What do blue-difference and red-difference chroma components mean in YCbCr color space?

Question

I'm currently working on a project that involves color space. I don't really understand what Cb and Cr means in YCbCr color space. Wikipedia says "blue-difference and red-difference chroma components", but I don't understand what they mean. And what's the advantage of YCbCr over RGB or HSV(HSL) in image processing? How do we decide which color space to use?

Answer 1

The Cb and Cr components of the YCbCr, or YCC, color space are not immediately physically intuitive. Cb and Cr respectively represent the blue-difference and red-difference chroma signals as you noted. Chromaticity represents the perceptual deviation from the neutral, i.e. grayscale, so the greater the deviation, the higher the intensity of chroma. An image with low chroma is less colorful, and conversely. And so, Cb and Cr are one representation of changes in blue and red "colorfulness", respectively.

Each colorspace has different advantages, mostly related to how sensitive channels to changes. For example, YCbCR is widely popular for its compressibility in the Cb and Cr channels by subsampling. Or, when doing things in computer vision like image matching using color-based methods, it is important to use an rg-chromaticity space with illumination invariance so that accuracy doesn't fluctuate dramatically when lighting changes.

You might also want to check out this question for more discussion on stability:
Most "stable" color representation : RGB? HSV? CIELAB?

And also this on HSV popularity in computer vision:
Why do we use the HSV colour space so often in vision and image processing?