Decoding AVIF: Deep dive with cats and imgproxy
evilmartians.com
Decoding AVIF: Deep dive with cats and imgproxy
1–10 of 29 posts
Re: Decoding AVIF: Deep dive with cats and imgproxy
#2"Commonly, three numbers are used to specify downsampling:
the first is always 4, don’t ask me why"
The first number represents the luminance.[0] Even if they know the first number represents luminance, the "don't ask me why" is just horrible on its own. The detail in the image is preserved through the luminance channel. The subsampling in the chroma is much less perceptable to humans, but more more noticeable in the luminance. Therefore, some very smart people learned to cheat the data saved for chroma, but not the luminance. "don't ask me why" in detailed write ups is just bad in so many ways.
Re: Decoding AVIF: Deep dive with cats and imgproxy
#3"Commonly, three numbers are used to specify downsampling: the first is always 4, don’t ask me why" Wow. Someone going to this much detail on explaining how a video/image codec works, and cannot bother learning what the numbers of chroma subsampling mean? The first number represents the luminance.[0] Even if they know the first number represents luminance, the "don't ask me why" is just horrible on its own. The detai…
Not sure that explains why the first number has to be 4, which was their point.
Re: Decoding AVIF: Deep dive with cats and imgproxy
#4"Commonly, three numbers are used to specify downsampling: the first is always 4, don’t ask me why" Wow. Someone going to this much detail on explaining how a video/image codec works, and cannot bother learning what the numbers of chroma subsampling mean? The first number represents the luminance.[0] Even if they know the first number represents luminance, the "don't ask me why" is just horrible on its own. The detai…
I don't think you're being generous with the author's statement, especially since this is in the section within which he's describing chroma subsampling. The author is stating "We use 4 as a convention. why is that the convention? No one really knows". That seems accurate to me. Do you have a clearer answer? Your Wikipedia link doesn't provide any enlightment AFAICT, although maybe I missed explanation?
Re: Decoding AVIF: Deep dive with cats and imgproxy
#5"Commonly, three numbers are used to specify downsampling: the first is always 4, don’t ask me why" Wow. Someone going to this much detail on explaining how a video/image codec works, and cannot bother learning what the numbers of chroma subsampling mean? The first number represents the luminance.[0] Even if they know the first number represents luminance, the "don't ask me why" is just horrible on its own. The detai…
You have not explained why the first number is always 4. (In fact, it's not always 4, it just usually is.)
Re: Decoding AVIF: Deep dive with cats and imgproxy
#6"Commonly, three numbers are used to specify downsampling: the first is always 4, don’t ask me why" Wow. Someone going to this much detail on explaining how a video/image codec works, and cannot bother learning what the numbers of chroma subsampling mean? The first number represents the luminance.[0] Even if they know the first number represents luminance, the "don't ask me why" is just horrible on its own. The detai…
Just after he says: "4:2:0 is the most popular case. Four luma samples per one chroma" so he does understand and write what it means. That does not explain why the value is 4.
I assume it is because with 4 you have 3 different subsampling ratios (if you want to keep factors of two, which you typically want to keep algorithms simple)
Re: Decoding AVIF: Deep dive with cats and imgproxy
#7"Commonly, three numbers are used to specify downsampling: the first is always 4, don’t ask me why" Wow. Someone going to this much detail on explaining how a video/image codec works, and cannot bother learning what the numbers of chroma subsampling mean? The first number represents the luminance.[0] Even if they know the first number represents luminance, the "don't ask me why" is just horrible on its own. The detai…
I don't think you're being generous with the author's statement, especially since this is in the section within which he's describing chroma subsampling. The author is stating "We use 4 as a convention. why is that the convention? No one really knows". That seems accurate to me. Do you have a clearer answer? Your Wikipedia link doesn't provide any enlightment AFAICT, although maybe I missed explanation?
Just before this section the author discusses how the image is broken down into blocks. This section is where the definition of the 4s could have come from, but they left out, for brevity's sake I'm assuming, how those blocks are shaped.
"Now, let's break it down the differences between 4:4:4; 4:2:2 and 4:2:0:
The number of pixels that share color is determined by what type of chroma subsampling it is. Each sample is defined by a block of 8 pixels. The first number refers to the size of the sample and its pattern, which is typically 4 pixels wide. The second number refers to how many pixels in the top row will receive color or chroma sampling. The third number shows how many pixels on the bottom row will receive chroma samples"[0]
The block sizes and sub-sampling methods are also why there are warnings issued when trying to scale an image when the dimensions are not divisible by the block sizes. If you try to scale to an odd number, then the sampling within the blocks is broken. If you scale to a number not divisible evenly by the largest block sizes requested, then you also get issues.
[0] https://blog.westpennwire.com/what-is-chroma-subsampling
Re: Decoding AVIF: Deep dive with cats and imgproxy
#8"Commonly, three numbers are used to specify downsampling: the first is always 4, don’t ask me why" Wow. Someone going to this much detail on explaining how a video/image codec works, and cannot bother learning what the numbers of chroma subsampling mean? The first number represents the luminance.[0] Even if they know the first number represents luminance, the "don't ask me why" is just horrible on its own. The detai…
Not sure that explains why the first number has to be 4, which was their point.
Then they did not look/research very hard. See my response above. I provide a link to someone else's blog that was easily found with a DDG search.
Re: Decoding AVIF: Deep dive with cats and imgproxy
#9"Commonly, three numbers are used to specify downsampling: the first is always 4, don’t ask me why" Wow. Someone going to this much detail on explaining how a video/image codec works, and cannot bother learning what the numbers of chroma subsampling mean? The first number represents the luminance.[0] Even if they know the first number represents luminance, the "don't ask me why" is just horrible on its own. The detai…
Edit: While the reason for 2 vertical lines (interlacing) is correct, it seems that the 4 horizontal lines is from a compromise between NTSC and 525-line systems' frequency (explained in length here: https://news.ycombinator.com/item?id=28203942, thank you keithwinstein), standardised as Rec. 601. One of the proposals is 3:2:x, but it was both worse analogue-speaking and harder digitally-speaking. 3:1:x was used for HDVS, which was the uncompressed storage format of Sony. It was essentially inherited for the MUSE, D-MAC and unreleased prototype American Analog HD systems (note that ATSC was essentially a reboot, trying to outcompete MUSE and DVB).
Okay, the real reason, as far as the bundles of paper I have* is accurate, is that digital chroma subsampling was first invented for MUSE, a Japanese analogue HD video standard (with pre-broadcast digital components). They chose four for horizontal because it's relatively easy to manipulate using their digital systems at the time and two for vertical so that it's easy to handle interlacing stuff. Unfortunately, I'm not Sony or NHK so I can't say for certain why not eight or any other powers of two. Also, Americans (aka the SMPTE) set the 1,080 lines (the Japanese standard is 1,025), the 16:9 compromise (between the European and Japanese 15:9 and cinema 21:9) and the "limited RGB" dilemma that is experienced in digital video systems (that's literally from the days of NTSC signalling!). Both the Japanese NHK/Sony MUSE system and the British IBA (adopted as European) D-MAC system uses the full-range 8-bit system that is used for JPEG (pre-broadcast to analogue, of course).
Analogous to this, the reason why CD audio is 44,100 Hz is because that's the commonality between NTSC (System M, 525-line 480-visible 60-Hz) and 625-line (576-visible 50-Hz) systems. Digital audio was literally stored on U-matic systems at the time, and it was initially only 14-bit PCM rather than the 16-bit PCM of CDs.
* or rather, my employer's mini-library.