Green Screen filming and Compositing
When you go and watch a film at the movies it is almost guaranteed that they have used special effects or a green screen in it. Using green screens in films has become a enormous part of film culture, and not just in big budget super hero films but in shows like ‘Law and Order’ and ‘Ugly Betty’ .Green Screen filming and editing has never been something I have thought about, I always assumed that you just chuck up a bright green sheet and hope for the best when the edit came around. But I found out with the help and guidance of Cinematographer Nick Paton and experienced Editors Steve Cooper and James Miller.
We began the process by shooting the interior shots in a room set with the green screen set up behind where the window would be. We set up a dolly to move across the room and we set up markers on the green screen behind the set with little tape crosses. The purpose of the markers was to ensure that in the edit and compositing phase we would be able to track the movement of the camera so that the background image will work logically to fit the perspective of the foreground image.
We did several run throughs of what the take would look like and with direction from Steve Cooper I placed five tracking markers on the green screen. This meant when they rolled the dolly through the shot the tracking markers were in the background. We also made sure to monitor and note the distance of the camera to the set window and that to the green screen along with all the camera settings including the f-stop, fps, the type and length of lens we were using and the height of the tripod from the ground. These notes were used later in the editing and compositing process to ensure the background footage was spaced and layered correctly in correlation to the foreground image.
After all our interior shooting was complete and we were happy with the way our shot looked we moved on to the exterior shot that would be used to show the view from the window, replacing the green screen. This type of shot is referred to as a Plate shot. We went onto the roof and set up our shot; we made sure our tripod was the same height as it was in our interior shot and also that we had the same camera settings so that it would match in the composite. We took notes on where the sun at what time so that it would work in coordination with the lighting set up we did in the interior shots and for when we move into post production adding our light filters. This was probably the most difficult aspect of the filming production process as the angle of light had to match for both shots and required a lot of planing.
We then moved into post production with the shots. Using Adobe After Effects we followed a step by step class by editor James Miller. We began by importing all our footage into the program, we then dragged the exterior rooftop plate shot behind the interior green screen image. Using the tracking markers we used the very handy Track Camera feature to key our composite. This ensured that both shots were moving at the same time and speed in alignment with one another and adding these to the 3D tracker. When the basic green screen composite was saved and rendered we masked the window, adding a new layer modifying the light coming through into the room and add light to the image. We played with the opacity and the feathering tool on the image until it matched the setting and looked organic. Finally we added a layer of dust just to really make the scene look authentic and real.
Overall I’ve learnt that green screen filming and compositing is an exercise that requires a lot of organisation, time and patience. It is harder work than I thought and I found my biggest struggle in keeping motivated to keyframe as it tedious and takes a lot of focus but in the edit it was all worth it.
