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8 bit vs 10 bit for chroma keying?

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Brian Ken
8 bit vs 10 bit for chroma keying?
on Mar 5, 2017 at 4:14:32 pm

Hi there.

I'm asking this because I understand the new GH5 camera will record 422 10 bit internal, however my main use for this camera will be green screen work. I know 422 will make a huge difference but I don't think 10 bit will help that much compared to 8? It's just extra information that I probably won't use? On many Youtube reviews, there seems to be a reduction in video quality when switching from 8 to 10 bit due to the strain it puts on the camera processor I guess. I just wanted to confirm if 10 bit actually has any advantage in chroma-keying at 150mbps or maybe an external monitor with my gh4 might be best.

Thanks.


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Chris Wright
Re: 8 bit vs 10 bit for chroma keying?
on Mar 5, 2017 at 6:56:30 pm
Last Edited By Chris Wright on Mar 5, 2017 at 7:00:10 pm

10 bit doesn't technically exist with h.264 compression at 150mbps or 18 MB/sec. Sorry GH5 guys!
Actually, trying to store 10 bit in the same bitrate compression as 8 bit causes the same data loss. It's just the laws of physics. For example, Prores 422HQ 10 bit 4096 x 2160 24fps is 209MB/sec and RED 4k 150MB/sec. That's about a 10x higher bitrate.

capturing 4:2:2 over 4:2:0 natively(which is twice the chroma resolution of what you need for good keying) is more important than how many grey values you have(8/10 bit luma precision) I would take a 4:4:4 8 bit camera over a 4:2:2 10 bit anyday.

Bit depth is how much precision you get per luma value. In the RED, they use 16bits to cover linear RAW dynamic range of 16. If you research a lot into this, you'll start to see a really interesting pattern. A general rule of thumb for holding linear dynamic range is a 1:1 ratio. ie. If you want to store more dynamic range into less bit depth, you'll have to do a compressed luma curve like
slog2. That's where all this crazyness came from. You steal bits from shadows and highlights which in turn, gets turned into noise. Native ISO affects noise as well.

All in all, you'll need to wait for GH5's new 400mbps codec to see any benefit for 10 bit. also, resolution is much more important than bit depth as a 10 bit camera at HD cannot resolve as well as a 8 bit at 4k.
Also, vlog with green screen is not recommended because it steals bits from the luma curve. Normally this works well with 10 bit cameras, but very low bitrates create noise that negates this feature realistically back down to 8 bit.

Higher quality would be to shoot with camera's neutral profile. It's like trying to export a cineon file at 8 bit. Won't really work.


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Brian Ken
Re: 8 bit vs 10 bit for chroma keying?
on Mar 7, 2017 at 7:49:06 pm

Great reply thanks. Yeah I was thinking that the 10 bit feature may do more harm than good! When the Gh5 does offer 400mbps it will be All-I as opposed to the initial IPB though.

I hear you on the vlog, the last thing I want is the put MORE pressure on the codec, so I was thinking if I were to conceal all areas of the green screen not in use with black fabric to reduce the amount of information in each frame and instead of boosting the shadows with master pedestal, just leave it alone, hopefully it will create footage that can be keyed easily.


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