Chroma Key and Compositing

[Jason Murphy]

The 1950's War of the Worlds was mostly shot in stages at Paramount, even many of the "outdoor" scenes at the beginning of the movie, but its easy to overlook because the movie is such a classic and really set records for number and quality of effects shots in a movie.

Another example of this: the effects work and rotoscoping on 2001 still looks fantastic 40 years on. Helps that Kubrick/Alcott was shooting 65mm, and much of the compositing was done against the blackness of space; but it's still great work, and hold up really well projected in 70mm.

 

[Jim Arthurs]

How about that Dragonslayer? That was really great blue screen work by Brian Johnson (Empire and Space 1999). I remember the article on that in American Cinematographer. They had a huge blue screen that was backlit with DC operated, blue tinted fluorescent bulbs. DC of course to eliminate any chance of flicker as electronic ballasts didn't really exist in 1981. Very clean keys. Look at the shot of the wizard on the mountain top at the end. With his white hair blowing in the wind. Not that real looking mind you (because of subject lighting and background too perfect) but very clean.

I still own a small Stewart transmission blue screen, the same material they used. Now THAT is blue, at a very narrow 450 nanometers. Put a red gelled light behind it and see NO light transmission on the front side. Had to be pure, as front light blue paint or fabric just didn't provide the color separation needed for good film optical work. It produced a ton of blue spill compared to front lighted screens, but that was what you had to deal with. I think Willow was the first major film project to use front painted cyc walls and it KILLED them in optical getting good mattes. Green screen? Very rare in film before digital compositing... only a small handful of poor shots in B-movies...

ILM did the actual opticals on DragonSlayer, with their super duper VistaVision one-of-a-kind quad head printer. Used low contrast separation stock instead of high-con B&W for the mattes to get that nice edge characteristic. Trade-off was transparency in the body of the matte... it's always something...

 

[Gavin Greenwalt]

I like to underexpose half a stop just because it seems to seperate from skin tones nicer but not enough to be noisy which is probably why they recommend 2 stops over.

DV = Seperate-Rotoed-keys. Unless everyone is wearing a hat.

 

[Jim Arthurs]

From hard life lessons with the HVX I find that the perfect green is IRE 60-65 which is a bit much from a conventional perspective and hard to get with good quality chroma paint or fabric unless the screen is backed up and lit seperate from talent. The HVX will generate noise in fine hair and motion blur at lesser levels. Using blue, 45 IRE seems the perfect level, and surprisingly, a bit more refined than green.

These are the same levels I've found with HDCAM, at least with the Sony 700a and ilk... lesser green IRE will result in noise and roiling grain in hair... with talking heads, I've found my green at IRE 70 quite nice if proper seperation from talent is achieved...

 

[Mathew Mackereth]

was testing some side by side footage between 35mm camera and the JVC HD-250 yesterday, and found the lack of colour space in the hdv format to be a problem, just like DV. Although the big killer IMHO with all digital formats is the "detail control", I've keyed with HDCAM, Digibeta, and DVCPROHD100 and the biggest killer is not colourspace but the sharpening that happens in the head of the camera before it hits the tape - essentially this seems to act as an "unsharp mask" type of convolving filter - which basically creates more contrasty edges - maybe good for eng or quick turnaround broadcast - but bad for keying - so with anything i shoot or supervise now i ensure the detail is off (not just turned down to zero!)

I imagine with the raw format this should be less of an issue - depends what the wavelet compression does.

with DV/hdv or any bad chroma source you are never going to pull a great key - but i've found that a better key can be pulled from this material if you convert the rgb image back into YUV - blur the UV channels slightly and then recombine with the untouched Y (luma) signal - this removes the unsightly chunks of colour and can go a long way toward stopping the noisy edges seen on images with a poor colour space.

 

[Rocket]

Man, I hate the fact that I have to stay up all night to be a part of the conversation. Damn time zones.

Well, there is one other option I can look at with this shoot that may still fit in the budget. A company here called Atomic VFX built thier own 2K camera system and I can get RGB 4:4:4 out of that straight to hard disk.

That might be the best bet end the end. Or hire a Genesis and record 1080p RGB 4:4:4 to HDCAM-SR, but there is no budget for that.

 

[Gavin Greenwalt]

I've also found something called "Neat Image" Run your footage through that and it'll remove a LOT of the DCT compression artifacts. It can be a life saver. But once you get your matte throw away the RGB and refer back to the original, otherwise you can get some de-noise artifacts in your plate.

 

[Rocket]

I'll look into that, thanks Gavin!

 

[Richard Andrewski]

Another example of this: the effects work and rotoscoping on 2001 still looks fantastic 40 years on. Helps that Kubrick/Alcott was shooting 65mm, and much of the compositing was done against the blackness of space; but it's still great work, and hold up really well projected in 70mm.

I had the book about the Making of 2001 when I was a kid. Did you know that many of those spaceship shots were produced on an animation stand with simple photo cutouts of the spaceships? The tracking shots were simply the animation stand head moving up and down toward or away from the spacecraft photo.

Also, I believe that was the first film where they successfully matted moving people into window shots on the outside of a spacecraft. Looked pretty darn good. Also I love that shot of the moon shuttle descending down and then landing on the pad. Then when you think it's over, the shuttle drops on an elevator into a huge cavernous miniature set with tons of optical work showing people moving around in windows. Great! Many of those people went on to work on Space 1999 which really was one of the first sci-fi tv shows with really great effects (for the time).

 

[Richard Andrewski]

ILM did the actual opticals on DragonSlayer, with their super duper VistaVision one-of-a-kind quad head printer. Used low contrast separation stock instead of high-con B&W for the mattes to get that nice edge characteristic. Trade-off was transparency in the body of the matte... it's always something...

Yup. A lot of that stuff built by Richard Edlund as I recall. I believe he built the EmpireFlex 65mm camera with motion control head too which may have been the first field oriented motion control camera to allow you to bring tracking data back to the studio and repeat shots for other elements. The printer was really advanced stuff for the time allowing super complex numbers of simultaneous mattes without so many generations. They had it for Empire but really showed it off in Return of the Jedi.

I wonder whatever happened to him after his company went out of business--I believe he did Air Force One and then quit?

And how about that sodium vapor process at Disney? Used a camera with special optics and a prism that was so special that it supposedly never could be duplicated after the original. Can you imagine how hot the stage must have been with those sodium vapor lights lighting the screen? Really worked great for some of the shots in Mary Poppins. I wonder if they still use that today sometimes? I remember they may have used it for The Black Hole which was 1980 or so but not sure after that.

 

[Jim Arthurs]

I imagine with the raw format this should be less of an issue - depends what the wavelet compression does.

The RED has no sharpening or detail feature... and nothing like sharpening in RED CINE when de-Bayering either... so it's a soft edge image perfect for keying. All indications are that the wavelet compression isn't a barrier to good matte work.

with DV/hdv or any bad chroma source you are never going to pull a great key - but i've found that a better key can be pulled from this material if you convert the rgb image back into YUV - blur the UV channels slightly and then recombine with the untouched Y (luma) signal - this removes the unsightly chunks of colour and can go a long way toward stopping the noisy edges seen on images with a poor colour space.

This is a great trick, and what should happen automatically at the codec stage, but usually doesn't. Codecs are either "generational" or "presentational". Generational codecs keep the color info as/is so that it will survive multiple generations. In the case of the DV and DVCRPRO codecs, this reveals itself in images where the color is blocky and literally "steps". Presentational codecs tend to blur out the color into a nice smear over the luma, which looks better to the eye and your keyer, but isn't ideal for going another generation. When keying, the analog outs of a DV deck usually key better than the data transfer via firewire for this very reason... the chroma is smoothed for you.