Green Screen with Nano
 

Recently got a Nano. What are your suggested settings for shooting green screen with the Nano? Does I-frame offer any better keying vs. Long GOP? I am very experienced with shooting/lighting green screen, I'm just asked regarding the Long GOP recording vs. individual frames.

I've only used Nanoflash files once for a special fx shot. I talked to the post house prior to the shoot and we both agreed that the max data rate (220Mb/s at the time) I-frame was the best as each frame would be intact. We didn't take the time to shoot tests as I normally would but they had the original HDCam tape (144Mb/s I-frame) to use as backup. In the end they were happy with the MOV's.

Theoretically only I-frame files should key properly but I just did a quick greenscreen test using h.264 files out of a Canon HDSLR and was totally shocked at the results.

My suggestion would be to stay with I-frame over 100Mb/s unless you have the time to test with whoever is doing your post.

Dear Ernie and Paul,

In order of importance: (to do better green screen work, as it applies to the nanoFlash).

1. 4:2:2 instead of 4:2:0. 4:2:2 is automatic, in the nanoFlash, if you record 50 Mbps or higher.

2. High Bit Rate. 220 is a nice bit rate for green screen, less or more will also work nicely.

3. Personally, I feel that Long-GOP or I-Frame Only should both work great, but I have not run any tests in the area. Others may have more knowledge and disagree.

[QUOTE=I just did a quick greenscreen test using h.264 files out of a Canon HDSLR and was totally shocked at the results..[/QUOTE]

Paul - was this 'shocked' in a good way or a bad way?

Ernie - I've done some keying - both blue and green screen - with the Nano recording at 100Mb Long GOP and the production companies / post houses have been very pleased with the results. (ie - no complaints from them whatsoever) Settled on the 100 Mb rate because of the length of the interviews, not wanting to burn through too many cards too quickly. One client keys with Boris Fx, the other uses After Effects, and as I said, both were very happy.

In my way of thinking, Green or Blue screen would be a pretty good candidate for Long GOP, since a large area of the image (ie- the background screen) is basically the same from frame to frame, so the compression can work a lot harder on the details in the foreground people/ objects. --- Well, at least it makes sense to me...

Dear Dave,

Thanks for sharing your experience.

I've done a couple green screen shots running 100 Mbps Long Gop...(XDcam only records 4:2:2 at data rates above 50 Mbps).

It has keyed very well in our experience...of course as with any green screen keying, the lighting, exposure, and shutter speed make as much difference as anything...

Hi Tim

Can you explain how shutter speed makes a difference for Green Screen.. never heard that before..

Thanks

Hi Robin,
My sense of shutter speed with green screen:
Higher shutter speeds tend to be better because any kind of movement with a low shutter speed (or no shutter in place for progressive scan) means that each frame is essentially a still picture taken at - say - a 30th or 24th of a second, and there is blurring on the edges of any movement. This blurring is essentially a combination of the (green or blue) background and the (non-green or blue) foreground. What happens when you create your key? -- this blurred area acts essentially as a 'polluted color area' and is almost impossible to pull a real hard-edged key.
I consider maximium shutter for matting to be 180 degrees, but much prefer 90 degrees or less if enough exposure is available.
Remember, you ideally want each frame to have essentially no motion blur, since the matting process treats each frame independently.
Think about what kind of shutter speed you would need to make a clear still photograph of the action. This shutter speed will give you a clean key.
For a series of green screen elements I shot with fast-moving dancers for a Cingular campaign a while back, we settled on a shutter speed of a 1/250 of a second after doing some tests at various shutter speeds. Of course the down side was that to light the studio for this exposure level, combined with using a polarizing filter to minimize 'backlight kick' coming off the floor (since we were seeing feet) We had to use multiple 10k's and maxi-brutes as keys, along with a dozen mighties through large silks for fill light. Luckily the air conditioning system was good, or the dancers would have melted!

Shutter speed is somewhat dependent on application in my view.

Most keying software does handle motion blur reasonably well...and compositing some short-duration, razor sharp footage into an existing piece of footage that has a vastly different shutter speed and trying to make the two look like they aren't two separate video layers (say for a special effect in an indie feature or something similar) adds some work to the process, certainly...

But then, you'd want to know something about the lens that was used as well to get something that really fits together.

If you're keying people in front of a graphic, or you have some other scenario which is obviously synthetic, then super sharp, high shutter-type motion isn't as out of place for the viewer...

High shutter speed may be required if you have a lot of fast motion. Try shooting a green screen shot at 180 deg shutter and wave your hand with fingers spread out quickly in front of the camera. Look at hand in the individual frames, you'll see some pretty extreme motion blur in the fingers from frame to frame.

Now when you try to key this scene, the extreme motion blur will make this a really hard scene to key and you'll likely get fingers that partially disappear. This looks horrible. They way to "fix" this to shoot with a faster shutter. Of course, that introduces all the problems of fast shutter.

There is some software that can take the high shutter footage after it's keyed and add the motion blur back in to make the shot look more natural, like it was shot 180 deg.

Ernie, I would say that I-Frame vs. Long GOP makes no difference with keying if your software supports Long GOP, you should be fine. I'm still using software which doesn't work very well with Long GOP (Shake), but I usually shoot Long GOP for green screen anyway.

Shocked in a good way. I've seen horrific greenscreen results with DVCam and HDV so at first glance the data rate and long GOP compression of the files coming out of a canon HDSLR camera look like they'd deliver the same results. Not so, I gotta say the results were pretty good. I'm not an editor and I'm waiting on my new CS5 machine just so you understand where I'm coming from. I just used a digital greensceen thrown over a fence, unlit with some wrinkles and with a click our two got a decent key.