I don't think it really counts as compression. It's like taking the statistics of a text and generating a new one with a Markov chain.
In other words, it is complete nonsense, texture synthesis, like the way some people go for printing a digital image on a fake "canvas" for a "textured, 3D look"... or maybe one of those flickering flame light bulbs.
Can anybody here imagine an audio codec that added simulated hiss and crackles to make it sound like you were listening to a vinyl record? That's what this is.
Some artists will intentionally put scratching or crackling into their music for atmosphere. Imagine an audio codec that eliminated all scratching and crackling from music. Some people may not mind but many artists would hate it.
Film is the same. Directors choose the film type and grain patterns that match the look and feel they want. A more grainy film feels darker and more serious. Splotchiness contributes to an unrefined or maybe even psychotic feel.
You don't want your encode removing that. You want the atmosphere of the visuals to match the original, because the director likely had that atmosphere in mind and made other decisions around it.
Well, the problem is that the encoding process in AV1 does remove grain, only to add fake synthetic grain back in. My objection is not to the idea of encoding grain, it's with replacing it with fake synthetic grain.
It's kitschy, ersatz, whatever you want to call it. I used to spend a lot of time with artist filmmakers (many of whom are very nostalgic about celluloid) and there is no way in hell they would accept this solution. It wouldn't meet their standards of authenticity. It also "fakes" a medium-specific property in a way that is unprecedented in audio or visual coding. Their solution would be to either code at high enough bitrate to capture the grain, or insist on screening on physical celluloid.
That might sound extreme and unrealistic—it's what artist's film people are like—and even a bit snobby. After all, a lot of the options out there for adding film grain effects (or scratches etc.) to video are not intended for use by professional filmmakers, depending on how you define professional. There is definitely an element of snobbery to the statement that film grain should never be faked, whether by compositing something onto your home video or in a hidden way inside AV1.
I hope you can see that I respect the director's prerogative in choosing grain, I just don't think this AV1 grain synthesis methodology is sound from an aesthetic/authenticity point of view. It's digital faking of an chemical/analog effect, which IMO makes it unavoidably kitsch for reasons to do with old modernist ideas of "medium specificity".
I couldn't easily pull up any screenshots, but I've been a video encoder for some years and I can tell you that psy-rd (the x264 option to add film grain) does wonders in terms of grain fidelity.
The problem with matching the grain perfectly without the randomization is that it drives the bitrate crazy. A movie with 99% fidelity grain and 0 psy-rd will have a bitrate of 30mbps+.
If you use psy-rd though, you can get to 99% fidely grain at closer to 20mbps. The two screenshots will be visually indistinguishable, even when you are rapidly switching between the source and encode, even though you know that the grain is being randomly generated you can't tell.
If I get a chance later today I'll drop some comparison screenshots for a film encode that used a lot of psy-rd, I think the results will surprise you.
In other words, it is complete nonsense, texture synthesis, like the way some people go for printing a digital image on a fake "canvas" for a "textured, 3D look"... or maybe one of those flickering flame light bulbs.
Can anybody here imagine an audio codec that added simulated hiss and crackles to make it sound like you were listening to a vinyl record? That's what this is.