following two lines of four pixels each:
[Yo0 Uo0 Ve0] [Yo1 Uo0 Ve0] [Yo2 Uo0 Ve0] [Yo3 Uo0 Ve0]
[Ye0 Uo0 Ve0] [Ye1 Uo0 Ve0] [Ye2 Uo0 Ve0] [Ye3 Uo0 Ve0]de:Chroma Subsampling