Comment by ionwake
3 days ago
Sorry for the noob question but I think finally someone in this thread can answer this for me. Sometimes when I see a youtube short video it looks like its HDR is whacked up by like 500% as per the image in this page, but Im confused how this could be done. Is video processing on the video before it is uploaded somehow giving it some sort of encoding which chrome just wacks up? Or is it the hardware doing it and encoding it a certain way?
I am not talking about a slight brightness increase, I am talking Ill be scrolling youtube and suddenly this video is like a portal into another dimension its so bright.
Can anyone explain how its done?
Screens can't often do full brightness on the whole screen so if you come across a video or image that is supposed to have a higher contrast ratio, the system will darken everything and then brighten up the pixels that are supposed to be brighter.
Yes, there are formats that able to store a higher contrast ratio so that's why it doesn't happen on non-HDR content but the actual brightening of a portal on your screen isn't because of the format but because of your hardware (and software) choosing to interpret the format that way.
For more a practical example, if you had an 8-bit HDR image, 255 on the red channel (after inputting this number through a math function like HLG[1] to "extract" a brightness number) might mean "make this pixel really bright red" whereas 255 on a SDR format would mean "just regular red." However, each red channel is still a number between 0 and 255 on both formats but your hardware decided to make it brighter on the HDR format.
(Although in reality, HDR formats are often 10-bit or higher because 256 values is not enough range to store both color and brightness so you would see banding[2]. Also, I have been using RGB for my example but you can store color/brightness number many other ways, such as with chroma subsampling[3], especially when you realize human eyes are more sensitive to some colors more than others so you could "devote fewer bits" to some colors.)
[1] https://en.wikipedia.org/wiki/Hybrid_log%E2%80%93gamma
[2] https://en.wikipedia.org/wiki/Colour_banding
[3] https://en.wikipedia.org/wiki/Chroma_subsampling
> Screens can't often do full brightness on the whole screen so if you come across a video or image that is supposed to have a higher contrast ratio, the system will darken everything and then brighten up the pixels that are supposed to be brighter.
There's no system that does that. The only thing that's kinda similar is at the display level there's a concept known as the "window size" since many displays cannot show peak brightness across the entire display. If you've ever seen brightness talked about in context of a "5%" or "10%" window size, this is what it means - the brightness the display can do when only 5% of the display is max-white, and the rest is black.
But outside of fullscreen this doesn't tend to be much of any issue in practice, and it depends on the display.
> There's no system that does that.
You mean the darkening of everything else to highlight bright HDR areas? All recent Macs do, including the one I'm typing on right now. It's a little disconcerting the first time it happens, but the effect is actually great!
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I use YouTube on my rather inexpensive TV. When a thumbnail of an HDR video starts playing, the whole screen brightens up significantly. I don't know as much about HDR as you do, so maybe they are using some other perceptual trick. It might also not be "full brightness." BTW, why can't screens do full brightness on the whole screen?
Thank you so much for your reply - I will look into it!
The video is marked as containing a different color space with a higher brightness/color range. That could either be because the initial camera recorded it that way (e.g. iPhones can do that) or because someone took a "normal" video and edited it.
Would this be specific software on the iphone used to record a video ? Or a default setting on a certain iphone? I ask because I only very rarely see this whacked up HDR youtube short, like super rarely.
Fairly sure a stock iPhone can do it, but you might need to enable it explicitly for compatibility reasons? And depending how you edit or upload the video it could get lost there too, it's still something where support is not really universal.
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Does this video look super whacked up to you too? It is HDR: https://www.youtube.com/watch?v=ceSiK-0HX_I
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There's many factors in play from what your SDR white point is at, how your OS handles HDR video, what the content contains, and finally what your brain is doing.
HDR10(+) & Dolby Vision, for example, encode content at absolute luminance, so they are basically completely trash formats since that's an insane thing to expect (the spec for authoring content in this format literally just goes "lol idk do X if you think it's going to be seen in a movie theater of Y for TV and hope"). Sadly, they are also quite common. Mobile phones (both Android & iOS) are instead pushing HLG, which is better. Although then hilariously MacOS's handling of HLG was atrocious until the latest update which fixed it but only if the video contains a magic flag that iPhone sets, but isn't standard so nobody else sets it (the "avme" tag https://developer.apple.com/documentation/technotes/tn3145-h... )
There's then also just how your eyes & brain react. When HDR shows up and suddenly the white background of a page looks like a dim gray? That's 100% a perceptual illusion. The actual light being emitted didn't change, just your perception of it did. This is a very hard problem to deal with, and it's one that so far the HDR industry as a whole has basically just ignored. But it's why there's a push to artificially limit the HDR range in mixed conditions, eg https://github.com/w3c/csswg-drafts/issues/9074
You clearly know alot about this, but I think there could be a misunderstanding. Not trying to offend but when I see the youtube link mentioned above in the other comment, my macbook screen literally goes darker AROUND the video , which gets brighter. I am not making this up. I think its how chrome on macbooks handles raw HDR encoding.
Can someone else confirm I am not mad?
PS - I am not trying to shut you down, you clearly know alot in the space I am just explaining what Im experiencing on this hardware.
> my macbook screen literally goes darker AROUND the video , which gets brighter. I am not making this up
This is almost certainly your eyes playing tricks on you, actually. Setup that situation where you know if you scroll down or whatever it'll happen, but before triggering it cover up the area where the HDR will be with something solid - like a piece of cardboard or whatever. Then do it. You'll likely not notice anything change, or if there is a shift it'll be very minor. Yet as soon as you remove that thing physically covering the area, bam it'll look gray.
It's a more intense version of the simultaneous contrast illusions: https://en.wikipedia.org/wiki/Contrast_effect & https://en.wikipedia.org/wiki/Checker_shadow_illusion
Eyes be weird.
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