Okay, so you’ve read the articles on Luminance vs Chrominance, the meaning of Y’CbCr, and the meaning of the numbers in chroma sub-sampling. But you’re still confused about the difference between RGB 444 and Y’CbCr 444.
Are these two different things, or the same thing?
These are two different things.
Let’s think of three pipes, each designed to carry orange juice over many miles. The engineers who designed these pipes took into account the acidic nature of orange juice, and all the other miniscule but important problems.
Feeling thirsty? The cool thing about these pipes is that they can also transport apple juice, and many other beverages – like coffee, beer or bourbon (Check the Paypal link below, I have my own pipe going).
When data is encoded in the Y’CbCr system, the three pipes contain information as follows:
- Pipe 1: Luma Y’
- Pipe 2: Chroma Cb
- Pipe 3: Chroma Cr
If you encode at 4:4:4, all the three pipes are at full flow. If you encode at 4:2:2, the second and third pipes only contain half the juice. At 4:2:0, the last pipe is empty.
However, in a Y’CbCr encoding system, it is always understood that the three pipes will adhere to the Luma-chroma system as shown above. Nobody will dare to mix color in the Luma pipe. And nobody will interchange the chroma pipes either. It’s a standard.
RGB is similar, except now the three pipes contain information as follows:
- Pipe 1: Red
- Pipe 2: Green
- Pipe 3: Blue
Nobody is allowed to mix green in the red pipe, or any other combination. Similarly, nobody is allowed to mix Luma in the Red pipe either. This is another standard.
Each pipe is called a channel. Those who are proficient in node-based applications like Nuke know what these are. Both RGB and Y’CbCr systems happen to use the three-channel method, but mean totally different things.
Check this out:
You can’t have a 4:2:2 RGB stream. What would this mean? What’s the meaning of half green and half blue? You can’t have a 4:2:0 RGB stream either. What kind of image is it, if it can never have blue at all?
To help you remember, if you’re talking about RGB, all three channels must contain the same number, which usually is 4:4:4. There’s nothing to stop anybody from screwing up, mind you, just that it isn’t done.
Only Y’CbCr is allowed to have different numbers like 4:2:2, 4:2:0, 3:1:1 and so on.
So, anybody ever tried to mix apple and orange juice?