Chromakey question...

A

andy_starbuck

Well-known member
I have the Reflecmedia green lite ring and reflective backdrop for chromakey. Anyone that is interested should know that you need th LARGE lite ring for the DVX100a and not the SMALL lite ring, as the small one is exactly the same diarmeter as the lens. And no one mentions this in their literature.

I don't know if this is the right forum, but because it is "technical" I thought this might be the place to ask.

I shot several chroma tests. The backdrop is incredible and gives a perfect even chromakey shot. However, DV compression being what it is, a perfect edge is always blurred by 2 to 4 pixels by the DV compression algorithm. And I know this is a limitation of 4:1:1 subsampling color space. My question is: Is 4:2:0 any better for chromakey? I understand that PAL DV uses 4:2:0 color space, as does the MPEG2 compression in HDV. Does the PAL DVX100 record in 4:2:0 ?

Would buying a PAL DVX100A or say a JVC GRU1 HDV give me any better chromakey?

Thanks for your insight and information.
 
Re: Chromakey question...

4:2:0 would be different, but not necessarily "better". It has the same amount of color information, just in a different arrangement, so instead of having wide horizontal 4x1 blocks of color you'd have chunky squares of 2x2 blocks of the same color.

The JVC HD1 is a horrible camera for chromakey work, even though the resolution is 2.5x as high, the chroma noise makes for difficult keying. The upcoming Sony HDV camera may be a much better choice. I would never recommend anyone buy the HD1 for any purpose.

Do you perchance use Vegas to edit? You may be able to get spectacular keys if you used Vegas and applied the "Chroma Blur" filter prior to the key. If you don't have Vegas, post a few uncompressed frames or a second or two of footage and I'll see if I can key it with Chroma Blur and then you can decide if it's adequate for your needs.
 
Re: Chromakey question...

Just curious as to the price range of the ring and the cloth? No pricing info on the site nor on the Bogen site (U.S. Dealer) Thank.s
 
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B&H lists a kit with ringlight, dimmer, power supply and collapsible background cloth for just under $2000.
 
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B&H Photo Video online shows a number of kits. Just go to their site and search for reflecmedia. The small or large lite rings are the same price, and go for roughly $840 including the variable intensity power supply. And the backdrop depends on the size. 12x10' is about $3200, 8x8' is $2850, and they have a fold-up 7' ring that is about $1500. And that doesn't include anything to hang the backdrop.
 
Re: Chromakey question...

Just to add a few thoughts-

If you are using FCP- there are similar plugs to use as Barry mentioned for Vegas to "smooth" things out prior to keying. "Color Smoothing 4:1:1" is included with FCP4 and I think Adam Wilt wrote a plug to help with this- called "Steppy".

Also- are you sure that your special cloth and ring are not causing some of this? I saw either this product, or something like it at NAB last April and noticed that their even some of their "live" demo shots had crummy edges. I was "oohing" and "ahhhing" along with the crowd until a snooty fellow next to me pointed it out. I'm not sure what type of video they were using though (probably not DV). Anyway....

I have had good results with DV25 keying- but only when the greenscreen is lighted perfectly. Sure it's not perfect...but you get what you pay for. If you need PERFECT keys- jump up to DVC50 or even use component BetaSP into an uncompressed 8 or 10 bit system like Decklink.

Good Luck
Jon
 
Re: Chromakey question...

When I shot the chroma test I was in a confined space. So the figures are about 6-8 feet from the screen, and unfortunately the LEDs in the Lite Ring cast a shadow on the background. (Well, they are lasers, so it makes sense. I guess they are angled so that at a specific distance the shadow of one LED is eliminated by the light from another LED.) So you are right about the edges issue. However, I've tried it in a larger space with a fill light pointed at the screen, and the figure about 10-15 feet in front of the screen, and there wasn't a shadow problem.
I think what I'm seeing is the 4:1:1 compression artifact that turns the nice straight edge into a gradient. If I am shooting with 24p, then motion blur also messes with the edges if the figures are doing anything active.

I'm using Premiere Pro 1.5 with the Keylight Plug-in. And I've worked on the same footage with After Effects Pro 6.5. I have FCP HD on the way. Haven't used Vegas. Using all of those tools, and especially dropping in an edge matte in AE, I can get pretty good results. But there are still a lot of artifacts. And with two figures on screen, it is almost impossible to get both of them keyed properly. One of the projects I'm looking at would involve shooting 24pA widescreen with possible film out. And I think if I want to do chroma I'll have to go rent something with 4:2:2 as suggested.

If you want to play with some stills, I've uploaded them here:
http://www.zoomforce.com/dvxuser/photos/images//260092346.jpg
http://www.zoomforce.com/dvxuser/photos/images//260092405.jpg
http://www.zoomforce.com/dvxuser/photos/images//260092417.jpg

They are each about 50k.

Thanks everyone for your considerate and informed ideas. I really appreciate your insight. I'll assume that I need to budget for renting 4:2:2 if I am going to do the chromakey in this project. Thanks for saving me from investing in something that wouldn't help.
 
Re: Chromakey question...

Yeah, that's really tough source to key from.  The shadows are pretty bad.  I got a sort of decent key off the guy, using a combination of bezier masking, chroma keying and chroma blur, but there's not much at all that can be done with the girl.

http://www.icexpo.com/dvx100/Image0.jpg

You really need to get some distance between your subject and the background, to make sure no shadows fall on the background, otherwise I don't think you'll get much better results from 4:2:2.  Better, yes, but maybe not enough to justify the rental.  From what I see in your footage, the problem is caused primarily by the varying shades of green, from very dark in the shadows to quite bright elsewhere.  Try a new test, get the subjects at least 10' away from the screen, and try that in DV -- you may be surprised at how well it can do, or you may have to rule out DV totally and just go with 4:2:2 (which, by the way, you can't edit/composite in DV or you'll end up with the same problem!)

If the ringlight itself is what cast the shadows, that would be bad news.  I haven't used that system, but -- how can it cast a shadow that it can see?  If the light is going directly out of the ring, 100% perpendicular to the plane of the lens glass, then theoretically shouldn't it be impossible for the camera to see any shadow caused by the ringlight system?
 
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The hair in the second capture will be almost un-greenscreenable without pulling a lot of seperate mattes.  I often am forced to rotoscope the head and greenscreen it seperately.

I did greenscreen this, sucessfully, though:
image013.jpg

But most people I've shown it to think it looks horrible.  I don't, but I'm not my target audience, sadly.  (Though this is from a total vanity project.)

There's a plug in FCP that approximates 4:2:2 colorspace from of DV, btw.  And if you just do chroma smoothing it fixes 4:1:1 issues pretty well, though you'll get blurry edges which are a nightmare to fix.
 
Re: Chromakey question...

Yep- I think more distance from the screen would help.

Another trick I've learned is to use less light on your subject, get that iris opened up and get the cam back a ways and zoom in. With enough distance you will get a rack focus that will naturally smooth some of the imperfections of your green screen out.

Also maybe just a little "back light" on your subjects might help clean up the line between subject & greenscreen- some folks even use a subtle magenta gel on this to further push the separation.

Also give Digital Film Tools plugin "zMatte" a try- I tried the demo a while back and it was awsome for the DV tests I ran at work.

DV is keyable- just a pain sometimes.
 
Re: Chromakey question...

Barry, Yes. I think you are right. I was confused by the shadows when there were two figures on screen. Notice that the shadow from the guy falls to his right. But the shadow from the girl falls to her left. I had thought originally that the shadows were caused by the ring, because how else could I have two figures with shadows going in different directions? But now I see that it must be the lighting that was casting shadows. I guess the key light is causing his shadow and the fill light is causing her shadow. So... distance, controlling the lighting, shadows... I have a bad test here. I'll need to try another test.

Policar, That looks great to me. I think I see some artifact to the top left of his ear.

Thanks again!
 
Re: Chromakey question...

Hey andy_starbuck,

Interesting grabs. I've been wondering how that reflecmedia stuff really held up. Do post some more pictures once you get the lighting as good as you can. I'm really curious.

And what kind of background is this stuff supposed to go in front of?
 
Re: Chromakey question...

Have you tried Ultra from Serious Magic? I believe they have a downloadable trial version. I have not worked with it, but have with other products from them. I have read good reports about Ultra's ability to key cleanly on DV.
 
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Tried to use ULTRA. With stills posted above it's nearly impossible. 2-shot was completly useless with Ultra. Other images were hardly keyable but with some twaking i was able to get some more or less usable shots.
 
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I use FCP's 4:1:1 smoother (Graeme Nattress's G-Nicer is better if you have 100 bucks), then export to lossless then use Keylight in After Effects. Keylight takes a little work to get used to, but it's remarkably simple and yields awesome results. If I don't get what I like, though, I do some rotoscoping and seperate the hair from everything else and use the matte choker seperately. This way I can get soft hair and not-very-soft other areas. If worse comes to worse, color that has been dropped from areas can be painted on later.

Still, I get lots of issues with spill that color correction won't take away and edges are never quite perfect.

And the thing to the left isn't an artifact; I think it's a weird thing in the background. I think.....
 
Re: Chromakey question...

Those images have already been smoothed....looks like you're using Avid or another codec that smooths for you.

I'd just use keylight, but only because that's why I'm used to.
 
Re: Chromakey question...

This is the result using AE with Keylight and throwing a Matte Choker on after.
greenfinal_04.jpg


The reflecmedia can work really well. The LiteRing has an intensity control, and as you turn it up it is amazing. The background goes from grey to green and at some point all the detail (folds/lines) in the background curtain just drop out. You set all your exposure with the LiteRing off, then turn the intensity up until the backgound overexposes.

I worked on an educational interview video using this technology. We had two experts that were supposed to be talking about the same thing. Problem was that their schedules wouldn't allow them to be interviewed at the same time. So we shot it using reflecmedia and combined it in post. We cut it so that each person showed reactions to what the other was saying. No one ever knew they were interviewed weeks apart. That was shot in a studio with the background pinned to a wall. Someone else did the post production work.

So I've seen it work really well. I'm still getting a handle on how to use the technology. In these tests I had the subjects too close to the screen and my lights were too harsh and casting shadows.
 
Re: Chromakey question...

To me the main reason this matte looks like a matte is that the fg lighting doesn't match the bg lighting. The bg is bright sunny daylight and the fg is soft almost underexposed. a harder key light on the face from camera right would make it more realistic
 
Re: Chromakey question...

Playing around with posted above images in Ultra I decided to give a shot to other Chromakey options. The one I have installed on my Avid is Boris Red. Boris gave me amazing results in just two clicks. Which brings me to conclusion that there is hardly any unusable images, there is only not enough experience or time to experiment.

just keep trying, folks!!!
 
Re: Chromakey question...

Here is an example of what I was able to pull using the ULTRA software:

http://www.seriousmagic.com/soule/girl.bmp

This was done by taking the JPG posted here, brought into ULTRA, and keyed using the "Key points" method. It was then outputted to an uncompressed AVI file, and converted to an uncompressed 1.1 MB BMP file.

An interesting note - the green Chromatte ring appears to throw up more of a "speckled" green than the blue ring I've tested with. This does mean that I needed to use more key points than on a usual green screen (20-30 points, instead of the usual 5-10.) I've not seen this behavior using the blue ring with ULTRA.
 
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