From what I've read online, there is a general perception that using a green (or blue) screen creates enormous problems and that the effect is fiendishly difficult to get right. It is said that you need to have a perfectly even shade of green, which means that you need perfectly even lighting so that there are no shadows; and if you're using a fabric green screen, it needs to be completely crease-free, so they say. In my experience, however, chroma keying is actually quite effective even in seemingly adverse conditions. We were using a green screen that was effectively just a massive green cloth, and it got exceptionally creased, but this did not ever cause any great difficulties with keying. We used two soft box lights on either side of the screen, which effectively removed dark shadows, but we were unable to eliminate shadows altogether; again, chroma keying was still remarkably successful. And yes, the two soft box lights were the only lights we used apart from the room's own lighting.
Nevertheless, there were still problems. The soft box lights generally only pointed at the space behind the actors' upper bodies, so darker shadows were often found further down. In particular, the floor did not have a soft box pointing at it, so the actors' feet were often the most difficult parts to key. Since they generally had dark shoes, there wasn't much contrast between their feet and the shadowy green around them, and this presents the biggest challenge for chroma keying. The most frequent problem is that I ended up with a lot of flickering at the edges of the actors. This is particularly severe around the dead ninjas, and that's precisely because the horizontal part of the green screen did not have a light pointing at it (and was probably significantly creased because of their death throes). Since the ninjas and their shadows are both very dark, separating the screen from the actors is very difficult. I could minimise the flickering only by replacing the parts where they weren't moving with a still frame.
Flickering was also an issue in many parts of scene 52, but I was able to neutralise this quite successfully by adding the rain, smoke and fire effects. (Yes, the rain is only there so that you don't notice the flickering at the edges of the actors; I suppose it happens to add a bit of interest to the scene as well.) Also, the fact that the scene takes place at night helps enormously in covering up chroma keying anomalies, because both the foreground and background layers were dark enough that it's difficult to notice problems.
A further problem with the green screen technique was the difficulty in getting a good wide angle shot. If the actors aren't moving on and off the shot, then it technically doesn't matter if the green screen doesn't fill the whole frame of the video, because you can simply crop the non-green parts straight off. However, if one of the actors goes just slightly too far to the edge of the screen, or even leans off it, as occasionally happens in scene 42 (Loch Ness), then the part of them that goes off shot will seem to vanish, as though they were leaning through an invisible portal. To further complicate matters, the fact that our camcorders didn't show the area outside the action safe region meant that the wide shots of the green screen almost always had at least some non-green border around them. For those parts where an actor has to come on shot or go off, such as when Jeanne and the ninjas come on shot on the shore of the loch, or when the panicked passengers dance into the limelight on the sinking ferry, this can be a headache-inducing problem. There are basically three fixes. Firstly you can move the scene so that the troublesome edge is right on the edge of the frame. This may not work if both of the edges are troublesome and it may not be desirable if it puts one of the actors half off-shot. Secondly, you can simply scale up the shot so that the edges of the green screen match the edges of the background. I was wary of doing this, but frankly, you can hardly tell. Thirdly, you can use animated garbage mattes - i.e. little shapes that selectively remove the non-green stuff that you don't want to see. This is the most difficult because you have to go through each frame individually and set the right parameters. I used a combination of these fixes to solve the problem, but my advice to anyone else is just to make sure that the green screen fills the frame of your shot at all times during filming (unless you're absolutely certain that the actors won't drift to the edges or go partially off-shot).
There are six stages to the chroma keying process. My descriptions here will refer to the process in Final Cut Pro, but I imagine it's similar in other editing applications:
- Start with the chroma key filter itself. In FCP, you simply use the colour picker tool to click on the colour you want to remove, and then hold shift to pick other colours or shades of it. You then have to fine-tune the key by adjusting the sliders to include other colours, and to take into account different saturation and luma levels of those colours.
- At this stage, most of the green screen is gone, but there's generally a green border around the actors and possibly a few awkward splodges of green elsewhere. Increase the edge thinning and they'll go away.
- Sometimes the edge thinning removes too much or too little, and so you can try adding the matte choker filter, which does practically the same thing but is a bit more subtle. I almost always had to use this.
- The edges are often a little jagged at this point, so adjust the softening slider in the chroma key filter and the corresponding slider of the matte choker filter; this softens the edges.
- Now you've got rid of green edges, but the edges of the actors themselves tend to have a green tint to them, and a slightly subtler green tint is often noticeable across the whole shot. This is due to the reflection of green light back onto the actors. FCP's chroma key filter has an 'enhance' slider that desaturates the edges of the key so that this is less noticeable, but in my experience it generally isn't very helpful, and I use the spill suppressor filter instead. This filter adds magenta to the shot, which has the effect of 'cancelling out' the green tint because magenta is opposite green on the colour wheel. In my experience, this filter is highly effective.
- Now, you need to use colour correction so that the lighting conditions of the background appear to match those of the foreground. Technically you should try to light your subjects when they're on the green screen in a manner that corresponds to the background's lighting conditions, but we didn't have the lights to do this. Scene 52 took place at night anyway, so it was enough to reduce the brightness and saturation of both foreground and background; I did use colour correction, however, to take account of the red glow that comes from the hypermarket's gem. Another useful idea is to apply a blur filter - only a subtle one - to the background, which gives a differential focus effect. I did this for the Loch Ness scenes and for the 'Saving the World' sequence at the end; in the latter case I think it was particularly effective, given the smoke effect in front of it.
Sound Problems
We put up our green screen in a village hall, yet the scenes were supposed to take place on a ferry, on the London Underground, on the shore of a loch and on a grassy common. Acoustically, these environments are nothing like a village hall. Hence the reverberation that you hear on the actor's voices during these scenes. Even more noticeably is the sound of their footsteps as they walk on the hard floor of the village hall - not really the sort of sound you'd expect pebbles or grass to make. I cunningly replaced some of these footstep sounds with more appropriate ones for Loch Ness, but other inconsistencies may remain. Another interesting part was when Jeanne (Amber) threw her hat away when she removed her disguise in scene 52: that hat actually made quite a reverberating thud as it hit the hard floor of the village hall, which is not what you'd expect from the grass of Rodborough Common. I replaced this sound with something more appropriate, but this would not have been possible if someone had been delivering a line at the same time as the hat's impact, because you can't reliably separate a "hat impact" sound from a speech signal in a single audio stream.
Advice
Green screen advice for amateur film-makers:
- Carefully consider whether or not the green screen technique is workable for your scene and what the implications might be.
- Think about what type of green screen is best for you: we used fabric but you can also use a plain board or even just paint the wall of your garage.
- Lighting is important, but don't obsess about getting rid of every last shadow. Only the really dark shadows are evil.
- Similarly, don't obsess over creases; they aren't that problematic.
- Make sure that the green screen covers the whole frame of the video unless you're sure that the subject isn't going to exceed its borders at any point.
- Consider the acoustic implications of your green screen studio - any impact with the ground is likely to make the wrong noise, for example.
If you are thinking of doing green screen stuff and you would like more detailed advice, do feel free to contact me (and the same goes for anything else I've written).
The next part in the series is somewhat trivial.
The next part in the series is somewhat trivial.
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