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Why is chroma subsampling so popular?


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#1 james smyth

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Posted 11 December 2006 - 05:30 PM

Why do people use chroma subsampling? Does 4:4:4 really use THAT much more space? I mean, when something as simple as color bars becomes a yucky mess, what's the point? In some situations it can be hard to tell, but most of the time it jumps out at me. Chroma subsampling and interlacing are the two reasons I hate producing on video.
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#2 Phil Rhodes

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Posted 11 December 2006 - 05:49 PM

Hi,

4:2:2 pictures take up two thirds the space of 4:4:4 pictures - think about it - half the resolution of two of the planes and you have saved a third of the space. It's done because getting video onto tape is a squeeze at the best of times.

I fully agree that it's best avoided for higher-end production; just be aware that the pictures produced by things like Red, Dalsa and Genesis are adulterated in a very similar way (though through an entirely different process).

Phil
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#3 james smyth

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Posted 11 December 2006 - 05:57 PM

I fully agree that it's best avoided for higher-end production; just be aware that the pictures produced by things like Red, Dalsa and Genesis are adulterated in a very similar way (though through an entirely different process).

Phil


You mean the bayer filter? It pretty much is the same thing, but at least it can be scaled down to a usable resolution (shoot at 2k and scale down to 1080p for an HD production). There's nowhere for NTSC to hide its flaws.
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#4 Michael Collier

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Posted 11 December 2006 - 06:14 PM

Well there are only 3 ways to compress video. 1. remove chroma sampling. 2. compress within the frame (cosine type-compression) or 3. compress across a group of frames (MPEG style)

Chroma sub-sampling is popular for a number of reasons. First is you can loose the most amount of data with the least visual impact. Not to say there is no impact, but from 4:4:4 to 4:2:2 there is very little visual signature, unless your planning extreme color correction or keying. Not so when you add compression. artifacts are very clearly visible, as whole pixels are changed (usually in high-frequency areas, where its more important to compress cleanly) . With chroma, only the accuracy of each pixel is compromised, and the luma channel does a lot to make sure the chroma isn't too far off.

The other reason is spead. Its incredibly easy to discard chroma to make for a lower-res channel. For cameras that have very taxed proccessors (dealing with all the new color correction tools being installed on them) dropping chroma is a quick and painless problem. For computers that need quick render speed and high quality, its painless to wait through a render.

In the end yes, 4:4:4 is a lot of data. Around 30MB/sec with audio. put the video in 4:2:2 and your around 15-20 MB/sec. then if needed you can add cosine (and get DVCPRO25) or add MPEG and your down to size. Its so popular because its the first method employed to drop the data down so the cosine compression isn't so high (think really bad JPEG, or really bad MPEG, faced between those two or some subsampling loss, which would you take?)

Edited by Michael Collier, 11 December 2006 - 06:17 PM.

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#5 Thomas Worth

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Posted 11 December 2006 - 06:21 PM

(shoot at 2k and scale down to 1080p for an HD production)

1080p is, for all intents and purposes, 2K (1920 as opposed to true 2K, which is 2048). I think you meant to say "shoot 4K and downsample to 2K."
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#6 james smyth

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Posted 11 December 2006 - 06:34 PM

I think you meant to say "shoot 4K and downsample to 2K."


I thought 2k was the horizontal resolution. 2000 vs 1080.
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#7 Will Earl

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Posted 11 December 2006 - 07:51 PM

I thought 2k was the horizontal resolution. 2000 vs 1080.


2K is typically defined as 2048x1556 (full frame) or 2048x1157 (1.77) and is used where film is involved.

HD uses 1920x1080 or 1280x720 (both 16:9). The visual difference (resolution wise) between 2K@1.77 and 1080HD is minimal. The main visual differences come from other factors such as the lens used, the sensor, film stock, compression, etc.
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