How to Avoid Common Green Screen Mistakes

I love using my green screen for visual effects. If used properly, it allows for a vast number of cool effects to be created with relative ease However, if you are not careful during filming you could end up with a number of problems in post production that will hinder if not completely block the visual effect you had in mind.

To not let all my failed attempts go to waste, I decided to create a video tutorial on the most common green mistakes and how you can avoid them!

Here is a quick summary of the 5 most common green screen mistakes and some tips on how to avoid or fix them

1. Shadows on the Green Screen

The Problem

If your subject is too close to the green screen or the light from the front is too strong, a dark shadow might be cast onto your green screen.

This can cause unpleasant artefacts when you finally try to apply the chroma key. Because the dark areas on the green screen are too close in colour to the dark areas of your subject it may be hard to key them out cleanly.
On the right side it’s hard to tell where the arm ends and the shadow on the green scren begins.

The Solution

Position your subject a little bit at a distance to the green screen to reduce the shadows.
Try to avoid a single hard light on your subject which will cause strong shadows.
Add lights behind your subject that are aimed at the green screen to brighten up the dark areas.

2. Unevenly lit Green Screen

The Problem

This problem is very similar to the previous one. While the green screen is now better lit, the lighting is very uneven, creating dark and bright areas in the background.

Again, this can make the application of the chroma key difficult without clipping away parts of your subject. Here is an image of what the unmodified screen matte looks like.

The Solution

Sometimes you can rectify the solution in post by carefully controlling the ‘deposit black’ and ‘deposit white’ parameters in your Keylight Effect (if in After Effects). Other compositing software chroma key effects will have similar options, so try this first
To prevent getting into this situation to begin wtih, try to add lights aimed at your green screen to even out the lighting before you shoot.

3. Motion blur in front of Green Screen

The Problem

If you shoot at a slow shutter angle, you may end up with motion blur in front of the green screen.

This is something that is really hard to fix in post production since the areas where the motion blur occurred will be blended together with the green screen. It can be hard or even impossible to set up a clean chroma key without losing parts of your subject.

The Solution

Try to have your subject move slowly to avoid motion blur to begin with.
If you need fast movement, use a higher shutter speed (a smaller shutter angle) to avoid motion blur. Because this will impose a certain hypersensitive onto your footage, you can add the motion blur back in post production – AFTER you applied the green screen

4. Clipping out of the Green Screen

The Problem

You shot in front of your green screen and the subject was out of the screen for just a short moment. Not a problem, right? Fix it in post?

This can be very annoying to fix as you will have to manually rotoscope out all the elements that left the green screen.

The Solution

While the rotobrush tool in After Effects may help make the job easier, I strongly recommend reshooting the footage if you can

5. No backlight

The Problem

This is not really a problem, more a tip on how to make your subject stand out a little cleaner against the green screen. This will make application of the chroma key a lot easier and it will help give the actor or object a bit more shape by lighting them smartly

If you just shoot your subject against a green screen with light from the front, the outline of the subject is not very well defined.

The Solution

Add a light between the green screen and your subject but aim it towards the camera at the back of your subject. This will create a nice highlight outline around your subject and give it shape and make it stand out nicely against the background

I hope these tips will be helpful when you’re doing your next green screen shoot! I certainly learnt them the hard way hehe