In part one of this article, I talked about how VFX can add scale and depth to your production, can often help you avoid unexpected and expensive re-shoots. If you’re properly prepared, it needn’t cost the earth.
How to make it work, and keep the budget down
This is where it gets a little technical. Most VFX shots will not be using a locked down camera therefore you need some way of anchoring your VFX object into the scene, so that its movement about the screen exactly matches the part of the set or person it’s supposed to be attached to. If you don’t anchor it, the effect will float about and be screamingly obvious.
The thing that anchors your VFX object is a very clever piece of software called a camera tracker. It analyses your 2D film or video frames and generates a 3D camera track which can be transferred to a 3D animation package like Maya or 3DS Max. Once the effects object has been built and rendered, it can be overlaid on your original footage, and will move about as part of your scene.
For this to work, it is essential that you and your set are properly prepared. So here are some tips to help you avoid that awful moment when you find the VFX you’d planned can’t be done:
1) Talk to your VFX person BEFORE you plan your scenes. He or she will be able to tell you what’s possible within your budget and what’s not. You might be wise to have a chat before you work out your budget. That way you can plan for what’s possible and what’s affordable.
2) If the effects are within moving camera shots, you need to be sure the camera tracking software has enough information in every frame to be able to generate a reliable track. There are a number of things you can do to help this:
a) if possible, place tracking markers (small tape crosses of a contrasting colour to the background, or special tracking targets which can be bought online) around your set, especially on any green screens or other areas you want to replace – these can be removed in post.
b) try to avoid using zoom lenses and shifts of focus: they make tracking much more difficult;
c) take note of the type, focal length and horizontal lens angle of the lens you’re using, as well as the camera make and model.
3) Try to ensure that any green screen you are using is well lit. Poorly-lit green screens can make creating the hold-out areas for VFX objects problematical. Any green light overspill onto objects in the scene can be filtered out in post.
4) If you’re using a green screen, avoid having actors wearing green clothes, otherwise they may be difficult to isolate in the travelling matte (see below). If your actors must wear green, try using a blue screen instead - but then you mustn’t have any actors in blue!
5) Take a series of photographs from the centre of your set or from the position of any VFX object you intend to be included, so that they can be assembled into a 360º panorama. Make several passes so that you include the ceiling and the floor (see here for a quick and dirty method using an iPhone), be sure you have all your lights on when you do this. This can be used to generate an image with which to light the VFX scene so that the lighting matches your real scene, it can also be used to provide reflections on VFX objects if you want them. At worst, take careful note of the position, height and power of all your lights, and also the size of any soft boxes you’re using. If you happen to know the colour temperature of each light as well, that can be useful.
6) If you have the budget, get hold of one of those large chrome balls-on-a-stick, place it in the middle of your set or where your VFX object is going to be, and take a hi-res photograph of it from roughly where the camera will be most of the time; as with the 360º panorama, this can be used to generate an image with which to light the VFX scene so that the lighting matches your real scene, and to provide reflections on VFX objects if you want them. You’re probably going to want to hire one of these, as they cost from £450 to over £1,000 to buy (see VFX store or Akromatic gadget shop). They often come either half silver and half a known colour of grey, or in pairs with one silver and one grey ball. The grey can be used to get an idea of the overall colour of your lighting. As with the 360º panorama, you need to be sure all your lights are on when you use these balls.
7) Make a plan of your constructed set and measure the actual sizes and distances between significant objects; this can help the VFX artist to line up elements in your scene and be sure everything fits, especially if you don’t have some of the other information above (please note that this has to be your actual physical set; production drawings won’t be enough).
The picture above shows an apartment set with a green screen outside the balcony where a New York vista could be added. Note that the green screen is well lit and it has tracking markers placed on it at regular intervals. A chrome sphere has been placed in the middle of the room to capture the lighting information (this can also be done by creating a 360º photographic panorama). Notice also that there are quite a lot of nice sharp angles in the set (corners of pictures and doors etc.) which make it easier for the tracking software to work.
Not all of these points will be necessary for every occasion, but talking to your VFX person in advance will help you decide what you actually need.
I’m very happy to talk further if anyone needs more explanation, so do feel free to e-mail me!