DVX and Chromakey Performance.

L

LUCAS_HITCHBERG

Active member
Hi, I'm back again to cause trouble. Actually, I just have a techical question stemming from Adam Wilt's review of the DVX100A. He reported a "chroma patterning problem" (see www.adamwilt.com for specifics.) The upshot is that some colors may pulse during certain conditions and this may make greenscreen work much more difficult. Can anyone confirm or dismiss this potential problem?

Also, has anyone used Serious Magic Ultra with the DVX100A?

Did it make chromakeying much better and easier to achive?

Thanks in advance for your feedback!

LHB
 
Re: DVX and Chromakey Performance.

No specific comment other than to sidetrack the discussion... chroma keying is always going to be more difficult on DV than you may otherwise want, because of the 4:1:1 color sampling. Does anyone have any significant experience with luma keying? Luma keying should deliver pixel-perfect keys (since you're keying against all 720 horizontal pixels, instead of just 160 like chroma keying relies on).

Serious Magic's Ultra looks like serious software though, their demo is just riveting.
 
Re: DVX and Chromakey Performance.

Barry,

I posted the same question to another forum and got many (all positive) replies on the DVX100A's ability to do greenscreen. More than one believe the maxim that "good DV25 chromakey is not obtainable" is false and stems from the old analog days of yore. Some people expect Hollywood results from prosumer equipment and for prosumer equipment the results are still very good. No replies on Ultra. Maybe it's just not needed? That's a good indicator to me that proper set-up is the key!

When Ultra lets you create your own sets and adds a few more features, it will be even better. Wish I could aford it - I'm saving for a DVX100A.

LHB
 
Re: DVX and Chromakey Performance.

I've seen good keys off DV, it's just nowhere near as easy or clean as it could be. You need to do a lot of "matte choking", etc. It just seems like a luma key should be incredibly cleaner and easier (provided your subject doesn't have a full spectrum of brightness).

I'm not saying you can't pull good keys off DV material. I'm just questioning whether it's the "right" way to do it, for DV. With 720 luma samples per line, vs. only 160 chroma samples, it sure seems like you should get a cleaner key off of a luma key vs. a chroma key.

But then again, what do I know... I rarely do keying...
 
Re: DVX and Chromakey Performance.

Hmmm. That's interesting. What's different on a lumakey set-up as far as limitations vis-a-vis on chromakey. What I mean is, in greenscreen "don't wear a green pair of green pants, keep light flat". Are there sets of does and don't and can't for lumakeying?

I don't plan on keying a ton of stuff either, but once in a while it's handy. And it may be necessary for a couple of short features I'm dreaming up. BTW I know what lumakeying is, just not the pitfalls and proper setup parameters.

LHB
 
Re: DVX and Chromakey Performance.

I'm no expert either, I'm just saying that on theory, luma keying should be much better for DV than chroma keying.

The main thing to control on a luma key is to make sure that the background is substantially brighter (or darker) than anything in the foreground. You'll obviously be dealing with a compressed exposure range (vs. chroma keying, where you're dealing with a compressed color range).

Some testing is indeed in order.

But, I bet you a nickel MichaelP could pop in here and answer everything -- that guy is a tech wiz.
 
Re: DVX and Chromakey Performance.

I'm about to shoot a massive (for me) blue screen project this weekend with a DVX100, so I guess we'll find out how good it is, but I gotta say I'm not too worried. The Panny delivers a sharp picture without too much trouble, which is helpful for matte work.

My first ever green screen shoot was done in my basement with my girlfriend, one silked 1000 watt light, and a small 40 watt for fill. I'll see if I can dig up a still, but it was done with a cheap Panasonic DV51, and I thought the results were incredible.

In other words, its all in how you do your setup.

Quick tips:

- Light your background screen as evenly as possible, or if you can't do that, at least try to get even lighting on the background behind your talent; you can garbage-matte the poorly lit edges of your screen out in post.

Put your talent half-way between your camera and the screen, and make sure you are better than twenty feet away from your screen. The farther away, the better, because you don't want your camera to pick out details on your background, only your talent. Some poeple go so far as to hang clear plastic between their screen and the actor', to help blur the background, although I've never found this to be necessary.

- It helps to light your talent such that the lighting on them is what they would get if they were standing in the scene you plan on compositing in later. If you don't know what that is going to be, I suggest you try hard to get the background shot first! Also, shoot some shots with a person standing where your talent would be, if you can manage it. Shows you where all the lighting is coming from, and what kind, and in what quantity...

- A softened key light on your talent, with some sort of hair light helps to separate them from their background. See 'three point lighting.'

- Some software tools are better (and easier!) than others for compositing work. I got lucky with my first green screen experience, and got a key I was VERY happy with after about 5 minutes with an old RT2500 system. Your milage may vary. After Effects does a GREAT job with compositing; in fact that is what it really does the best. A plugin called DVMatte is worth getting because it is a real timesaver and its cheap, if you are going to use AE.

- If you have a monitor that can do B&W or blue only, you can look at your image and get a better idea where your screen is not as evenly lit as you thought. Take 15 extra minutes to get the lighting as good as you can, because every minute spent in setup is 10 minutes saved in post.

- Obviously, don't let the light you throw on your subject cast shadows on your screen. This usually necessitates 3/4 placement, or a lot of bounce and/or softening.

- If you have a Matrox RT100, and are doing a 60i production, this hardware is really great at pulling a reasonable key, even when you didn't take time to light your set correctly.

- Just like everything else, the better your source material, the better your final product will be.

- Finally, remember what your distribution medium will be. You don't need pixel-perfect keys if your presentation is only going to be displayed on the Web in a 160x120 window... on the other hand, if you plan to transfer your epic to film, Perfect isn't nearly good enough!

;D
 
Re: DVX and Chromakey Performance.

Time for me to chime in...

First off... After Effects is NOT the best thing for pulling keys. Most compositing programs let you use 3rd party keyers which are really excellent, so from a technical standpoint it doesn't matter one bit which compositor you're using. It pains me that AE is such a standard, cause I think the interface is a backwards piece of *&^%, but that's really a matter of personal preference I suppose, and has little to do with the quality of the final composite.

The very very very best chroma-keys from 4:1:1 source pretty much look like crap from a critical standpoint. i.e. It gets good enough that you can do 'virtual set' stuff like Ultra and it looks nice enough. But it's not going to fake anyone out in a situation where they're looking for the key. Particularly when you get into motion-- the matte lines really become evident. The most sophisticated software on the planet can not make something one-quarter the resolution (a chroma-key in 4:1:1) hold up when put up to full res.

That said, there's a lot you can do to get an impressive key despite the technical limitations. 'speedbump's advice was all good. One other thing I do for DV... is set up a lot of keys for one single shot. In other words-- if there are a variety of edges between your foreground and green/blue-screen, quickly draw some masks to divy-up the keying task amongst a few keyers. Then you can set each one appropriately for the consistent piece it deals with. Do a feathered edge to transition between the different sections of the matte, and voilá. It may seem counter-intuitive: doing MORE keys on one shot to save time. But once you get the architecture set up it really goes super quick 'cause each key is soooo much easier.

I've also tried some luminance-based keys, trying to capture the better res in 4:1:1. Unfortunately, I haven't had much luck. I seem to get a "halo" around everything I do. I'm experimenting with some stuff now that may be helpful... setting up an architecture that lets you sorta selectively choose between how much you want the key to be based on the chroma vs. the lumi. That way [the theory goes] you'd be able to find that sweet spot between the two.

OK enough rambling for now. If anyone else has had better results off a lumi-key, share share share!
 
Re: DVX and Chromakey Performance.

[quote author=speedbump ;D[/quote]

My first ever green screen shoot was done in my basement with my girlfriend, one silked 1000 watt light, and a small 40 watt for fill. I'll see if I can dig up a still, but it was done with a cheap Panasonic DV51, and I thought the results were incredible.


I'd love to see it. Please post the clip

LHB.
 
Re: DVX and Chromakey Performance.

Excellent site that talks about the issues with 4.1.1 color-sampling, and the best ways of attempting to deal with it:

http://www.neopics.com/bluescreen/

Some of the newest keyers, like Ultimatte's Advantedge, already incorporate the CrCb blur they're talking about. But the other keyers can get results just as good if you do it manually.
 
Re: DVX and Chromakey Performance.

01.jpg

02.jpg

03.jpg


Sorry bout the crappy JPEG compression
 
Re: DVX and Chromakey Performance.

Boogerg, nice key! What software did you use for that? Check her hair on the right side, that's tough to strike a good balance, but you did so.

Some software handles keys better than others. Here's some footage from a shoot I did this weekend.

http://entertain.com/movies

Click on the test link for blue screen.

There were several things that weren't optimal here. We shot the head in 24P, and I imported it as Matrox DV, 60i. I wanted to see how well my RTX100 does with keying in realtime, so this is what I got. I'm willing to bet that AE with DvMatte would do better, and Vegas should certainly be able to get better results with 24P footage.

Secondly, both the background footage and the foreground were colorized in post, so there was some loss of resolution around the edges. The background was stock off the Adobe Encore DVD, but it and the foreground were just too cheery, and this production needs more menacing overtones.

Third, that head shot was a last minute opportunity thing; I didn't have any plan for it, so any kind of lighting match between it and a background will have to be done in post, further degrading the image.

One more thing: since the foreground is a static shot, I could have just used a single frame, but since most of my material consists of live performances, I went with both background and foreground as motion video.
 
Re: DVX and Chromakey Performance.

That's a nice key booggerg. What trickery did you pull to do it?

That is to say... what did you do between the second framegrab and the third?

Nice screen too. You actually set up a big screen lit all evenly like that? Or is the first grab already after some sort of processing to smooth it out?
 
Re: DVX and Chromakey Performance.

These two clips are the before&after of a Cosby Intro spoof I created for student cultural show.

Original

Finished(no names of course)
 
Re: DVX and Chromakey Performance.

boog.. thanks that cosby thing was very enjoyable.
 
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