Keying Out A Bad Green Screen
by Ayman Farahat
on
May 23, 2008 at 2:04:30 pm
Hi Everyone,
I have a major keying task to attend to over the next several weeks/months. It’s a feature film that was entirely shot on green screen, which was not the ideal green screen that we all see in demos and manuals. It was not always uniformly lit, sometimes had many creases and folds in it, and some other times the talent/objects were too close to it and bathed in its lovely green spill.
These are some project specs to give an idea about the footage and system used:
Production:
- Camera: Sony HDW F-900 HDCAM CineAlta
- Tapes: Sony BCT-40HD
- Shooting format: 1920x1080 25p
Post-Production:
- Workstation: Mac Pro 1 (Dual-Core Intel Xeon 3GHz processors –16GB RAM – NVIDIA Quadro FX4500 graphics card – MAC OS X Tiger10.4.10 operating system)
- RAID Storage: G-TECH GSPEEDXL12 Dual Host 4Gb FC with ATTO Celerity FC-42ES host adapter card
- External monitor: JVC DT-V24L1D broadcast LCD monitor
- Software: Fincal Cut Studio 2, Shake 4, Studio Artist 3, Adobe Creative Suite CS3 Production Premium.
- Capture card: Blackmagic Decklink HD Pro PCIe
- Capture VTR: Sony HDW-M2000
Now, I have been looking for the most effective keying applications for the past few weeks and I ended up with the aging Primatte (in Shake) as well as the better Keylight (in both Shake and After Effects) and Zmatte (as a plug-in to After Effects). I’m also using Power Mask from Digital Film Works/Tools, which comes handy with shots with minor/no camera/talent movements where I can easily and effectively mask without having to struggle with keying a bad green screen. Adobe Ultra is not compatible with my Mac system and I’m not sure how helpful/superior it is to the other keyers like Keylight, zMatte and Primatte. I’m also thinking about Nuke’s IBK keyer, but again I’m not sure about how superior this keyer is to the other ones.
Going with Adobe Ultra or Nuke IBK is an expensive solution for me, given the system I already have, but I’m willing to take the plunge if they will add significantly more power to my keying operations beyond what I already have (Keylight, zMatte, Primatte, Power Mask). I’m also wondering if there are any hidden (or so obvious) keying gems out there that I’m missing here.
Any thoughts/experiences/comments/tips on this will be much appreciated.
Re: Keying Out A Bad Green Screen by Arnie Schlissel on May 23, 2008 at 6:17:23 pm
Ayman, even with perfectly lit backgrounds (which is almost impossible to achieve), keying is almost never a simple one click process.
It's not unusual to have to apply several keys to different parts of a shot, and not unusual to use more than one keyer, either.
These 3 tutorials are all based on Shake, but the lessons can be applied to any VFX application:
Re: Keying Out A Bad Green Screen by Andrew Shanks on May 25, 2008 at 2:37:11 am
Hey Ayman,
a big chunk of my work is keying, and as Arnie says, its almost never a one click keying solution. Tips with HD and film keying work:
1. degrain/denoise before pulling the key (then apply the generated matte back to your grainy/noisey original footage, ...even the purest of HD sources will have some form of digital noise in my experience).
2. do a rough roto around the foreground object you wish to key (to get rid of any non-greenscreen items around the border of the shot, and also so your keying operation is only dealing with the character part of the screen, not edges where lighting might fall away).
3. most importantly, do a proceedural key (edge, core and garbage mattes combined). I will often use primatte in shake to pull the garbage and core/holdout mattes, pipe those directly into the garabge and holdout inputs on a keylight node and use that keylight to pull the edge matte (due to it having core and garbage mattes ported into it, it in effect produces my final key).
My tutorials cover one way of achieving a proceedural key (the way I have done it is not exactly how I do it in practice, the method i just mentioned is often the way I do it, ...but it makes the process a bit clearer I think).
If you do this, using more than one keying pass to create the final matte, ...you'll find you can key most wrinkled or badly lit screens (its no one click solution, but its pretty darn good and quick when you're used to it). It is standard practice with keys, even if the finer points of the techniques vary, the way it works is you will pull a key for the edge (trying not to crunch the matte much, you want a nice edge with a little bit of grey graduation from the white of the foreground element to the black of the background screen), then pull a key to act as a garbage matte (i.e. a crunched up matte, concentrated on having a pure black background) and another (or a branch off the garbage matte's key) concentrating on getting a nice pure white inner, to act as the core matte. You erode the core matte and max that over the edge matte (so the white from the core matte is combined with the white of the edge matte, ...important to erode the matte enough that it doesn't contaminate the nice edge that the edge key has produced. A little feathering/blurring of the core matte before combining it with the edge matte can help integrate the mattes better), and then expand the garabge matte before using min to combine it with the edge/core matte (this combines the black region from the garbage matte with the black of the edge/core matte, again making sure its expanded enough as not to destroy the edge the original edge matte produced).
Couple of additional notes in regards to the tutorials. Firstly, instead of using a normal blur for the edge blend, use an iblur node (not that it seems to make much difference, but that is the prefered method). Secondly, if you are working in Floating point colour, remember to clamp mattes so funky things don't happen (shake4's nodes matte inputs clamp by default, but some operations can get upset if your matte levels don't sit between 0 (black) and 1 (white), ...in float of course your white could be something like 7 which can upset the apple cart somewhat. Also using the screen node (like I do in the tutorial for lightwrap) with floating point colour won't work (as the maths is based again on a range of 0 to 1).
Re: Keying Out A Bad Green Screen by kris tomi on Oct 28, 2009 at 8:24:10 am
question.
HOW CAN YOU INTERGRATED TO YOUR PIPELINE a degrain version of your clip INTO keylight.
i am workin on nuke.
have a very grainy shot that I degrained in an other soft.
so now I have my original shot+ degrained shot which I need to use just for making tha matte.
DO u know a tip how can I use my degrained clip for making screen alpha in keylight but using my original clip for RGB.....
