Chroma Key Filter (Green Screen / Blue Screen)

Overview

Chroma keying, is a special effects technique for compositing (layering) two images or video streams together based on color hues (chroma range). The technique is used to remove a background from the subject of a photo or video.

Available Chroma Key Colors: Green, Blue, and Red. Green Screen is explained here, but Blue Screen and Red Screen are also available Color Key selections (Figure 4) and work in the same manner. Red is not a good choice when people are the video subjects, but may be useful in specialty cases for objects that do not contain red hues.

Description

Removes the key color (green) to expose a background video or image behind the foreground video.
Remember that filters are applied to the foreground video, never to the background video.

Figure 1. Green Screen Video or Image

Figure 2. Image with Key Color Removed

Figure 3. Media Screen With Background Video or Image Added

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How to Use the Green Screen Filter

These Instructions assume you have a basic understanding of how Video Mill works and how to create a basic Media Item with a Filter. Image samples are screen shots from actual Media Screens. Your results will vary based on the quality of your green screen backdrop. You can test this procedure using the Green Screen Girl image (Figure 1). Right click on the image and pick Save Image As from the context menu. Then you can drop the file into Video Mill and try out green screening.

This procedure is a one-time set-up task for a Media Item. The next time you want to play the green screen video, you will simply click Play.

Steps

Record your video in front of a Green Screen backdrop. Or you may stream your video from a camera.
Create a Media Item with your Green Screen video as the Media Item's Media File.
In the Video Filters control section, check the Apply Filter check box. Click the Apply Live button so filter settings will be applied to the live Media Screen. Then click the Filter Edit Tool button to open the editor.
Set the Media Item Background color to White. While you are adjusting the filter to remove the green key color, a white background will help show stranded green pixels.
Click Play to load the Media Item to a Media Screen so you can see the effects of changing the filter settings. You can Pause the video while adjusting these settings to make it easier to fine tune the filter settings.
Select Green Screen from the Key Color pulldown menu. (Figure 4)
Move both slider controls to the high end (value=1.0, right end)(Figure 4).
Now slide the Key Lightness slider (Figure 4), backward until all of the Green background disappears (becomes invisible). (Figure 2)
Fine tune the color masking with the Proximity slider (Figure 4). This will clean up the green pixels along the edges of the subject. And any pixels on the subject that are very close to green may have been erased by the Key Lightness adjustment.
You should now have a fully masked image as shown in Figure 2. The Video Filter Edit Tool screen shot (Figure 4), shows the settings used to mask the sample image used for this tutorial.
Now add the background video or image. Go to the Screen Layout Attributes. From the Style pulldown, pick Image on C1 or Video. Then click the Video/Image File Browse button to select the background source.
Unload (close) the Media Screen, then click Play again to see the Media Screen with the background video or image. (Figure 3)

Figure 4. Video Filter Edit Tool - Chroma Key Settings

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Limitations

Chroma Key Filters DO NOT WORK when using Load Templates mode or Cross-Fade mode.
Fade-In/Fade-Out may interfere with the filter rendering (somewhat) during the seconds a fade is processing. Fades and some filters manipulate video Opacity.

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Green Screen Tips

Try to get a Green Screen backdrop as close to the Pure RGB colors as possible. Darker key color backdrops make the color masking more difficult to completely mask out. Pure RGB colors are: Red: (R=255, G=0, B=0); Green: (R=0, G=255, B=0); Blue: (R=0, G=0, B=255) (Figure 5).
Don't use Red Screen backdrop to record videos with people as subjects. Facial tones have an affinity to red tones and will conflict when masking out the key color.
Dark gray clothing can make it harder to mask against Green and Blue screens.
Search "Green Screen" on YouTube for short sample Green Screen videos to test with.
Here's a short YouTube video on "Green Screen Tips, Tricks and Materials"

Figure 5. Pure RGB colors

Footnotes:

^[1] - Sample Girl in Hood image (source link) used under Creative Commons Attribution-ShareAlike 2.0 license. Original image was modified to demonstate video filter effects.
^[2] - Sample Beach image (source link)

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