Chroma Subsampling is a method of encoding an image or video with less chroma data than luma data. What this effectively means, is that the levels of black and white in an image are retained, whilst reducing the number of colors the image can display. That retains most of the quality of the picture or video, whilst reducing its overall file size, making it easier to transmit. The disadvantage is that this does affect image quality, especially with more extreme chroma subsampling formats.
What makes chroma subsampling particularly clever, however, is that it exploits the human visual system’s weaker ability to interpret color differences than luminance differences to inhibit the image in ways in which it’s harder for a human eye to notice. That said, some chroma subsampling formats remove enough color information that the effect is obvious, even to those who don’t know what to look for.
Although it’s most often talked about in the context of video compression, chroma subsampling is arguably used most often in JPEG images. They use chroma subsampling to convert red, green, and blue data into luminance, chroma blue, and chroma red, reducing overall image file size, whilst retaining most of the original image’s clarity. This wouldn’t be possible without chroma subsampling, and the internet as we know it would be very different without JPEG images.
What is Chroma Subsampling?
Chroma subsampling is a method used to reduce the amount of data in a photo or video. Where technology like Display Stream Compression uses compression algorithms to perform a similar function, chroma subsampling cuts down on the amount of information in the image or video stream in the first place.
Like DSC, though, chroma subsampling is designed to retain as much of the original image or video’s quality as possible; at least from the perspective of the viewer. Since the human eye can very clearly distinguish differences in brightness – IE. contrast – but not so clearly distinguish differences in color, chroma subsampling limits the color resolution in a way that is designed to be as imperceptible as possible.
That reduction in color information means that a greater quantity of luminance information can be transmitted instead, allowing for higher resolutions or refresh rates to be displayed than might otherwise be possible. This enables older generations of HDMI and DisplayPort cables to transmit video at resolutions and refresh rates that might otherwise fall outside of their specifications. It can also be used to help reduce the total bandwidth of images and video transmitted over the internet, like in the creation of JPEG images.
There are different types of chroma subsampling, typically referred to as chroma subsampling formats. They involve different levels of chroma subsampling, from standard, to mild, to more extreme examples with very limited color information.
Chroma Subsampling Formats
There are four typical chroma subsampling formats, designated by a string of three numbers in the form of a ratio, such as 4:4:4, or 4:2:2. These numbers equate to the quantity of information of a particular component of the image.
The first number denotes the number of horizontally sampled pixels, so in both of the previous examples, that would be four. The second number denotes the quantity of chrominance (or color) samples in the first row, including both red and blue chromatic data. The third denotes the number of vertical samples taken.
So in our previous example, the 4:4:4 has four horizontal and vertical samples, with four chrominance samples within that defined range. It’s a signal without any kind of color reduction and is effectively, uncompressed. The 4:2:2 example, however, has just two chrominance samples within those four horizontal and two vertical samples, which reduces the amount of color data in the image or video, reducing its file size in turn.
The most commonly used chroma subsampling formats are:
- 4:4:4 is a standard video or image signal without any kind of color reduction. It contains all the horizontal and vertical color resolutions without any loss. Although this signal can still use compression in the form of DSC, it isn’t leveraging chroma subsampling to limit color data. It’s typically used in film post-production.
- 4:2:2 cuts the amount of color data of 4:4:4 images and video in half, which has the effect of reducing the amount of overall data in the stream by a third. Thanks to the way the human eye processes luminance and color data, however, this reduction in color resolution will be almost imperceptible for most viewers. This can be used in combination with DSC to allow cables, like HDMI 2.1, to deliver higher resolution or refresh rates than they would otherwise be capable of. It was also used in high-end digital video formats, like Digital BetaCam, and Digital-S.
- 4:2:0 chroma subsampling has the same amount of color data as 4:2:2, but only samples it every other vertical line, cutting the vertical color resolution dramatically. Despite this severe reduction in overall data, the end result is still hard to distinguish from a 4:4:4 image or video, so 4:2:0 chroma subsampling has been used extensively. It’s often used in DVD-Video and Blu-ray discs but is most commonly used in the creation of JPEG images.
There are other chroma subsampling formats, such as 4:1:1, and 4:4:0, but they are far less common as their effects on visual quality are far more obvious, and fall below the level considered acceptable for broadcast quality. While it’s still hard to spot much of a difference in general viewing, in specific scenarios, particularly with on-screen text, or high contrasting colors, it’s more noticeable.
Even though the 4:4:4 native video or image without any kind of chroma subsampling is unquestionably the higher quality image, the visual difference between different chroma subsampling formats is actually quite small, as the comparison image above shows. While more extreme chroma subsampling with reduced chroma information does lead to a much lower resolution of color within the image, the human eye just cannot distinguish it in the same way as lowering the overall resolution.
Ultimately, this means that chroma subsampling is an effective way to reduce the required bandwidth to transmit video or imagery either online or down a specific cable, without having an overt negative impact on visual quality.
Chroma Subsampling in Video Compression
Compression is an important component in all manner of digital media, but it’s especially impactful with video since that can be such a demanding medium to transmit, be it over a network, or a mere video transmission cable like HDMI or DisplayPort. Compression is the process of reducing the overall file size of the media being transmitted, and there are various ways to achieve it.
Chroma subsampling is a form of compression, in that it takes a native video stream or image and removes color information in it, before sending that now-leaner signal onto a display or other device. It is, however, considered a “visually lossy” form of compression, in that the resulting signal does lack some of the visual detail of the original native image or video.
This is different than a technique like display stream compression, for example, which while mathematically lossy, is considered “visually lossless,” since viewers cannot tell the difference between a compressed and native image. Although there are formats of chroma subsampling that make it hard to tell the difference between that and native uncompressed media, more extreme chroma subsampling is much more obvious, making the standard a lossy one.
Since the compression systems work differently, however, they can be used together. Indeed HDMI 2.1 is specifically cited as being capable of 10K or higher resolution video transmission when using DSC and 4:2:0 chroma subsampling. Without either form of compression, HDMI 2.1 is merely capable of 8K resolution video at up to 30Hz.
Chroma subsampling is an important component in achieving ultra-high resolutions and refresh rates on TVs and monitors. It’s also a huge part of making the internet functional with the ever-increasing demands placed on local connections and international infrastructure as more and more people consume higher-resolution content.
Newer networking technologies are enabling faster and faster internet connections, and new generations of local video transmission cables, like HDMI 2.1 and DisplayPort 2.0, are enabling ever higher detailed content, including HDR and 10-bit color. That said, there will always be a place for chroma subsampling. Not only does it let these already-incredible connections do even more, but it helps reduce the overall load on networks. When there is visually very little lost in the process, the reduced overall bandwidth is a real benefit.
Frequently Asked Questions
Are you just here for some quick answers on Chroma Subsampling and what makes it a useful technique for video and image compression? We’ve got you covered. Here are some of the most common questions about chroma subsampling.
What is Chroma Subsampling?
Chroma subsampling is a compression technique used to reduce the quantity of chroma, or color detail within an image, whilst retaining the information related to lighting and contrast. This results in an image that, to the human eye, is almost virtually indistinguishable from the original picture or video.
Chroma subsampling is typically used in compressing video to allow for higher resolutions and refresh rates, but it’s also used to reduce the file size of digital images, like JPEGs.
What are the different types of Chroma Subsampling Formats?
Chroma subsampling formats are displayed as a trio of three numbers in a ratio. They relate to the horizontal sample size, the number of color samples taken, and the number of vertical samples. This can result in chroma subsample formats like 4:2:2, or 4:1:0, depending on the level of compression.
A chroma subsample format of 4:4:4 is effectively an uncompressed video, whereas a chroma subsample of 4:2:2, cuts the color information in half, reducing the overall file size of the video dramatically.
How does Chroma Subsampling Impact Image and Video Quality?
Chroma subsample is a “lossy” visual compression technique, in that the end result does have a reduced overall visual quality, even if it is rarely dramatic or obvious. The reason chroma subsampling is so effective is that the human eye can discern far greater differences in brightness and contrast than it can in color. Reducing the color resolution of an image is quite hard to spot, in comparison to lowering the overall resolution or the luma data within the sample.
With some of the more extreme examples of chroma subsampling, it can be more obvious that some form of compression has been applied, but more subtle forms have been used in commercial media like DVDs and Blu-rays for years, with most viewers being completely unaware.
How do I choose the right Chroma Subsampling Format?
In general, you’re unlikely to need to use chroma subsampling in day-to-day viewing. However, if you find that your cable can’t deliver the resolution you need it to, or you want to reduce the overall strain on your network, you can use chroma subsampling to fix it.
Picking the right chroma subsampling format for your particular needs depends on how much compression you want or need. If you only need to reduce the file size of your image or video a little, then you could use 4:2:2, as this is almost completely visually lossless, but it cuts the file size by a third. For more intense compression, you might opt for 4:2:0, though this will start to show some image degradation in high-contrast scenes.
How can I minimize Artifacts while using Chroma Subsampling?
The best way to minimize artifacts with chroma subsampling is to use less extreme subsampling formats. Even so, with specific streams with alternating colors, thin black lines, or high-contrast color scenes, artifacts can be prevalent no matter which formats you use. With that in mind, you can avoid chroma subsampling artifacts for the most part, by only applying chroma subsampling to appropriate video or images that are less likely to be negatively affected.
Chroma subsampling motion artifacts are also more prevalent in interlaced content, especially if progressive sampling is used. Using either interlaced sampling, or purely progressive media for chroma subsampling can get around this issue.