Expand on Chroma subsampling about digital_video_introduction HOT 6 CLOSED

Of course :)

from digital_video_introduction.

I don't fully understand the Chroma subsampling ratios so I went online to find more info but that just ended with me becoming more confused. This blog post shows an example of 4:2:0 subsampling which seemingly conflicts with the statement given in this introduction

y is the number of changes of chroma samples between the first and seconds rows of a pixels.

I assume that if the number of sample changes between the first and second and row (i.e. y) is 0 then the chroma subsampling of row one and two should be identical, which is not the case in the mentioned blog post.

All that aside, the only reason I started looking up other resources was because I found it hard to understand the explanation of the different ratios. Maybe having an example image similar to the YCbCr 4:2:0 merge but for 4:2:2 would illustrate what the y value does. I personally found the illustration from the blog post more intuitive.

My current understanding of the subsampling system presented in this introduction might be completely wrong, I can't come up with any suggestions for improvement in that case.

from digital_video_introduction.

@Waranoi the visualization from Wikipedia is clearer in my opinion:

• the 4 in 4:2:0 is the width of the grid taken as a reference
• the 2 in 4:2:0 is the number of chrominance values for the first row
• the 0 in 4:2:0 is the number of changes in chrominance between the first and second row. In fact, the second row doesn't have new chrominance values, but has the same chrominance as the first row.

What might be confusing in that article is the fact that it says:

one line has Cb samples for every other pixel, and the next line has Cr samples for every other pixel.

But as the article says, it's just "one of many ways" to achieve 4:2:0. In that case, instead of saying that the first row has certain Cb and Cr values and the second row "inherits" them (hence the 0 in 4:2:0), it says that the chrominance is "split" in different pixels. So the Cb and Cr values are put in different rows, but are indeed used together to compose the final "colour", which is common for all the pixels in the 2x2 sub-grid.

I actually find that explanation a bit more confusing, but I think it does it that way to better highlight how chroma subsampling is implemented (so putting Cb and Cr in multiple pixels, but using the values together to have the actual chrominance for any pixel).

I hope it makes some sense :)

from digital_video_introduction.

Indeed, after looking it up I also think that the visualization from Wikipedia is clearer. Having multiple example images and J, a and b indexes to the side makes it a lot easier for me to understand. Also your expanded explanation of the b index helps a lot.

So if I understand correctly, the 2 in 4:2:0 does not mean that it only uses chrominance values found in the first row, because the 0 in 4:2:0 says that the final colour for row 1 and row 2 are identical. That's why both row 1 and row 2 can be used to sample the final color? If it's like this then I would understand why the article and this intro's explanation are not conflicting each other.

from digital_video_introduction.

That's why both row 1 and row 2 can be used to sample the final color?

Yes. As you say, in 4:2:0 the final chrominance is identical for row 1 and row 2, so you can spread the chrominance information (Cb and Cr) in which row you want, and then use row 1 and/or row 2 at your choice to revert the subsampling

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Nice! If it's ok for you I might create a pull request later with the stuff you told me because I think it's much easier to understand than the current subsampling section.

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