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Utilizing a Green Screen

Incorporating a green screen, also referred to as chromakey, into your work can expand your projects immensely. The Learning and Professional Development team at Northern Arizona University uses our green screen frequently for different things ranging from simple photography to interviews to full videos that we release once a month.

Before You Start… Lighting!

Before I go into the specifics of post production with green screen footage it is important to note that lighting is very important in production and is unfortunately often overlooked. You want to make sure that the green screen itself is lit correctly but also that you are lighting your talent separately to help eliminate shadows and reflections.

Using three point lighting for your talent and two separate lights just for your background is ideal practice when lighting a green screen. Three point lighting is exactly as it sounds, you use three lights. These three lights are the key light, fill light and back light.

  • The key light is your main light and the strongest light. You want this light set up on one side of the camera facing your talent in a way so that the side of your talent is well lit and the other side opposite of the light has a soft shadow.
  • Your fill light will be placed on the other side of the camera. This light you do need to manipulate a little. You want this light to light the shadow that the key light causes on your talent. In our studio we have our fill light faced towards a white wall to bounce the light back onto our talent rather than facing directly at the talent. However, depending on your setup you might need to move the light closer or further from your talent, face it directly towards your talent or possibly away from your talent. Keep moving the light until you fill the shadows caused by the key light.
  • The third light is your back light, sometimes also referred to as the hair light, and is placed behind your talent. This light is used to separate the talent from the background and create a three-dimensional look. It’s purpose isn’t for lighting the subject like the key and fill lights but rather to give the talent definition and highlights.

You want to make sure that your talent is a few feet away, in front of course, from the green screen or else you will get a full on shadow. Have you ever been watching the news and noticed the meteorologists shadow is behind him/her? That’s because they are too close to their green screen. A quick fix is to just move a few steps forward, away from the green screen, and that will eliminate most of that shadow.

Shoot Your Scene!

Our Tech Time segments are all done with the use of a green screen in our studio. For this we shoot our episode in our studio then upload the footage to our computers so we can edit it with our desired image for the background.

A lot of people wonder “why green?” It doesn’t necessarily have to be that awful shade of green to work for chromakeying. The reason behind this shade of green is that it is not a common color that people wear, and you absolutely cannot have your talent wear the same color as the background if you will be using chromakey. A lot of studios have blue screens which I have mixed feelings about because I love wearing the color blue so I would have to be careful not to wear specific shades that are even close to it to avoid being keyed out (disappearing). These two colors are both used rather than orange or red because they are easier to light and work with in post-production.

There are exceptions to this however, where you might actually want to wear the same color, for example one year when I was reporting the weather I wanted my head to be “detached” from my body. I had a friend with a similar body type as me cover her head with the same green color as the green screen and I covered my body. This was of course for our Halloween show but it was a fun, quirky mix-up from our everyday routine but not something I would want to happen on a normal day! So just keep in mind to avoid picking the background you work with (typically green or blue) is not the color your talent wears basically ever.

 Time to Edit!

Our team uses Adobe Premiere Pro, which in my opinion is very user friendly, especially when it comes to using the chromakey functions. For our Tech Time videos, I use a garbage matte and ultra key effects. A garbage matte allows you to select which pixels will be visable on the screen and which ones will not be based on a region. You will use the garbage matte to essentially outline your talent as close as possible to get the best image and reduce the amount of green screen behind the talent. You can do this with a 4 point garbage matte, 8 point or 16 point. The ultra key is the process of digitally replacing an area of an image, wuch as a green screen background. Once these two effects are applied to the video of our talent, I move them around and tweak them so that the greenscreen is keyed out and I have just my talent. If you are using Adobe Premiere Pro, on the video lines you will want your talent above your desired background. So for example, you will want your desired background on video line 1 and your talent on video line 2.

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When working with your ultrakey in post production you want to make sure to pay close attention to detail. This is not the time to try to cut corners or do a quick editing job. You have two primary sets of mattes preferences to alter for each project to see what works best. You have your matte generation which includes transparency, highlight, shadow, tolerance and pedestal. You have your matte cleanup that includes choke, soften, contrast and midpoint. Along with those two mattes to work with you have spill suppression that consists of desaturate, range, spill and luma. Then finally you have color correction that consists of saturation, hue and luminance. Each one of these needs to be altered individually to get your talent to look as perfect as possible, casting off shadows, fixing their skin tone, etc.

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Let’s take a quick, deeper look into the two primary mattes and what they specifically affect. Under Matte Generation there is transparency, which is essentially the baseline for the opacity of the foreground. Next is highlight, this alters the opacity of the bright areas whereas shadow alters the opacity of the dark areas. And finally under this category is pedestal; this affects the darker pixels. The next category is Matte Cleanup. The under Matte Cleanup is the choke, which brings in edges and then the soften tool will soften the edges. Contrast will decrease or increase the contrast and midpoint adjusts the rotation point of the contrast.

 This will help any beginner learning to work with green screen videos and chroma-keying get started. Once you get comfortable with the format you will realize the benefit of these wonderful tools and I wouldn’t doubt you try to utilize them as much as possible, as I know I already do.

 

Audrey is a senior Instructional Designer with the Learning & Professional Development team at Northern Arizona University. She has a B.S. in Electronic Media and Film with an emphasis in Entertainment Management and an M.Ed. in Educational Technology from NAU. With her experience as a newsroom weather director and on-screen talent, as well as multiple years of experience teaching in a university classroom setting, Audrey brings a unique perspective to the LPD team.

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