Yesterday Scott Wenger,
Eva Pfaff (collectively known together as
Solar Light Films), and I went into the shop to experiment with green screen chroma key and the Canon C100. I originally intended to compare the keying of footage from the internal codec to the Atomos Samurai, but that didn't work out. I'll get to that later. For now, we'll just concentrate on the C100.
The internal C100 codec is an 8-bit, 24Mbps, 4:2:0 codec - i.e. not *the* best when it comes to pulling a key off a green screen. We wanted to test this codec under some of the worst possible conditions. Eva has long, blonde (meaning semi-transparent), wispy hair. To make things worse, we even threw in a modeling fan as is common in any glamour goddess shoot. It's one thing to key a static bald person (easy) and another to key hair flying through the wind.
The first thing I ever learned about chroma key is that you want an evenly lit green screen with no folds. The screen should be lit between 50 and 60 IRE; a little higher is OK. It greatly helps if you have a waveform monitor available. The C100 does have a luma waveform monitor built in, but we were lucky to also have a Panasonic production studio monitor sitting idle that day which has waveform monitors built in (woohoo!). The green dipped around the edges, but those are easy to crop out in post.
The camera was equipped with a Zeiss CP.2 35mm lens set to T6.3. The Atomos Samurai has an HDMI to SDI converter, so we were able to send a feed to the production monitor. As a side note, the monitor does accept DVI, so you can run a feed directly from the camera to the monitor through a simple passive HDMI to DVI adapter.
Our lighting setup consisted of five daylight balanced lights. The two large Kino-Flo 4x4' banks were placed about 8 feet from the screen and angled in as per the readings on our waveform monitor. In hindsight I could have removed the egg crates, but we already had more than enough light at ISO 850, T6.3. I let the Kino's define out exposure.
Eva was lit using all LED lights. We had a 4x2' Kino-Flo LED bank as her soft key, which worked really well when it was pushed in about 4 feet from her. We also used two Litepanels 1x1's: the first as an on-axis fill just above and behind the camera; the second just overhead as a hair/separation light.
The backdrop was the rental 10'x12' green/blue screen (see second photo below and look the ground level). It's made of a material more like felt rather than some of the thin polyester fabric ones I've seen. The chroma key screens made out of thin materials tend to develop folding creases, whereas this material doesn't have that issue. Folding creases create shadows that do bad things when it comes to pulling a good key.
There were two setups. In the first setup we placed Eva about 8 feet from the screen. The second setup you'll see in the video below has her placed 14 feet from the green screen. We wanted to experiment with how far she needed to be from the screen to avoid green spill. In the second setup we simply moved the key, fill, separation lights, and camera back 6 feet to follow Eva. The move wasn't exact so you'll probably see a difference in lighting, but that wasn't the point.
To make this test even more torturous I decided to fill the background of the final footage with magenta, i.e. the opposite of green. This would normally make any and all green spill exaggerated. In Premier Pro I used three effects per clip, two "Chroma Key" effects as the "chainsaw" key removal, and "Ultra Key" as the final detail. Yes, I could have done keying in After Effects using the "Keylight" effect, but after some experimentation "Ultra Key" worked equally as well for me and staying in Premier was just an easier workflow for what I wanted to produce. I am NOT a post production or VFX expert, so I recommend you do your own experimentation.
Surprisingly, I didn't have to give Eva too much of a "hair cut" to get her footage to work, even on a magenta background. The C100 footage is surprisingly sharp and detailed for AVCHD, but the codec didn't have to work very hard on a flat green backdrop. This quality is certainly more than good enough for a corporate video, web video, or television spot in my humble opinion. Would a 4:2:2 codec work
better?...yeah, probably. Would this quality get you a paycheck? Probably. The only thing I noticed is that the 8ft footage has a bit more green spill into Eva's hair than the 14ft footage did. Ultra Key was able to remove it to an almost unnoticeable degree. It was mainly a matter of experimenting with the controls in the effect until the footage looked decent.
You can download the 1080p version directly from our Vimeo account and look at the key in more detail if you like. I apologize in advance for the music. I was feel uninspired to scour ccmixter.org for just the right song.
So...what happened with the Atomos Samurai footage?... I learned a big lesson about the C100 and Samurai. When in NTSC mode the C100 only outputs 1080p 60i from the camera. If you have the camera set to 24p then the camera interleaves the 24p frames inside a 60i output stream just like television. The Samurai is then required to do a 3:2 pulldown to "recover" the 24p output. Well, sometimes the Samurai doesn't automatically recognize that it needs to do a 3:2 pull down and incorrectly records the stream as 29.97p. After
reading the manual online, it discusses this issue in detail in section 10 and how to fix it. So I don't fault the C100 or Samurai. This was just my inexperience. Now I know. I made the incorrect assumption that the Samurai would automatically use the correct recording frame rate since I didn't face this issue on the Sony FS-700 earlier this year.
If you record without pulldown the footage has interlaced-type jaggies all over the place: hair, clothes - anything that slightly resembles a curved or straight line. You also miss quite a bit of detail and the footage appears to have more noise. So I didn't bother to include this footage in the green screen test. Maybe I'll go back and do a comparison in the future.
If you'd like some advice on how best to use the Canon C100 camera and Atomos Samurai, please stop by the shop and talk to Salvador or Michael. They'll help you get your Godzilla tribute film rolling!
