Post Production: How To Use Green and Blue Screens
You will replace the blue and green screen you used in shooting during post production. This technique creates a composite of two images. So wherever you had a blue screen, a completely different image will appear. This is traditionally used for weather forecasting as well for many special effects in movies. Blue and green colors are used because they are the most different when compared to skin tones. This type of shooting is becoming increasingly common.
Step 1. Shooting
When shooting the film, it is best to have a green screen or blue screen present as the background. Make sure that the costumes do not blend in with the background. Green screens are very popular because they create a brighter color channel. However, the blue screen is best for outdoor backgrounds, water and certain motion capture.
Step 2. Import to Computer
When using color screens it is necessary to import your video into the computer program but also the image that will be used to replace the green or blue screen. Depending on the video editing software it may be necessary to have both clips open on the same timeline or to only work with the main video.
Step 3. Chroma Key
The function to replace the color screens is referred to as a chroma key. In many ways replacing the green screen is very similar to working with mattes. Select the color or screen that will be replaced. If you use a green screen then anything that is the bright green color in the image will be replaced with the new background or image. This is where poor costume choice can be a problem.
Step 4. The Replacement
Have the background replacement ready. Applying this to the clip may be as easy as merging the 2 images or you will need to go through and create multi matte layers. Each different software program will have its own tutorials about the best way to achieve this merging of the 2 images.
Step 5. Other Mattes
When applying a chroma key matte it is also a good time to work with any other mattes depending on the type of effect for that shot. Many times the light source and light usage needs to be changed. Those just starting to work with green screen tend to have trouble properly lighting the screen when shooting, though this can be corrected in post production.
Step 6. Preview
Before burning the video make sure you look at the preview to ensure that the green or blue screen replacement worked. If you are just starting to use this form of advanced editing it may take a few tries to achieve the desired result.
Step 7. The Video
Many films shoot entirely behind the green screen and blue screen as it is impossible to shoot in the real area. Many times blue screen and green screen is much less expensive than building entire sets. Stock photographs can be easily obtained and used as a background replacement. For those just starting out chroma key mattes offer and easy and affordable way to create great movies and videos without building up huge bills.