Dan Hume's Blog


green screen and compositing in after effects
February 26, 2010, 12:37 pm
Filed under: After Effects, Green Screen, Post Production Techniques

Green Screen

A chroma key, sometimes called a green screen, is a device used in film and video production. The screen itself can actually be either green or blue. The editing program will recognize the specific color of the green screen and replace anything in the image that is that color with whatever image you choose. So if the subject is a person, you can put that person in any and every location from the comfort of your studio.

Setting Up

It is important when setting up your green screen that you get the fabric as wrinkle free as possible. The most effective technique is to use a steamer. Make sure it is hung evenly and that there are no unnecessary folds. Wrinkles and folds can cause disruptions in the image once it is placed over the green screen in post production. Also make sure no part of the subject is green including clothing and props. If there’s a similar color it might display the background image instead of the subject. This is why there is a blue side. Also when you place the subject in front of the green screen, make sure there is enough separation so you can light the subject and the green screen separately. Lastly, it’s good to make sure you have a quite a large green screen so that you can have some extra space for a subject that moves a lot.

Lighting

It is critical to light the green screen and the subject separately. Light the green screen first. Any shadows on the green screen will not key correctly and will cause holes in your image. No part of the green screen should look brighter than any other part. This means you need multiple lights. The simplest lighting technique is to place one light on each side of the screen. Next light your subject, taking careful precautions to ensure the lighting of your subject does not interfere with the lighting of the green screen.

Preparing for Post Production

Besides proper lighting and set up of a green screen, there are also some important considerations in order to have a solid product after post production. Consider the perspective of the subject compared with the image that will be displayed in the background. Otherwise the subject might look to big or too small to mach┬átheir background. Also if the subject is lit brightly, but the background is at night time, it won’t look good. Match the lighting of the subject in the studio to the lighting of the background it’ll be in front of. Depending on the editing program you use there will be options that allow you to tweak the effect. Experimenting with a sharper contrast is usually one of these options and can make a big difference on screen.

Keying in After Effects

Last week we did some filming on green screening just to try it out. Today we used that footage to import into after effects and had to key out all the green background. There were two methods to this. The first was using the Colour key which requires you to adjust some settings until you get as much of the green to disappear. It’s quite fiddly to remove all the green without effecting the main subject of the footage.

The most effective way of keying out all the green was using the Keylight plugin which pretty much removes all the green on the footage that I have got. It all depends on how well the green screen was lit for the keylight to effectively work its magic.

Before keying out the green screen

Green screen keyed out using Keylight

I imported a photo I took of Bournemouth beach as the background for the video. I realized the lighting on myself, in the video, is not correct to how the light is prjected in the image. This is why i added a lens flare to make it look like there was strong light coming the top right.

Creating an iPod advert look

I created two solids, each a different colour. The first solid was orange and I’ve used this as my background colour. The other solid I used was black and I made that my alpha layer which then the main subject from the video footage becomes like a silhouette. It’s a bit rough at first but you can adjust the Screen Matte which removes any of the details that is showing through.

Here is a clip:

I did another version using Sam’s video of him dancing.

This was just a very simple exercise.

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