BT.709 (or Rec.709) and BT.2020 (or Rec.2020) are RGB Color Spaces
. RGB Color Spaces, also known as Color Models
, are generally labeled R’G’B’ (pronounced “R-prime, G-prime, B-prime”) by color scientists because they are “gamma-corrected” spaces. Color scientists reserve RGB to refer to non-gamma-corrected (or “linear”) spaces that use red, green, and blue primaries.
[For more on Color Space, see my post >> HERE
With the exception of a few rare cases, every pixel on your screen is made up of Red, Green, and Blue sub-pixels. Therefore, everything sent into your display becomes RGB at some point. However, not everything sent to your display starts off as RGB.
Even though video cameras physically measure RGB values, and displays are made using RGB primaries, video is stored, transmitted, and processed in a color encoding system called YCbCr (or Y′CbCr). When your TV receives a YCbCr signal (e.g. from a Blu-ray player), it processes that signal and converts it to RGB - since that is what the TV uses to display images.
So why do we have YCbCr and RGB? It’s a long story. Let me just say that it involves and goes back to Black & White TV, the transition to color, and human visual perception. In practical application, YCbCr is no different than RGB in terms of quality since humans are far more sensitive to black-and-white information (luminance) than they are to color information (chrominance) - especially in moving images.
is a process where color information (chroma) is “sacrificed” in order to reduce bandwidth and file sizes. By a convoluted method of trial and error, and lots of testing, they discovered that they could actually throw out half or even three fourths of the color information and most people wouldn’t even notice. Subsampling (or downsampling) is what happens when you don’t sample 1:1 - which means you throw out some of the data.
Note: Chroma subsampling only applies to YCbCr encoding systems. An RGB color model implicitly does NOT use any chroma subsampling.
You can’t have a 4:2:2 RGB stream. What would this mean? What’s the meaning of half green and half blue? Nor can you have a 4:2:0 RGB stream. What kind of image would it be if it can never have blue at all? When you’re talking about RGB, all three channels must contain the same number (i.e. 4:4:4).
is the color encoding system used for video
sources. It has a native "limited" color range of 16-235 (for 8-bit sources), where everything below 16 is considered "Blacker-than-Black" and everything above 235 is "Whiter-than-White". All video discs, including Blu-rays and UHD Blu-rays, are encoded as YCbCr. Most HDMI devices (Blu-ray players, Satellite receivers, Cable boxes) will output a YCrCb signal. It is usually best to set these source components to output a YCbCr signal not RGB.
For instance, Blu-ray movies are encoded in YCbCr 4:2:0. If you set your Blu-ray player to output an RGB signal, the player needs to first upsample the YCbCr 4:2:0 encode to YCbCr 4:4:4 and then convert it to RGB before sending it to the TV. When you set it to output a 4:2:2 signal, the player first scales the 4:2:0 encode up to 4:2:2, sends that signal to the TV, and then the TV does the upscaling to 4:4:4, converts it to RGB, and sends it to the display controller. If you set it to output a YCbCr 4:4:4 signal, the TV doesn't need to do any upscaling at all since the player takes care of the upsampling, but it still needs to process that signal and convert it to RGB.
In practice, resampling (whether upsampling or downsampling) the color portions of an image is very much like scaling an image from one pixel size to another. Subsampling is in effect a very simple lossy compression scheme, throwing away portions of the image that are less important for perception (the detailed color information) in order to devote more resources to the important stuff (the black-and-white details).
In a Y’CbCr encoding system, the luminance information (or luma
signal) is transmitted separately from the color information and a "color difference system"
is used to derive green. Y′ (pronounced Y-prime) is the luma
component and Cb & Cr are the blue-difference and red-difference chroma
components - they are derived from B-Y’ and R-Y’.
Therefore, the luma signal (Y′) can be stored in high resolution and transmitted at high bandwidth, and the two chroma components (Cb and Cr) can be bandwidth-reduced, subsampled, compressed, or otherwise treated separately for improved system efficiency.
If the maximum possible quality of a full color image of a YCbCr signal is "12".
- 4:4:4 = 4+4+4 = 12, or 100% of maximum possible quality
- 4:2:2 = 4+2+2 = 8, which is 66.7% of 4:4:4
- 4:2:0 = 4+2+0 = 6, which is 50% of 4:4:4
Therefore, if a 4:4:4 uncompressed frame is 24 MB, then a 4:2:2 frame will be reduced to 16 MB, and a 4:2:0 will be 12 MB. Note: This is before any compression has been applied
For 4:4:4, each of the three YCbCr components have the same sample rate, thus there is no chroma subsampling
. For every 4 Y pixels, there are 4 Cb and Cr pixels on the even scan lines and 4 Cb and Cr pixels on the odd scan lines. This scheme is sometimes used in high-end film scanners and cinematic post production.
For 4:2:2, the two chroma components are sampled at half the sample rate of luma: the horizontal chroma resolution is halved but
the full vertical resolution is retained. For every 4 Y pixels, there are 2 Cb and Cr pixels on the even lines and 2 Cb and Cr pixels on the odd lines. This reduces the bandwidth of an uncompressed video signal by one-third with little to no visual difference.
For 4:2:0, both the horizontal and
the vertical chroma resolution is halved since the Cb and Cr chroma channels are only sampled on each alternate line. For every 4 Y pixels on the even scan lines, there will be 2 Cb and Cr pixels and for every 4 Y pixels on the odd scan lines there will be 0 Cb and Cr pixels. For television broadcasts, videos, and movies, 4:2:0 subsampling reduces the file size by half - before any compression has been applied.
However, while subsampling can easily reduce the size of an uncompressed image by up to 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.
The important thing to understand and remember is, when it comes to viewing movies, TV shows, or videos, very few people will be able to notice any difference in quality between 4:4:4 and 4:2:0 subsampling.