YUV Pixel Formats Overview
The YUV model defines a color space in terms of one luma (Y) and two chrominance (UV) components. The YUV color model is used for analog encoding of color information in television transmissions (PAL). Y stands for the luminance component (the brightness) and U (Cb) and V (Cr) are the chrominance (color) components.
The requirement for a compatible signal transmission method for both black-and-white and color information led to the invention of YUV pixel formats. The UV signal was added to the existing luma component (Y) black-and-white signal. This enabled color television transmissions within the black and white infrastructure.
The YUV formats are divided into two groups:
- Interleaved formats in which the Y, U (Cb) and V (Cr) samples are packed together into macropixels that are stored in a single array.
- Planar format in which each component (Y, U (Cb) and V (Cr)) is stored as a separate array. The final image is a fusion of the three separate planes.
Subsampling intervals
Chroma subsampling is the practice of encoding images by implementing less resolution for chroma information (U (Cb) and V (Cr)) than for the luma (Y) information.
The following table shows how U and V are sampled relative to Y.
| Subsampling scheme | Description |
|---|---|
| 4:4:4 | No downsampling of the chroma channels. |
| 4:2:2 | 2:1 horizontal downsampling, with no vertical downsampling. Every scanline contains 4 Y samples for every 2 U or V samples. |
| 4:2:0 | 2:1 horizontal downsampling, with 2:1 vertical downsampling. |
The subsampling scheme is commonly expressed as a three part ratio J : a : b, where,
- J is the horizontal sampling reference (the width of the conceptual region). This is usually, 4
- a is the number of chrominance samples (Cr, Cb) in the first row of J pixels.
- b is the number of (additional) chrominance samples (Cr, Cb) in the second row of J pixels.
The horizontal subsampling interval describes how frequently across a line a sample of that component is taken. The vertical subsampling interval describes on which lines samples are taken.
For example, the UYVY (4:2:2) formats have a horizontal subsampling period of 2 for both the U and V components. This indicates that the U and V samples are taken for every second pixel across a line. The vertical subsampling period is 1, which indicates that U and V samples are taken on each line of the image. The following table summarizes the horizontal and vertical subsampling intervals for the YUV 4:2:2 formats.
Contrast this to a subsampling scheme of 4:4:4. This has a horizontal subsample period of 1 for both the U and V components and a vertical subsample period of 1 for both the U and V components.
This reduction results in almost no perceivable visual difference.
- YUV Pixel Formats
This topic provides detailed information about the supported YUV pixel formats.
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