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CHROMA SUBSAMPLING in RESOLVE 4K to 2K

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Devinda Fernando
CHROMA SUBSAMPLING in RESOLVE 4K to 2K
on Oct 15, 2015 at 5:37:33 am

I have seen a few YouTube videos claiming that when you shook 4K on most Phones (Samsung Galaxy, iPhone, etc) and on low end cameras like GoPro, or the DJI Phantom 3's own gimbal cam, or even the Sony a7s or the Panasonic GH4... you basically get 4K at 4:2:0 Chroma sub-sampling. But if you downscale that to 2K in Davinci Resolve, or even Adobe After effects or Premiere, you end up with 2K at 4:4:4 (RAW) as your end result. The explanation given is that 4K at 4:2:0 is 8 Million Pixels, of which every 4 pixels shares the Chroma data of 1 Pixel, but when you downscale to 2K (2 Million Pixels) those 4 Pixels are merged into 1 pixel and thus each pixel has its own Chroma Data - and thus making the 2K 4:4:4 (RAW). Can anyone validate this claim?


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Marc Wielage
Re: CHROMA SUBSAMPLING in RESOLVE 4K to 2K
on Oct 15, 2015 at 11:25:47 am

They may pad out the bits with empty data, but you don't get "real" bits. The color damage has already done with 4:2:0 or 8-bit cameras. You'd have to do tests to determine how visible the damage will be, but one slo-mo shot won't generally hurt the project, assuming good exposure.

There are issues trying to pull clean keys off of 4:2:0 or 8-bit images, just because the keys break up more due to lack of information. Those cameras would not be a good choice for green screen VFX, at least to me.


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Devinda Fernando
Re: CHROMA SUBSAMPLING in RESOLVE 4K to 2K
on Oct 15, 2015 at 11:39:01 am

I totally agree with you on the keying and VFX... Obviously going from 4K to 2K would not give accurate pixel chroma data when you merge them together. I was mostly interested in being able to match footage taken from good cameras (Sony Fs7) that record in Apple ProRes or even 2K Raw with footage taken from a GoPro Hero 4 and the DJI Phantom 3. Both the action cams only take 4K 4:2:0 8 Bit, whereas the Sony Fs7 does 4:2:2 or 4:4:4 10 bit.


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Michael Gissing
Re: CHROMA SUBSAMPLING in RESOLVE 4K to 2K
on Oct 21, 2015 at 11:21:05 pm

Yes I heard this argument last night from a Panasonic rep but it doesn't make sense in the real world. You don't get RAW of course because the original is mpeg codec so there is no way to change colour temp, ISO etc.

The grey scale limitation of the 8 bit signal cant be made 10 bit just by combining four pixel information into one in my experience. As others have indicated the codec will bake in certain characteristics.


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Marc Wielage
Re: CHROMA SUBSAMPLING in RESOLVE 4K to 2K
on Oct 22, 2015 at 12:33:26 am

[Michael Gissing] "Yes I heard this argument last night from a Panasonic rep but it doesn't make sense in the real world. You don't get RAW of course because the original is mpeg codec so there is no way to change colour temp, ISO etc. "
That's true, but if it's a log recording and you have the range, you can generally get there. I would rather have a true raw file to work with if possible, but you can still work with a high-resolution MPEG to the point. There are varieties of H.264 that are not ideal and will bog down the works due to overhead. 8 bits is a bigger problem, particularly in keying and qualifying.


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Devinda Fernando
Re: CHROMA SUBSAMPLING in RESOLVE 4K to 2K
on Oct 22, 2015 at 1:47:58 am

*** There are varieties of H.264 that are not ideal and will bog down the works due to overhead. 8 bits is a bigger problem ***

I guess its also must be how you Export the final output too? I mean if you import a 4K 4:2:0 8 Bit to your Editing software, then export it at 2K at also 4:2:0 8 Bit, then you are losing the additional Chroma Data from the downscaled 4K image. You would have to Export the final Output with a Codec that preserves the 4:4:4 Chroma data like Apple ProRes 4444.


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Devinda Fernando
Re: CHROMA SUBSAMPLING in RESOLVE 4K to 2K
on Oct 22, 2015 at 1:54:50 am

*** You don't get RAW of course because the original is mpeg codec so there is no way to change colour temp, ISO etc. ***

True... I used the word "RAW" slightly incorrectly here. I was really referring to uncompressed Chroma Subsampling; you are right as it cannot logically be possible to create detail data from thin air, so if the 4K footage is only 8 Bit Color, then the resulting 2K cannot be any greater. So I guess the best I can hope to get from a pro-sumer 4K camera is 2K (actually 1080p) at 4:4:4 8 Bit.


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