What is chroma subsampling?

Chroma subsampling is one of the most effective methods for viewing high resolution, high refresh rate content without the huge bandwidth required to transmit such signals. Like display stream compression, chroma subsampling is a compression technique that, when used effectively, is nearly indistinguishable from the original image.

It works by removing some color information from a picture or video, while retaining all luminance, or brightness, information. This reduces bandwidth, opening up more space for higher resolution and refresh rate signals. This makes high refresh rate UHD monitors possible, and is a technology without which, platforms like Disney+ and Netflix wouldn’t work as well.

What is chroma subsampling?

Chroma subsampling removes some color information from a signal image or video, while retaining information related to brightness. This reduces the overall bandwidth required to transmit the signal, while not substantially affecting the final image. Unlike display stream compression, which is visually lossless, chroma subsampling produces artifacts on occasion, although it is usually hard to detect unless you are looking for it.

In fact the reason chroma subsampling is so effective and so widely used is a trick of the human eye. While humans are excellent at recognizing contrast – that’s the difference between light and dark – they are not so good at recognizing differences in color. This is why in chroma subsampled images and videos, full brightness data is transmitted, but color data is restricted. Although this reduces variation in color within an image, even for a trained eye it is very hard to identify unless they are really looking for it.

This is why most movies are broadcast with chroma subsampling used on all major streaming platforms, and is also used with UHD Blu-ray, which is largely considered the highest quality way to watch movies. goes.

The above image shows how little difference it makes when removing large portions of color resolution with more extreme chroma subsampling techniques.

How does chroma subsampling work?

A standard video signal consists of two main components: brightness information, and color information. Fully uncompressed, such a signal is called 4:4:4, where each pixel has its own set of chrominance (color) values. This is the highest possible quality for a video, with no sub-sampling involved.

A moderately compressed signal is called 4:2:2, which contains color information for every other pixel, with those other pixels copying the color information from their neighbors. So in a two by two block of pixels, each pixel has its own luminance data, while each group of two shares color data. It is often used in professional video editing, where it reduces the data rate without affecting the visual quality.

Broadcast, streaming and UHD Blu-ray use the more aggressive chroma subsampling of 4:2:0, where there is only one pixel containing color data every four pixels. Blocks of two by two pixels will contain luminance data for each pixel, but they all share color data. Although this seems dramatic (and has an impact on bandwidth Is Dramatic) It is still almost impossible to distinguish the end result from 4:2:2, or 4:4:4.

Chroma subsampling is not usually enabled by default on PC monitors, as it can cause problems with the legibility of small text – something that is more easily seen on monitors than TVs.

Should you use chroma subsampling?

Chroma subsampling is one of several compression techniques that can be used to reduce the overall bandwidth required to transmit video. For consumers, DSC has largely replaced chroma subsampling, as it is completely visually invisible and unlocks the additional bandwidth needed for higher refresh rates and resolution.

However, this does not mean that chroma subsampling is unnecessary. In fact, it is still a key component in the functionality of modern streaming services and modern UHD Blu-rays, which take advantage of it to store the masses of chroma and luminance data required for 4K resolution video.

Chroma subsampling is still useful when using older displays that do not support DSC. It is also a key component of some still image algorithms, such as those by which JPEGs are created.

Suffice to say, chroma subsampling can still be useful and is routinely used in a variety of industries and specific settings. However, for general consumers, DSC is arguably a more effective compression system. This is far more modern and in the cases where you can use it, you’re more likely to find it enabled by default, so you shouldn’t need to manually use chroma subsampling.