Doubts about color spaces and chroma subsampling

Question

Hy everyone, I have some questions about color spaces and chroma subsampling that I haven't been able to solve just by searching the web.

1. Are YUV and YCbCr the same thing? When I take a look at the information of a random MP4 video file with ffmpeg or MediaInfo, I see that the color space is indicated as YUV. Is that actually YCbCr since we're talking about a digital file?

2. It doesn't make any sense to say RGB 4:2:0, right? Because chroma subsampling only applies to the YCbCr color space, taking the (Cb, Cr) components, and deciding how many of them should be but in a 4x2 pixels region. Is that correct?

3. When ffprobe outputs that a video has a bits_per_raw_sample (or bit depth) of 8 bits with a yuv420p pixel format, what does it mean? What is the "raw sample" here? Is the 8 bits value referred to the number of bits used for each component (Y, Cb, Cr) before doing chroma subsampling? In that case, how should I calculate how many bits per pixel (or per 4x2 pixels region [?]) are used after chroma subsampling?

I hope it's ok to have multiple questions in the same post. Thank you.

Answer 1

Are YUV and YCbCr the same thing? → strictly, no. The former is applied to analog signals, and the latter to digital. But media apps (like Mediainfo) use YUV as the designated term.

It doesn't make any sense to say RGB 4:2:0, right? → Not theoretically incoherent, but I haven't seen any codec that implements subsampling for RGB pixels except for Bayer RGB streams possibly. Don't know its details.

What is the "raw sample" here? → a stored single value of a pixel component i.e. any of R,G,B,Y,U,V (or alpha).

Is the 8 bits value referred to the number of bits used for each component (Y, Cb, Cr) before doing chroma subsampling? → it is the resolved bits per individual sample i.e. a 12-bit value has to be stored as a 16-bit datum, so 12 bits is the raw sample size and 16 bits is the coded sample size. 4 bits are for padding.

how should I calculate how many bits per pixel → in terms of luma/chroma resolution, it's the raw sample size. In terms of storage space for a raw uncompressed stream, you sum up the counts in the sampling scheme and divide by maximum count. Multiply by (no. of components x coded sample size).

So, for 4:2:2, that's

(4+2+2) / (4+4+4) = 2/3 and then 2/3 x (3 x 8 bits) = 16 bits per sample

Answer 2

2nd question: RGB JPEG can use all kinds of subsamplings, including 4:2:0 with "center" G, B siting. It makes little sense, since we have better vision in green.