HDV and keying
 

Much of our recent work has been done using PD-150s and DVX100s. And we have always had problems getting good keys out of the DV codec. This is because, as I understand it, DV (not professional DVCAM) generalizes color information across 4 pixels square, resulting in keys that are not sharp.

I'm wondering if this will be true of HDV also. I'm considering getting one of the Sony cameras for a broadcast documentary we're working on but don't want to do this unless I can be guaranteed that shooting interviews with a blue screen will result in solid keys. Has anyone tested this and/or knows how HDV treats color information? How does HDV compare with HD?

Loch

NTSC DV, DVCAM and DVCPro are all 4:1:1

HDV is 4:2:0 which is as bad as 4:1:1 imho.

Graeme

You could shoot HDV and downsample to SD. You will get near 4:2:2 quality which would be ideal for keying work.

Thanks guys. 2 potentially stupid questions. In 4:1:1 and 4:2:0, what do the different numbers stand for? And what does "imho" stand for?

Yup, I'm getting too old for this biz.

Loch

4:1:1 means that for ever 4 luma samples horizontally there's 1 sample for Cb and one for Cr (Cb and Cr being the proper names for the chroma components of digital video, normally and incorrectly referred to as U and V)

4:2:0 means that for every 4 luma samples there are 2 samples of chroma horizonally - but none on the next row, alternating with Cb on one row and Cr on the other - it's rather messy, but you still end up with chroma pixels being 4 square rather than 4 long as with 4:1:1.

IMHO - in my humble opinion.

Graeme

<<<-- Originally posted by Davi Dortas : You could shoot HDV and downsample to SD. You will get near 4:2:2 quality which would be ideal for keying work. -->>>

I would aruge you get a better key by keying at HD resolution the downconverting to SD..

Here's a still image from a chroma key I did from a blue wall.

http://www.miketiffee.com/hdv/chromakeytest.jpg

Thanks for that Mike. That does look good. Any special tricks or keyers that you used? And you're right that I wouldn't want to down convert to SD because the final product would be in HD, that being the point.

I've had mediocre luck doing keys in the AVID, haven't tried FCP. Boris does a good job through AfterEffects, but that's time consuming.

Loch

I tried a key in After Effects using a bluscreen shot Kaku Ito had posted to the boards about the start of November. He shot it using a Chromatte background, which is where you have a reflective glass beaded material and an LED light ring around the lens.

I wasn't terribly impressed with the quality of the shot, as the blue background was less than smooth, and the talent was too close to the backing, throwing additional shadows onto it and making it hard to key. I also had to convert the footage using MPEG Streamclip, which may have introduced additional artifacts into the footage. Kaku also admitted that it was a quick, down-and-dirty test, and it was his first ever bluescreen shoot. I actually liked that, because it means more of a real-world test in less-than-perfect conditions.

I will say that even with all that, Kaku's FX1 footage was no worse than DV, and I would say that the additional resolution actually made it easier to fine-tune the key. For interview situations, I would have no hesitation shooting with the FX1. When shooting interviews, you typically have a very controlled environment to shoot good plates. There is also minimal movement. You also don't need hollywood quality keys. All of these are pluses when trying to pull a key.

If you already have the AfterEffects Production Bundle, you have awesome keying tools without additional plugins. The Color Difference Keyer is suitable for almost every well-shot plate, if you take the time to learn how the filter works. The book Creating Motion Graphics with After Effects has a whole chapter dedicated to using the Color Difference Keyer, and I have found it possible to pull very good keys, even from DV. If you are finding keying to be too detail-oriented and time consuming. you may want to look at the Primatte keyer, which allows good results quickly, and with minimal tweaking.

I can email you a few before/after stills from the Kaku footage if you are interested. I'm afraid I don't have a website I can post them to from work.

<<<-- Originally posted by Loch Phillipps : Thanks for that Mike. That does look good. Any special tricks or keyers that you used?
Loch -->>>

I used a linear color key in After Efffects. The Key was very clean even when played back in RAM- it wasn't dancing or crawling.

Thanks Mike. Yeah, Scott, go ahead and send me the Kaku keys you did. I've never heard of this Chromatte method. I was thinking of just running around with a bluescreen fabric, throwing it up and doing my interviews in front of it. We do doc work and don't have studio type luxuries.

Not sure if you're supposed to post your email here or not but:

loch@offrampfilms.com

Thanks for all the feedback. Just trying to punch up our advocacy doc work for maximum effect and this helps. Any lighting advice would be appreciated as well. Is it a good idea to use a backlight for instance? I know you want as much separation as possible between subject and background, but this is something we don't always have, plus it means carrying a large backdrop and means of hanging it.

I've had good success with a Westcott Collapsible Illuminator background in a Chroma-Key Blue. It folds up into a small bag, then unfolds like an auto sunshade with a spring steel frame. The frame keeps the material under tension, smoothing out any wrinkles in the fabric. It can also hang off a single lightweight stand, and the 6'x7' is large enough for most interviews. The $350 for background and stand may seem pricey, but the time saved in setups in a variety of locations starts to make a lot of sense.

I don't want to pull this thread off topic with a discussion of lighting techniques for chroma key, plus there has been plenty of discussion in the Photon Mangement Forum. I suggest you do some keyword searches in that forum, and if you still have questions, post them there.

I'd have to go with the three light setup (front, back, and filler). Basically all you need to do is just make sure that the subject doesn't cast a shadow onto the blue/green fabric, which would obviously mess up the key job.