Monday, June 28, 2010

Color Subsampling, or What is 4:4:4 or 4:2:2?

From Adobe's Karl Soule: "In the video space, there's always a lot of talk about these number ratios - 4:4:4, or 4:2:2, or 4:1:1, but what exactly do they mean? Recently, someone argued with me that it was better to convert every video clip from my Canon Rebel T2i DSLR camera into a 4:4:4 intermediate codec before editing; that this would make the color magically "better" and that editing natively was somehow bad. They were wrong, and I'm going to explain why.

Before you read on, make sure you've read my earlier articles on 32-bit floating point and on YUV color, and look at the picture from the Wikimedia Commons site of the barn in YUV breakdown.

In the picture of the barn, try to look at the fine detail in the U and V channels.Typically, without any brightness information, it's hard to see any detail in the color channels. The naked eye just does a much better job distinguishing brightness than color. This fact holds true for moving pictures. If the video uses YUV color space, the most important data is in the Y channel. You can throw away a lot of the color information, and the average viewer can't tell that it's gone.

One trick that video engineers have used for years is to toss away a lot of the color information. Basically, they can toss away the color values on every other pixel, and it's not very noticeable. In some cases, they throw away even more color information. This is called Color Subsampling, and it's a big part of a lot of modern HD formats for video.

When looking at color subsampling, you use a ratio to express what the color subsampling is. Most of us are familiar with these numbers: 4:4:4, or 4:2:2, or 4:1:1, and most of us are aware that bigger numbers are better. Fewer people understand what the numbers actually mean. It's actually pretty easy."

Click here to continue reading this story.


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