Green Screen and Compositing

 

Last week we had the opportunity to take part in a green screen and compositing workshop. This entailed dressing and lighting the set and adjusting the green screen so that it was the appropriate size and distance away. This was followed by shooting and finally, compositing the interior and exterior shots together in post.

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Something I learnt in this workshop is the necessity of the white tracking markers we put on the green screen. These markers assist with the perspective of the plate that replaces the green screen. In the case of our exercise we were replacing the view out of the window. Depending on one’s position in the room their view out of the window will vary slightly. In order to get a realistic product the green screen plate should (ideally) do the same. This is where the tracking markers come in. As the dolly moves across the room, the perspective out of the window changes, however as it is a solid green it is hard to see this variation. By adding the four markers in each corner,  the varied perspective is evident. This is a great help in post production. Using the tools in Adobe After effects, you can automatically lock the plate onto the tracking points so that the perspective is translated onto the movement and position of the footage. These markers also help the program to estimate the distance between the foreground and background footage which contributes to the realism of the product.

Changing perspective of the markers as the dolly moved

One of the biggest things to consider when shooting green screen with both inside and outside elements is the lighting. It can be really obvious when the light is pointing in different directions in the footage and green screen plate! When setting up these shots, you need to pay attention to what time of day it will be and the surroundings, for example, if the footage is of an apartment surrounded by high rises, there isn’t going to be a lot of afternoon sun. The majority of the natural light occurs in the middle of the day from a high angle to get over the tops of the buildings so you need to know the time of day, where the sun will be and light accordingly with the appropriate intensity and angle. Another thing to be aware of is the colours of the set. When editing a green screen you have to ‘key’ out the green colour, which means it will want to get rid of every green (or colours close to green like yellow) item in the shot to varying degrees depending on their likeness to the original key colour. So, knowing this, it is good to make sure there are no green props or costuming overlapping the green screen. If they aren’t overlapping it is easy to mask them so that the key doesn’t affect them. If that isn’t possible or the size and positioning of the green screen (eg. edges are visible with camera movements)don’t work, you can manually cut an element from one piece of footage and impose it on another with a process known as rotoscoping. But, this is an expensive, lengthy and painstakingly intricate job that we haven’t learnt! In most cases it is better to work with a green screen for these reasons.

Setting up the lights and dolly

Aside from the setting of the scene, the settings in the camera can make a huge difference in the success of the greenscreen ie. how realistic it looks. The three primary settings to pay attention to are the ISO, F or T stop and shutter speed. ISO adjusts the sensitivity of the light sensor in the camera. A higher ISO makes a scene appear brighter but also adds noise or grain to the footage (this generally occurs with ISO’s higher than 850). This slight distortion can make it harder to get a clean key as it affects the colours and their quality – especially green. In saying this, if your background plate is noisy, adding noise to the foreground footage in post production can add credibility to the final product. Shutter speed can also affect the keying of the greenscreen as it affects the motion blur in the footage. If the subject blurs in front of the greenscreen it will be very hard to get a clean key. A higher shutter speed is generally recommended to avoid this. Finally, it is also important to have similar F or T stops. This setting affects the depth of field; having similar depths of field will make the footage more compatible and realistic. It is recommended to use a higher stop (deeper DOF), this will ensure that the green screen and subjects are in focus so that you can get the best possible key. All of these settings need to be optimised to achieve the best possible key. The lens size also plays a role in this as it determines the field of view.  Going between different lens sizes will result in varying fields of view which will not match when the plate and footage are composited together.

This workshop was my first experience filming green screen so it was really interesting to see everything that had to be taken into consideration to make sure we would have a good key and compatible footage. For this shoot, we were using the Sony FS7 camera, one we are only just being introduced to and learning our way around so there were some difficulties navigating the menus and finding the settings we wanted. Although we had managed to get it to work in previous workshops, we were unable to display the desired LUT on the field monitor so that we could light the set accordingly. Even after much experimentation and lighting to what we thought looked fairly decent, the footage was still too dark and very grainy. This caused many complications in the compositing class as the footage was too dark and the green screen wasn’t an appropriate colour to be keyed. This meant we spent the majority of the class trying to get a realistic grade on the footage that would match with the exterior plate.  Another new experience was follow focusing on the dolly. My job was to keep the focus on the kettle as the dolly pushed in on the set. What made it difficult was that I had to move with the dolly whilst keeping an eye on the pre-marked focus wheel, as well as the floor markings, whilst adjusting my speed with the speed of the dolly operator. It was a lot to think about for a shot that only lasted a few seconds! Despite the challenges, the whole experience was a fun and memorable one with so much to learn!

 

 

 

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