The time has come, I can finally say I am “Chasen Green” for I have finally done a green screen workshop!
This week I had the privilege to set up a green screen behind a set with the intention of having the set’s window view on to the lit green screen. Having worked with green screens in the post and only once with a green screen in person. It was amazing to actually see the production needed to go through to create and utilize a green screen on the camera.
First of all, I had no idea how much distance you had to have between your green screen and your set. Setting up the green screen we had to make sure that there was enough room for the camera to pan. Thus giving the parallax effect between the set and green screen effect.
When replacing a scene with a green screen, two separate pieces of footage are needed. This is the scene which includes the green screen, and the plate which is the replacement of the green screen. While setting up a scene and a plate, there are many factors to consider in order to ensure both the scene and plate will match. This includes; lighting and it’s tendency to spill, Camera and plate distance, Camera Blocking, and specs and the green screen markers. When lighting the scene and the green screen, you have to consider the spill of how each object reflect. Whether it be an actual prop with a gloss or even the green screen itself. Each one of these can affect the green screens removal later on, in addition to the aesthetic of the footage. Specifically looking at the green screen, with multiple lights projected on it can cause it’s green glow to spill onto the set. To fix this we can move the green screen back further away to reduce the residue from the green screen is lit. In addition to light spill, light matching is another key to look at. Lighting from the plate should be visible in your scene and vice a versa, this is to improve the continuity to support the realism of the plate working with the scene. For instances, if your plate is an exterior shot and is used as a window scenery for the scene. The sun theoretically would come through the window, and react with the scene. This can be solved with “artificial lighting” making the scene look like it is taking light from the plate. Yet when doing so, research of the location and the plate’s effect onto the scene should highly be taken into consideration when lighting the scene and or the plate.
The next factor to consider is the distance between the camera and the plate. When filming the separate footages pieces of the scene and the plate, the distance between the camera and the green screen for the scene will have to match the camera and the subject of the plate object. This is to ensure the actual subject of the plate feels in the right place when replacing the green screen. Paired with the camera setup, including the camera lens, aperture and depth of field, all these settings have to be duplicated or slightly modified from the scene and replicated for the plate. When first adjusting the aperture to the appropriate level for the scene, and then moving to the plate, the brightness of the room or environment in which is being used for the plate has to match the same kind of look of the scene. For instances, if the scene was interior shot and the plate is being filmed as an exterior shot, the light of the sun could potentially be too exposing for the open aperture therefore, the iris has to close slightly or be matched to the scene with the addition of a neutral-density filter.
When using lenses depending on how wide of a lens is selected and used for the scene is also has to be replicated for the plate. This is to make sure that the wideness and distance of the lens are identical to the plate, extending on from the scene to the plate. In addition to the lens choice, the lens has to have the scene’s green screen to be set in proper focus keeping both the scene and green screen in focus and then to be extended by the plate and its same depth of field settings. This gives the subject of the plate the shallow or deep depth of field behind them, giving the illusion that the scene is filming both the scene and the plate behind it and that it’s filming the real thing.
Once the set and plate have been set, the next is the physical markers and camera movement for the sequence. Even though the green screen is where the plate is going, it will not look right during the camera blocking unless it follows the similar movement of the green screen of the actual scene. This is where tracking markers come in, tracking markers are placed on the green screen to help track the plate onto the green screen during the post-processing. When setting up the tracking markers, they have to be set up in a way to be visible at all times. This is to ensure that the movement of the markers can be recorded and replace by the plate. The amount of the markers there is the more data you can extract. Without markers, the recreation of the plates position and movement with the camera, will take much longer and may not look right making the plate not match the movement of the camera. When adding tracking markers depending on the camera blocking, may have them disappear during a pan or if an object blocks it. To deal with this we can either add another marker to counter this or we can use multiple markers to our advantage. For instance, if the camera were to pan left or right and ends up clipping a set of tracking marker, adding another set to the opposite side the tracking can be paused midway during post-processing than redone with the new visible tracking markers. The minimum amount of key markers recommended to be used is four, this is to ensure that the rectangular plate can be matched the square points of the tracking markers on the green screen. Use editing programs like “After Effects” can track the tracking makers and collect the data of the scale, distance and movement of the markers during the camera’s movement in which can be replicated on the plates, placement behind the scene.
After the plate and scene have been filmed and marked as a print, it’s off to the editing suite to put it together. Entering the editing suite, after placing both pieces of footage (with the same camera lens and distance matching) into the timeline, the first step is to create the removal of the green screen. focusing on the main scene, using a keying tool we can remove the green screen and actually “key” out the green screen from the scene. Setting the colour options to target the same colour green as the green screen, we then can set our tolerance options and configure it to only target the parameters of the green screen. Although the green screen is now removed we still have the markers remaining, now the next step is the focus on removing the markers. By removing the green screen and markers first we will have quick access to view our plate behind our scene later on. Putting the time ruler to the beginning of the timeline, we create a mask around the markers currently visible. Setting the mask path the be keyframed, we adjust the time ruler through the scene slowly and stop and adjust the mask shape once the mask overlaps either the scene or misses a marker. Continuing this until the scene is ended, we then set the mask the subtract, this removes anything within the mask. Since the green screen is already removed it’s won’t affect anything but the remaining markers in the place of the green screen.
After removing the green screen the next part is adding in the plate clip underneath the scene clip. The plate clip should appear exactly through the green screen we had removed. The next step is to get the tracking and movement to parallax with the scene.
To do this we have to work in a 3d space, in addition, add a 3d camera to work with. To do this first we have to create a 3d tracker camera. After creating the camera we have to set it to analyse the scene. This analyses the scene and then created tracking points for further tracking in the future. Yet for this we use it as the physical camera we look through creating the parallax effect. After creating the 3d tracker camera the next part is creating the parallax distance effect. Returning back to the plate, you will convert it to a 3d layer, in doing so the camera will now see this as a 2d object in a 3d space. Which the camera can move and will make up the depth of field for the objects within the space. After making the plate part of the 3d space, we will then push back the plate in the 3d space. Under the Position option, we will use the 3rd set of number (z-axis) and push the plate back to match the same distance of where the green screen was. However there is no actual real conversion rate from pixels and metres, most of the time a guess will have to be used. Yet lucky there are a few rough ratios online for it.
After setting back the distance of the plate, it would have gotten smaller. As we can’t use a small image we now have to scale up the plate. Scaling up the plate to match its orignal height, then adjusting the plate to the position we desire. Then we are practically done, playing back the scene the plate should be inserted and look almost like the real thing. The only thing left to do is colour grade!
During the filming and editing of this workshop, I found the most difficult part to be the stabilisation of the camera. During the filming of the plate, it was very windy causing the camera to wobble a bit while we were shooting. Having to stabilise the camera multiple times in post-processing was frustrating as many times the plugin did not work. I think from this I will like to control the camera more when outside and either add more weight to it or hold it still manually.
This is the basic breakdown of creating greenscreen and using them, however, for more advanced steps, other factors should be used, such as camera stabilisation, extra tracking and keyframing of the plate. However, that’s a breakdown for another day!