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

[David Heath]

Originally Posted by Peter Moretti

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.

Firstly, the differences between interlace and progressive in this context must be stressed, and I don't think with a progressive system there is any connection between motion artifacting and colour space.

With interlace, then yes, there may be some motion effect, but I'd hazard that it is a secondary effect compared to the reduced resolution, and also that most motion artifacts are more likely to be associated with the intra/inter frame issue. But maybe my earlier sentence would be better reworded something like "Colour space ratios represent the ratios of colour resolution to luminance resolution and have less to do with motion artifacts - the latter are more a product of intra v inter frame compression. ..."?

A great quote from the link Glenn provides is "The reason this (4:2:0 on a DVD) is done is for efficient use of space on the DVD. Sure, 4:4:4 would be nice, but the DVD might have to be two feet in diameter. So, it's 4:2:0."

It would be great if we could all, always, record 10bit 4:4:4 uncompressed at the highest resolution in a camcorder sized package. That's not likely to be the case. Compromises are inevitable, and colour space issues are but one of many. If 4:2:2 means higher overall compression than 4:2:0, it may not be a better compromise.

But in the first post the question posed is:

".......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."

To answer that question directly, then if you are referring to a 720p HDV camera (ie JVC) then the speed of motion of the subjects will be irrelevant. If it's a 1080i camera, it may have some relevance, but I'd suggest that if you can get at the HD-SDI output, less compression will have significantly more effect than colour space issues on motion artifacting.

[Greg Boston]

Originally Posted by David Heath

If 4:2:2 means higher overall compression than 4:2:0, it may not be a better compromise.

That makes sense, David. However, everything using 4:2:2 that comes to my mind, is running a higher data rate to accommodate the extra chroma resolution, so that additional compression doesn't become necessary.

Examples:

DVCPRO25: 4:1:1
DVCPRO50: 4:2:2

XDCAMHD 35VBR: 4:2:0
XDCAMHD 50VBR: 4:2:2

-gb-

[David Heath]

Originally Posted by Greg Boston

That makes sense, David. However, everything using 4:2:2 that comes to my mind, is running a higher data rate to accommodate the extra chroma resolution, so that additional compression doesn't become necessary.

Absolutely true, and I stress everything I said was entirely theoretical, deliberately staying away from actual examples. Hence the earlier comment: "FOR TWO SYSTEMS OF THE SAME BANDWIDTH, 4:2:2 may not be such a good idea as 4:2:0".

DVCPro 50 should obviously be better than DVCPro 25 in quality terms - but that's due to lower overall compression as well as increased colour space. At the expense of double the bandwidth.

I really wanted to get away from statements such as "4:2:2 is better than 4:2:0" given without qualification. Imagine the choice was between 8bit 4:2:2 or 10bit 4:2:0, all else equal. Under those circumstances, the 4:2:0 option may well be the better one - especially for a progressive system!

[Glenn Chan]

To hopefully clarify things a little...

There are many different 4:2:0 schemes in use. They mostly differ in whether they operate on each individual fields (let's call this "interlaced sampling 4:2:0") or on both fields at once (let's call this "progressive sampling 4:2:0").

Progressive sampling 4:2:0 reduces chroma resolution (only) 2X in both directions. If interlaced footage is used in this scheme, motion artifacts can result.

Interlaced sampling 4:2:0 reduces chroma resolution 2X horizontally and *4X* vertically. No motion artifacts result if interlaced footage is used in this scheme.
*It gets slightly more complicated in that the chroma has a comb-teeth look to it; a good deinterlacer solves this.

2- Done poorly, chroma subsampling (any of the schemes, e.g. 4:2:2) can suffer from aliasing. Motion can make aliasing look worse. This is a separate issue.

[Greg Boston]

Originally Posted by David Heath

I really wanted to get away from statements such as "4:2:2 is better than 4:2:0" given without qualification. Imagine the choice was between 8bit 4:2:2 or 10bit 4:2:0, all else equal. Under those circumstances, the 4:2:0 option may well be the better one - especially for a progressive system!

No argument here, I'm shooting XDCAM HD and feel good about the quality. Others have used this system and reported pulling very clean keys.

I think something that sort of gets lost in all of this is the fact that we're talking about HD resolution. That, in and of itself, lends to a nice progressive picture, even at 4:2:0. Many folks that think they need 4:2:2 are basing it on what they've seen for years working with SD resolution. But when you render the same scene with 4X the number of pixels, the chroma resolution isn't all that bad.

-gb-

[Ken Hodson]

I think a consideration should be mentioned in regard to resoultion. DVCproHD records as a lower resolution (720p/1080p) regardless of being 4:2:2.
Your chroma rez is only related to your luma rez.

Second point, what about pixel shifting? Does a very low rez sensor eg. 960x540 deliver anything definative in regards to 4:2:2 resolution when it is displayed as 720p/1080p or even lower if it is DVCproHD? It is pixel shifting colour after all.

Third point is that you can not compare bit rate as a definitive for colour space comparison.
More colour space is not always more bitrate. The codec is a very important factor.
Eg.
DVCproHD 720p24: 4:2:2 960x720 40Mb/s
HDV 720p24: 4:2:0 1280x720 19.7 Mb/s (4-5 times more efficient codec)
So who is the clear winner?

[David Heath]

Originally Posted by Greg Boston

Many folks that think they need 4:2:2 are basing it on what they've seen for years working with SD resolution.

Very likely true. And whilst in PAL land there is very little 4:1:1 coding (DVCPro being the obvious exception) I understand that with NTSC DV25 is 4:1:1. The obvious problem being that when going to DVD (4:2:0) the resultant is effectively 4:1:0. With 4:2:2 that doesn't happen, but 4:2:2 is preferred not because of what it is, but because it is NOT 4:1:1, if that makes sense!

DVCproHD records as a lower resolution (720p/1080p) regardless of being 4:2:2.

Yes, and interesting to speculate whether the overall result would be better if it was 4:2:0 1280x720 - trade off chrominance samples for extra luminance. My understanding is that DVCProHD is what it is due to an initial derivation from DVCPro25, and using 4 chips in parallel. A single chip gives 480i/30, a pair gives 480p/60 (and 4:2:2 cf 4:1:1), and two pairs gives twice as many samples - 2x720x480, or 720x960. Can anyone comfirm or deny that?

Second point, what about pixel shifting?

Like a Bayer sensor, pixel shifting inherently gives a higher luminance resolution than chrominance - nothing wrong with that, it mimics the eye. But I do feel that if recorded 4:2:2, you may not be getting what you think you are. Although the recording format may be 4:2:2, the *system* is not giving equal lum/chrom vertical resolution.

Trouble is, the more you go in to the subject, the more complex it becomes. I believe that for some applications where bandwidth is limited (such as digital terrestial transmission) chrominance is allocated fewer bits/sample than luminance, and this is a major cause of the contouring that may be seen on areas of nearly uniform saturated colour. What is then needed is lower compression, not a higher colour space. (Which would need even more compression!)

[Bob Grant]

What's needed isn't so much less compression, it's better compression. Given that DCVPro HD is 100Mbits/sec it's possible today to fit stunning quality into that bitrate, modern wavelet codecs can deliver 4:4:4 full raster HD at that bitrate and not raise a sweat.
Not so long ago the problem was the silicon to encode and decode those kinds of data streams were the problem. Today that's no longer the case with off the shelf CPUs.

[Ken Hodson]

Originally Posted by Bob Grant

What's needed isn't so much less compression, it's better compression.

I agree that compression isn't the concern. The on board processors these cams carry improves every year. For example the "super encoder" on the JVC 200 series delivers twice the frame rate as the older 100 series (720p60 vs. 720p30) and offers better image quality, all in the exact same 19.7Mbs bit rate. On the older DVCproHD codec a jump from 960x720@24p to 960x720@60p results in a jump from 40Mbs to 100Mbs.

[Thomas Smet]

Here is a image I have shown before about chroma sub sampling.

Chroma sub sampling does not have to be a bad thing.

4:2:2 is about as good as you could ever hope for when it comes to keying only. 4:2:2 means the chroma pixels are 2x1 pixels in size compared to the luma pixels being 1x1 pixels in sixe. All it means is that the chroma pixels are twice as wide.

true progressive 4:2:0 (XDCAM progressive mode, JVC HDV1, Canon F modes) has chroma blocks that are 2x2 pixels in size. this creates very clean results because while it has larger chroma pixels then 4:2:2, there is a equal amount of pixe overlap in both the horizontal and vertical directions.

4:1:1 was bad because the chroma blocks were 4x1 pixels in size. This worked great for interlaced video but it creates holes that are 4 pixels wide which show up very well in keyed footage.

4:2:0 interlaced is better then 4:1:1 but it is a little complex to deal with because it has to alternate with the fields. When you key 4:2:0 interlaced material it is going to have lines that seem to be out of phase and it will look funky. As soon as you playback this footage however it will be corrected because a display will show only one field at a time either because the display is analog or via bobbing or some other method. So if interlaced footage is meant to stay as interlaced then 4:2:0 is not bad at all. The area that is a problem with interlaced however is when you start to have progressive inside of interlaced.


Think of keying a 4:2:0 progressive 1080 HDV image as keying a 720x540 4:4:4 image and then doubling it's size. but then being able to pull in extra luma detail.

Good keying also comes down to the software you use. The best way to key footage is to interpolate the chroma samples back in. My favorite way to do this is to blur or resample only the chroma channels. This gives me a soft 4:4:4 chroma channel to key with. Yes it isn't any more detailed but it doesn't have the jagged edges either. This method even works well with 4:1:1 material but it works very well with 4:2:0 progressive material.


http://www.dvinfo.net/conf/attachmen...0&d=1159424266

[Jack Jenkins]

Is there really a perceptual difference between 4:4:4 color and 4:2:2/4:2:0?

[Glenn Chan]

In some (rare) cases yes. It can happen if:
A- You have transitions between very saturated colors, like pure green text on a purple background (or yellow/blue or red/cyan). You get dark bands where the chroma transitions are. How dark the bands are is proportional to saturation; so in real world scenes (where colors aren't as saturated) this isn't really a problem.
B- Saturated text on a black or white background.

see the pictures at codecs.onerivermedia.com
In the top right part of that test pattern, none of the 4:2:2 codecs can get the red lines on the black background correct.

[Jack Jenkins]

So really the only drawbacks to 4:2:2 or 4:2:0 color is that the compressed colors can cause artifacting when pushed to the extreme saturation levels and that obviously keying an uncompressed 4:4:4 piece of footage is gonna be cleaner because there is more information. But basically for real world colors/contrasts there is no perceptual difference between a 4:4:4 and a 4:2:2/4:2:0?

[Glenn Chan]

If you are encoding something with very high saturation, then 4:2:2 can exhibit artifacts.

But basically for real world colors/contrasts there is no perceptual difference between a 4:4:4 and a 4:2:2/4:2:0?

In the big majority of situations, yep. In very saturated lighting, 4:2:2 can fall apart slightly (e.g. red LEDs, concert lighting, neon lighting, etc.).

[Jack Jenkins]

Whew, that makes me feel better. I did so much hand wringing over the Red's 4:4:4 and HVX's 4:2:2 vs the A1's 4:2:0. In the end I bought the CANON and am very pleased.