4:2:2 Color, is it Worth it?

[Peter Moretti]

There ARE ways to get 4:2:2 color out of HDV camcorders (by using their HDMI, component or HD-SDI outputs). But is it worth it?

I imagine for fast moving subjects, the answer may be yes. But I'm just not sure if 4:2:2 is worth the considerable extra hassel and expense.

Your thoughts on the subject?

Thanks much!

[Glenn Chan]

Well you also get to avoid lots of compression. If you are shooting blue/greenscreen or special effects work or lots of color grading, that can be helpful.

2- What are you shooting? If you're shooting a doc, then the higher quality recording is not that feasible (unless you get something like the Red camera).

[Peter Moretti]

Originally Posted by Glenn Chan

...

2- What are you shooting? If you're shooting a doc, then the higher quality recording is not that feasible (unless you get something like the Red camera).

The first step of my documentary is to shoot the talking head interviews. Not a lot of movement subject movement. But not a lot of camera movement either. So I COULD capture to a computer. Cineform would give me the color and a reasonable bandwidth/file size.

[Bob Grant]

If you're downscaling to SD you can get close to 4:2:2 as part of the process. If you're delivering on SD DVD it's only 4:2:0. I'm not suggesting it's not desirable to acquire 4:2:2 or better but from a practical point of view and depending on what you're shooting I think there's more important things to worry about.

As for HDV and the motion problem, overrated from what I've seen.

[Peter Moretti]

Originally Posted by Bob Grant

If you're downscaling to SD you can get close to 4:2:2 as part of the process. If you're delivering on SD DVD it's only 4:2:0. I'm not suggesting it's not desirable to acquire 4:2:2 or better but from a practical point of view and depending on what you're shooting I think there's more important things to worry about.

As for HDV and the motion problem, overrated from what I've seen.

I definitely want to have an HD final cut. I agree about other things being more important, namely SOUND. But I'll get that next.

[John Bosco Jr.]

Is it important? That depends on what you are doing. You are shooting interviews without a lot of movement. Are the interviews going to be shot on green screen for chroma key in post? Are you going to need a lot of color correction in post? If the answer is no, then 4:2:2 is not necessary in your situation. By the way, 4:2:2 has nothing to do with motion... bypassing HDV compression does. In my opinion, the extra color information is not worth it unless you need to do green screen or have to color correct in post.

[Peter Moretti]

John, thanks a lot for your insight, it's much appreciated ;).

Originally Posted by John Bosco Jr.

... By the way, 4:2:2 has nothing to do with motion... bypassing HDV compression does.

One of the aspects of HDV compression is using 4:2:0 luminence and color sampling. Reading up on this, it seems that 4:2:2 can actually have motion artifacts as well (just not as many as 4:2:0). Only 4:4:4 color should be truly artifact free, FWICT.

[Jon McGuffin]

So are we saying that 4:2:2 is really no better than 4:2:0 and might actually be worse? I always though a larger colorspace would be beneficial. Frankly, due to the mathmatic nature of these terms, I wouldn't think it would be possible for 4:2:2 to not be as good..

[Peter Moretti]

No, no, no. 4:2:2 IS beter than 4:2:0. It's just that from what I've been able to digest on the subject, it seems that only 4:4:4 would be truly motion artifact free.

[Bennis Hahn]

Motion artifacts from color? I don't really understand how that is possible.

[Jon McGuffin]

Okay, glad we are on the same page. 4:4:4 I believe is pixel perfect with essentially no color sampling being processed assuming that the camera is really recording a true 1920x1080 image (for example). Obviously ideal for chroma-keying.

[Glenn Chan]

1- There are different 4:2:0 schemes in use, and this makes a difference.

The intuitive approach would be to apply 4:2:0 to the entire frame (both fields at once). But if you are shooting interlaced and there is motion, this doesn't necessarily work well. Chroma from one field (a particular slice of time) will bleed into the other field (a different slice of time). The bleeding in chroma from different slices of time can appear as a visible artifact.

So to solve that, you have to apply the 4:2:0 chroma subsampling on each individual field. But this raises a new problem in that you see combing artifacts in the chroma. So to solve THAT problem, you could blur the chroma vertically (this does not hurt resolution). However, this brings you back to square one. Now the chroma is bleeding between different slices of time. So to fix that, you need a good de-interlacer that detects motion. If there is motion, then the de-interlacer doesn't blur the chroma vertically.

So essentially... if you are shooting interlaced, that really screws up 4:2:0.

See the Wikipedia article on chroma subsampling and
http://www.hometheaterhifi.com/volum...ug-4-2001.html
for some pictures. The home theatre article is described this problem (interlaced chroma problem) and a different problem, the chroma bug if you get confused like I do.

2- In some rare cases, 4:2:2 color has subtle visual differences when compared to 4:4:4. This will happen if you have VERY saturated colors in the shot (which is rare except for colored lights, certain stage lighting).

3- If it's an interview, I wouldn't worry about it. Better to keep it simple, keep the subject comfortable, and spent more time interviewing them.

4- 4:2:2 doesn't have the same motion artifacts that using interlaced material with 4:2:0 (applied to both fields as a whole) does.

You could argue that 4:2:2 has aliasing artifacts that are made worse by motion. Subtle movement will cause the aliases to move and cause distracting crawling motion. But you won't really see this on 4:2:2 done properly, with reasonable filtering (e.g. linear/triangle resampling both on encoding and decoding).

[David Heath]

Originally Posted by Peter Moretti

No, no, no. 4:2:2 IS beter than 4:2:0. It's just that from what I've been able to digest on the subject, it seems that only 4:4:4 would be truly motion artifact free.

Colour space ratios represent the ratios of colour resolution to luminance resolution and don't have anything to do with motion artifacts - the latter are more a product of intra v inter frame compression. But it's worth thinking about what the colour space numbers actually mean.

4:4:4 means that colour resolution is as good as luminance, period. 4:2:0 means that colour resolution is only half that of luminance - both horizontally and vertically. Is that a problem? Well, not as good for keying etc, but it matches the behaviour of the eye, which is also more sensitive to luminance detail than colour, so it may be seen as a pragmatic way of saving data.

So what about 4:2:2? In this case colour resolution is half that of luminance horizontally (as for 4:2:0), but the same vertically - the system is assymetric.

Which begs the question why. My understanding has always been that 4:2:2 dates from the days when video was inherently interlace. A 4:2:0 approach would lead to the interlace causing samples on lines 1 (field 1) and 2 (field 2), but none on 3&4 etc - hence 4:2:2 makes much sense with interlace TV systems. It halves the number of chrominance samples (relative to 4:4:4), but you don't get the effect of two adjacent lines without samples, and possible related PAL/NTSC coding problems.

But with a PROGRESSIVE TV system the point of 4:2:2 is much less valid. Why have this imbalance between the vertical and horizontal colour resolutions? For progressive TV systems 4:4:4 and 4:2:0 both have validity in their own ways (both have symmetry), but 4:2:2? It may be argued that more colour samples are always a good thing. Maybe, but it comes at a price - higher data rate, or higher overall compression - so hence, yes, FOR TWO SYSTEMS OF THE SAME BANDWIDTH, 4:2:2 may not be such a good idea as 4:2:0. (A 10bit 4:2:0 system may be preferable to 8bit 4:2:2, for example.)

And these numbers only refer to what the recording system is capable of, NOT necessarily what the imaging system can deliver - the colour resolution of a Bayer sensor is less than it's luminance resolution, for example. (And hence more closely resembles the behaviour of the human eye.)

It's not worth getting too hung up on colour space. It's only one parameter out of many that define relative performance of cameras.

[EDIT: Glenn goes into more detail above, but I think the main message is the same. The 4:2:2 v 4:2:0 debate is very different for interlaced and progressive systems.]

[Glenn Chan]

For production, you would ideally record 4:4:4. 4:2:2 or lower can be visually different. (And obviously for chroma keying, 4:2:2 is not completely ideal; though in practice 4:2:2 is not the biggest obstacle to good keys, other factors are.)

The wikipedia article partly covers that. The other big reason why 4:2:2 is not visually lossless is because it pushes colors outside the R'G'B' gamut. For example, it can end up creating colors with negative green or blue values. Monitors can't emit negative light- that's a problem.

2- For emission / distribution, chroma subsampling makes sense for DCT-based codecs (MPEG2, MPEG4, etc.). For progressive systems, 4:2:0 of course makes sense.

But a lot of broadcast is being done in 1080i60. So 4:2:0 interlaced with 1080i60 broadcast has poorer chroma than a theoretical progressive system.

[Peter Moretti]

Originally Posted by David Heath

Colour space ratios represent the ratios of colour resolution to luminance resolution and don't have anything to do with motion artifacts - the latter are more a product of intra v inter frame compression. ...

David, I have to disagree (with my newly found and probably incomplete knowledge, LOL). It's pretty clear to me from Glen's posts, and other materials, that the lower sampling of chroma versus luma can cause problems with motion.