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Difference between RGB and YCbCr

ac dit color space signals monitor

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#1 davide sorasio

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Posted 02 June 2015 - 06:41 PM

Hi everybody, I'm trying to really get the difference between RGB and YCbCR. The problem is that I cannot find a consistent answer to this question. From  what I know they are both signals. I know that RGB is always a 4:4:4 signal while YCbCr can be subsapled in different ways. I also know that YCbCr is originated by the RGB because this last signak is redundant so (what does ut exactly mean?). I also would like to know the difference between a 4:4:4 RGB and a 4:4:4 YCbCr, don't they both have the maximum chroma and luminance sampling possible?

 

Thanks so much!


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#2 Phil Rhodes

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Posted 03 June 2015 - 03:48 AM

RGB imaging is well understood by most people. All imaging systems designed to simulate colour as humans see it need three channels of information because we have three types of cone cell in our retinas, and RGB stores, roughly speaking, one channel of information which will be detectable by each type of cone cell.

 

YUV is a term that's commonly (if slightly inaccurately) used to refer to a set of techniques designed to separate colour information from brightness information. Because the human eye has more brightness-sensitive rods than it has colour-sensitive cones, we see most sharply in brightness, rather than colour. Separating brightness and colour means the colour information can then be stored at lower resolution without obvious image quality problems.

 

In YUV-type imaging, the Y refers to the brightness information, and the U and V refer to the colour information. Roughly, the U refers to the mathematical difference between brightness and the blue channel, and V refers to the difference between brightness and the red channel. Because of this, in computing, YUV-type signals are sometimes called YCbCr, or other similar terms, depending on the situation.

 

Given this information, an RGB result can be calculated from any YUV-type signal. In practical applications, YUV-type signals are often used with the UV colour information at half or one-quarter the resolution of the Y brightness information, which saves bandwidth without affecting the apparent sharpness very much.

 

There are significant mathematical complexities regarding the range of colours that can be encoded by RGB and YUV systems, as well as practical issues with standardisation of signal levels which can cause contrast and colour shifts if not properly done. 

 

Hope this helps

 

Phil


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#3 davide sorasio

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Posted 05 June 2015 - 03:21 PM

Thanks Phil. What I do not understand  if it 's correct saying that RGB and YCbCr are 2 different signals and if they are actually signals. My doubt comes from the fact that in a brochure given me by Arri they say that they are 2 different signals, and that YCbCr is originated by an RGB. Also some of my teachers mentioned the possibility of some cameras to record an uncompressed RGB signals. For example from the alexa studio camera manual when they talk about internal recoring they state that pro res 444 is a RGB color space, while pro res 422 a YCbCr, and again that the YCbCr is originated from the RGB (so this mekes me suppose they are 2 different signals. They also say so in the brochure where they talked about signals and color subsampling). Is what I'm stating correct or not?

Thanks so much,

Davide


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#4 Keith Putnam

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Posted 09 June 2015 - 10:35 AM

What I do not understand if it 's correct saying that RGB and YCbCr are 2 different signals and if they are actually signals.

I'm not clear on what you mean by "actually signals".

 

Video color information can be encoded in various ways. One way is what we call "RGB", where the full information in all three color channels is maintained*. Another way is YCbCr (or YUV), where the color information in the Blue and Red channels is thrown away.

 

That encoded information can be sent through a medium (like a coaxial cable or via a radio transmitter) as a signal of different types; SDI is a different signal than analog RF, for example. Or it can be stored as a digital file in various formats.

 

So, when recording ProRes on the Alexa you can choose 4:4:4 encoding, which preserves all of the color information, or 4:2:2 which throws away half of the blue and half of the red while preserving all of the green channel. This information is a "signal" when it is moving from place to place; otherwise it's just data.

 

 

*"RGB" is also used slightly differently to refer to a RAW image which has been debayered into something visible. But this doesn't refer to a difference between 4:4:4 and 4:2:2; it's just a distinction between "raw" and "not raw".


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#5 davide sorasio

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Posted 09 June 2015 - 03:31 PM

Ok, everything is way clearer now. I still have 2 doubts:

1) I've realized we've always talked about blue and red channel, what about the green?

2) I've been told that the difference between a raw image and an uncompressed rgb is the fact that in a raw image we store everything that the sensor has read, while in an uncompressed rgb we have the maximum info for an image on which a look has already been applied ( if it makes things clearer people were talking about the possibility of the sony F3 to record an uncompressed RGB with the help of an external recorder).

 

Thanks so much!

 

Davide


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#6 Phil Rhodes

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Posted 09 June 2015 - 03:47 PM

The reason people talk about blue and red is because the Cr and Cb components of a YUV (properly, YCrCb etc) image represent the difference between Y (brightness) and red, and between Y and blue. Given that information, RGB can be calculated mathematically. As a practical matter, the Y channel is made up mainly of green data because the human eye sees green with the greatest sensitivity, but it's not correct to think of Y as being green in any sense other than that.

 

"Raw" is a term with no universally accepted meaning, but it usually refers to the unprocessed output of a Bayer-type (or similar) single-sensor camera. Usually, though not always, this will be uncompressed. Raw formats can differ from manufacturer to manufacturer, and this doesn't have much to do with the difference between RGB and YUV.

 

RGB data can represent anything from the output of a camera before grading to a finalised production; it's used all over the place.

 

P


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#7 David Mullen ASC

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Posted 09 June 2015 - 11:49 PM

Keep in mind that converting raw to uncompressed RGB essentially triples the amount of data to record.


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