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Author Topic: How does BMPCC video/film compare to Canon DLSR with Magic Lantern raw video?  (Read 23819 times)
John Brawley
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« Reply #60 on: October 23, 2013, 05:33:28 AM »
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"Chroma subsampling is the practice of encoding images ...."

The conversion of some R'G'B' format to some Y'CbCr format.....

The point being that you're converting.

So why use terminology from what you're converting to when you're talking about what you're converting from ?

jb
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hjulenissen
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« Reply #61 on: October 24, 2013, 06:17:12 AM »
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You are confusing digitally obtained data as lossless. 0 = 1, so to revert 1 = 0. To obtain this data you must first sample, and then sub-sample. There are many analog sampling stages before you even begin to see data.

Digitization is lossy by definition.
You are confusing subsampling with sampling. Every digital signal is sampled (and thus carries some error compared to some analog source). That is irrelevant to this discussion imho.

The act of converting (digital) R'G'B' to (digital) Y'CbCr is not digitization. No ADC is involved. No subsampling is involved unless you choose to (spatially) subsample the chroma channels to e.g. 4:2:2 or 4:2:0.

There is (usually) some loss in converting R'G'B' to Y'CbCr 4:4:4, but this loss is not what an engineer would call "subsampling", it is quantization and clipping. I routinely do such conversions in MATLAB that is lossless (due to using floating-point numbers).

For further background, I recommend Charles Poyntons book "Digital video and HDTV: Algorithms and interfaces", p.p 90, "Chroma subsampling".

-h
« Last Edit: October 24, 2013, 06:24:44 AM by hjulenissen » Logged
Sareesh Sudhakaran
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« Reply #62 on: October 24, 2013, 07:32:06 AM »
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You are confusing subsampling with sampling. Every digital signal is sampled (and thus carries some error compared to some analog source). That is irrelevant to this discussion imho.

The act of converting (digital) R'G'B' to (digital) Y'CbCr is not digitization. No ADC is involved. No subsampling is involved unless you choose to (spatially) subsample the chroma channels to e.g. 4:2:2 or 4:2:0.

Well I guess the camera manufacturers told you when they do what, in what sequence, and how.

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There is (usually) some loss in converting R'G'B' to Y'CbCr 4:4:4, but this loss is not what an engineer would call "subsampling", it is quantization and clipping. I routinely do such conversions in MATLAB that is lossless (due to using floating-point numbers).

What is the definition and process of quantization?

Anyway, this argument is pointless. So I'm going to stop.
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hjulenissen
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« Reply #63 on: October 24, 2013, 08:10:42 AM »
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Well I guess the camera manufacturers told you when they do what, in what sequence, and how.
No, but the corresponding ITU documents as well as the reference litterature gives a pretty good description of _what_ should be done. If you want to claim that some manufacturer does it very differently, I think it is reasonable to expect you to support that claim.
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What is the definition and process of quantization?
http://en.wikipedia.org/wiki/Quantization_(signal_processing)

My attempt (at describing basic quantisation):
For some real number R (e.g. '4/3'), represent it by the nearest integer (i.e. '1').
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Anyway, this argument is pointless. So I'm going to stop.
Yes, I have found this branch of the discussion to contribute little to the discussion. I guess I have a hard time not responding whenever I see some claims that are (in my view) totally wrong. I shall work on my "let it go"-skills.

-h
« Last Edit: October 24, 2013, 08:13:35 AM by hjulenissen » Logged
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