Introduction
When the things comes in terms of video compression then we generally see 4:4:4 , 4:2:2 , 4:2:0 , 4:1:1 and 3:1:1. We also see “Y” , Cb and Cr. Why these terms are specified what does this ratio system mean? What Is Luminance and Chrominance how does these two affect human eyes vision?
So in this article we will cover all of these things.
What Is Chroma Subsampling
The Chroma Subsampling is a compression method denoted by the ratio like A : B : C . Where A represents Luminance Component for the frame Y. While other two represents the Chrominance Component Cb and Cr. Where luminance represents the brightness of video where as chrominance represents the colours. Here Cb is responsible for blue colour chrominance while Cr is responsible for red Colour chrominance. It reduces the bandwidth consumption upto 50 percent.
If there is fully uncompressed video then its Chroma Subsampling rate will be 4:4:4. Where 4 : 4 : 4 indicates that for every 4 luminance components it have 4 chroma components Cb and Cr. But 4 : 4: 4 chroma subsampling rate is not preferred because this video is uncompressed having full details which consumes lot of space required large bandwidth to deliver. Because if we record 1 hour of video then it takes approx 100 Gb space which is very high space for one hour video. So It can be called as raw data which needs to compress for efficient delivery.
Devices | Chroma Subsampling |
Computer | 4 : 4 : 4 |
TV Shows | 4 : 2 : 0 |
Sports | 4 : 2 : 0 |
Movies | 4 : 2 : 0 |
Video Games | 4 : 4 : 4 |
Chroma Subsampling Working
To make more efficient video for delivery purpose, the Chroma Subsampling technique gets the advantage of human vision system. Human eye vision is more sensible for luminance component rather than the chrominance component. A little bit drop of Y or luminance component will affect more to human eyes while we can do drop in Cb , Cr or Chrominance Component it will not affect that much.
So we are droping some chrominance component by using chroma subsampling to reduce the bandwidth of video which results 4 : 2: 2 or 4 :2 :0 rate of video. Now this reduced chominance video have lower bandwidth requirement by maintaining same quality of video.
4 : 4 : 4 Chroma Subsampling Ratio
As we have discusses above chroma subsampling ratio defined as A : B : C. So here A = 4 , B = 4 and C = 4. Where A denotes how many pixels wide is reference block. So we denoted it as 1 , 2 , 3 and 4 in the figure above. B denotes in how many pixels of top row are going to get the chroma samples. Here B = 4 so we filled chroma samples red and blue. Like this C = 4 dentoes in how pixels of down row are going to get chroma samples. We have indicated it figure above. This is uncompressed video , no any chorma subsampling is done here so it is denoted by 4 : 4 :.
4 : 2 : 2 Chroma Subsampling Ratio
As shown above in the figure value of A = 4 , B = 2 and C = 2. Where A defines the reference block wide pixel numbers as 4. B denotes that every pixel of first row have to filled with 2 chroma samples. While C denotes every pixel of second row have to filled with 2 chroma samples. We have shown these in the figure above 4 : 2 : 2. In this subsampling we have lost the information or detail but still we gets good quality.
4 : 2 : 0 Chroma Subsampling Ratio
As shwon above in the figure the value of A = 4 , B=2 and C=0. A denotes reference block is 4 pixel wide as we have shown in above figure. B = 2 denotes that there will be two colours in first row we have filled two colors in first row of reference block. While C denotes no any chroma samples so first row chroma have copied into the second row as it is.
Conclusion
Chroma Subsampling is compression technique used to reduce the bandwidth without impacting on quality of video practically. It is denoted in ratio where first part Y denotes the luninance or brightness of videos whereas second two parts denoted as Cb and Cr known as chrominance of video or colour in the video. Chroma Subsampling 4 : 2 : 0 is preferred for streaming online videos like Youtube and Vimeo.
Know More About H.263 H.264 H.265 Difference
reference : telairity
