I’m not an engineer.

I don’t have a degree in electrical engineering, or any other form of engineering. I’ve never sat on an ISO, IEEE or any other standards panel. I’m not a computer genius. Everything I know about cameras, color space and NLEs I either picked up in film school or learned on the job along the way. I do, however, work with cameras and post-production workflows on a daily basis. I do a lot of shooting, editing and keying and I’m always trying to grasp the concept of different color spaces across the SD and HD spectrum. 

If you’re reading this post, then that probably closely describes you, too.

What is 4:2:0 HDV color space? How does 4:2:0 color space work? 

As I have done research over the years, the one thing I have learned is that color space is a deep, complicated topic. To make things worse, there doesn’t seem to be a single, easy-to-understand post on the web that breaks the concepts of color space down into layman’s terms. This post won’t be all-encompassing, but I do aim to share with you the concept of 4:2:0 color space in clear english to the best of my ability.

I highly encourage conversation on this topic. If you read this post and think I’ve gotten something wrong, please post a comment. If you can help to further clarify a point, please join in. As I continue my research and learn more about color space I’ll write about it in future posts.

Okay… here are the oversimplified basics of color space…

4:X:X refers to the number of pixels being sampled for each component of a three color image. “4″ is the maximum number and “1″ is technically the minimum number. 

For digital images, the whole ratio is generally referred to as YCbCr, where “Y” represents luminance, “Cb” represents the blue channel (as a difference between that and the luminance channel) and “Cr” represents the red channel (also as a difference from luminance). In the analog world, “YPbPr” means basically the same thing.

The first number refers to the number of luminance pixels that are sampled for each block of 4 pixels. In almost all cases, the first number will be 4. The second number refers to the number of blue pixels that are sampled for each block of 4 pixels. The third number refers to the number of red pixels that are sampled for each block of 4 pixels. Thus, 4:4:4 color space is the best possible option, since every single pixel is sampled for luminance, blue and red information.

Enter digital video and HDV. In order to save bandwidth and bitrate during DV compression, most manufacturers made their standard def DV cameras work on a 4:1:1 system. This meant that for each block of 4 pixels, all four were sampled for luminance, 1 was sampled for blue and 1was sampled for red. This means that DV doesn’t take up much bandwidth, but it does lack a significant amount of color information, which means it’s not ideal for pulling keys.

With the 4:2:0 color space of HDV the color information issue gets just a bit better. HDV works by sampling every luminance pixel as well as 2 of every 4 blue and red pixels. But what’s that crazy “0″ all about? Apparently, when the third number is 0, it means that every other line is skipped. This means that we now have 2 blue and 2 red pixels of information out of every 4, but only on alternate scan lines. This effectively gives us 4:2:2 color space, but at half the true resolution. This is better than 4:1:1, but still not great, since it leads to issues when trying to key out fine edges, moving objects, fingers, etc.

I hope that this helps to clear up some of your basic questions about 4:2:0 color space. I’ll be updating this post and adding new ones as I learn more.

– Mike