Another area I’m getting pelting with questions about is the little YUV logo on some Premiere Pro effects. What exactly is YUV when talking about video?
Well, it’s a way of breaking the brightness and colors in the image down into numbers, and it’s a little different from RGB, which we discussed last time. Just as a refresher, most cameras take the light coming into the lens, and convert that into 3 sets of numbers, one for Red, one for Green, and one for Blue. This is called RGB, and we discussed how each of those numbers come in different bit depths in my last article.
There’s one big problem with RGB color - it’s tough to work with. If I need to lower the brightness uniformly on an image, I need to do it to all 3 colors. There’s also a LOT of redundancy in the data. To combat this redundancy, there’s a different way of storing the info called YCbCr, which breaks the signal down into a Y, or luminance channel, and 2 channels that store color info without brightness info - a Blue channel and a Red channel that don’t contain any brightness.
Now, the correct way to abbreviate this would be Y, Cb, Cr. However, I want you to try saying YCbCr 10 times, and compare that with saying RGB 10 times. Y, Cb, Cr, is a mouthful. Some engineer somewhere decided that saying YCbCr was just too inconvenient, and borrowed another color term, YUV, to use instead. Technically speaking, saying “YUV” to describe YCbCr is not accurate at all, but the name stuck, and so now, most people who are talking about YCbCr use the term YUV incorrectly. It’s like calling an American football team that plays in New Jersey the “New York Giants.”
Here’s a graphic from the Wikimedia Commons site that shows RGB breakdowns of a frame:
YPbPr is how we refer to analogue component video. YCbCr is how we refer to digital component video. As pointed out YUV is not correct, but it is easier to say! YUV refers to a stage in composite video processing for broadcast.
Posted by Graeme Nattress on 06/28 at 12:14 PM
To take it one stage further..
YPbPr is also know as Y, R-Y & B-Y. It was a very clever way of reducing the bandwith needed for transmission purposes whilst maintaining a black and white signal
Or .3R+.59G+.11B=Y therefor Y-R-B=G which means you don’t need to send a B&W signal and separate colour signal.
Taking the Y signal away from the R + B means the R+B signals go down as the Y goes up eliminating cross talk.
Not quite sure why YPrPb was adopted as a signal processing system in the component era over RGB but it was
Posted by MichaelSanders on 06/28 at 03:27 PM
Oh and as Graeme say’s U and V refers to the B-Y and R-Y signals after weighting has been applied at the last stage of the encoding process before the three signals are added together to form a composite signal.
Posted by MichaelSanders on 06/28 at 03:30 PM
Oh and that’s PAL! I AND Q are the respective NTSC signals (I think!)
Posted by MichaelSanders on 06/28 at 03:30 PM
YIQ was proposed, but never widely adopted for NTSC, but again, it’s a method for forming a composite signal for broadcast and nothing at all to do with component video.
Posted by Graeme Nattress on 06/28 at 04:15 PM
*sigh* - I have this annoying habit of associating PbPr with digital and CbCr with analog. It’s just backwards in my brain…
Thanks for the catch, guys. Updating now….
Posted by .(JavaScript must be enabled to view this email address) on 06/28 at 05:17 PM
Good stuff one and all - this and the previous article about color for PPro - I’ve bookmarked them for future reference - thanks!
Posted by Jim Hines on 06/29 at 07:18 AM
Is it really correct to say there is lot of redundancy in RGB? As far as I can see, every (r,g,b)-triplet in the RGB system corresponds to a unique color. So, where is the redundancy?!?
Need to check this, but it sounds like you try to explain that RGB and YUV are two different parametrizations of the color space.
Compare, the spatial space can be parametrized for example by a Cartesian coordinate system (the xyz-coordinates), by cylindrical coordinates (the radius, angle, and z -coordinates), or by spherical coordinates (angle, angle, radius). These just correspond to three different choices of how the spatial space is parametrized.
Posted by .(JavaScript must be enabled to view this email address) on 06/30 at 12:21 PM
Hi Larry,
I guess it’s more accurate to say that perceptually there’s redundancy in RGB when compared to YUV, since the chroma and luma information are mixed together in RGB, and the human eye is much more sensitive to luma differences than chroma.
Guys, here’s a quick disclaimer - I am not a mathematician. I’m a video editor, demo monkey, and evangelist for Adobe. The goal of this article was to explain exactly what YUV is in relation to the new effects icons in Premiere Pro. Color for film and video is a bit confusing for the newbie, and I’m trying to tackle little bits at a time.
There are a lot of articles on the specific math in converting between RGB and YUV, and understanding the different coefficients involved for 601/709/240M color spaces, There are some wonderful diagrams of the different color spaces, how they overlap, and what color ranges are out-of-gamut from one to the other. Do any of you have your favorite references?
One of my long-term standby’s is “Video Demystified: A Handbook for the Digital Engineer” by Keith Jack.
Posted by .(JavaScript must be enabled to view this email address) on 06/30 at 01:57 PM
It’s not the RGB space that has redundancy, it’s the images we represent in that space that have redundancy. Generally, a naturalistic image has a lot more measurable information in the luma than the chroma, so you can losslessly compress (remove entropy) to a smaller file when you first do a RGB to YCbCr transform before the entropy encoder.
Posted by Graeme Nattress on 07/01 at 02:40 PM
