Advanced Video Camera and Editing


Chroma Keying

 

The process of electronically removing colour from video is called Chroma Keying. The most common involves what broadcast news stations do during their weather segments when the meteorologist appears in front of the weather map. Actually, they are standing in front of a green screen (sometimes it’s blue), which is then keyed out of the shot and replaced with the weather map.

Digitally removing the colour is a way of “cutting out” the subject in the shot and placing them in front of any background you select. The process involves selecting a colour, or range of colours and making them fully transparent.


NOTE: The quality of the chroma key is dependent on the quality of the source material. The higher the quality of the video, the better the chroma key. Also, success depends on how evenly lit the background colour is, how close your subject is to the background, and how well you understand the tools.



Adobe Premiere removes the key colour using the Ultra Key filter. In the Timeline, the chroma key clip (foreground) is placed in the video track above the clip that contains the background. That way, when the colour is removed, it will reveal the background that is underneath.

Troubleshooting to improve the chroma key

In an ideal world, the colour background (green or blue) would be lit evenly without any variation in shades. But most of the time, you will end up dealing with problems associated with poor lighting, which could impact the quality of the key.

If the footage is not keying properly, you could always add a very light, soft blur effect to the clip. This effect blends pixel detail and can give a smoother-looking key. Once you’ve applied the blur, and then add the Chroma Key filter.

Another option would be to consider colour correcting the shot before you add the chroma key filter. Sometimes, adjusting the colour, using a Three-Way Colour Corrector or Fast Colour Corrector, will help create a cleaner key.

Another solution would be to define a smaller area around the subject in the footage, limited the amount of colour the filter has to remove. To define a select area around the subject, use a Garbage Matte.



Garbage Matte
The Garbage Matte is a user-defined area that you use to cut out an area that you want to be visible or transparent. Go back to the Effects tab in the Project panel. Type “garbage” in the search field. You should see 3 options that gives you different types of control when determining the shape of the garbage matte around your subject:

•       Eight-Point
•       Four-Point
•       Sixteen-Point


Make sure the clip in the timeline is selected and then double-click the garbage matte of your choice. Or, you can simply grab the garbage matte and drag it onto the clip in the Timeline.

In the Source monitor, under the Effect Controls, scroll down to reveal the Garbage Matte parameters. Select it and, depending on whether it’s a 16-point, an 8-point or a 4-point garbage matte, you should see the same number of crosshairs positioned around the image in the Program monitor. Click in the crosshairs and drag to reposition them around the subject.



NOTE: When you move the garbage matte points, be sensitive of motion in your subject. If the subject is in motion you also have the option to apply keyframes to your matte to move it along with the subject.

With the garbage matte in place, you can now focus on removing the colour that is contained within the matte.

 

Using the Ultra Key

In the Project Panel, click the Effects tab and then type in the search field, Ultra, to locate the Ultra Key filter.

Select the chroma key clip in the timeline and then double-click the Ultra Key filter to apply it. Or you can grab the filter and drag it onto the video that contains your chroma key shot.

In the Source panel, click the Effect Controls tab and reveal the drop-down items under the Ultra Key filter. You will notice an eyedropper where it says Key Colour.


Select the eyedropper and click in the Program monitor on top of the colour that you want to remove from the shot.

If the video is shot well, with the chroma key colour evenly lit, then removing the colour entirely can be done in this one simple step using the eyedropper.

To make sure that you’ve keyed out all the colour in the background of the shot, change the output to reveal the matte. Under the Ultra Key controls in Output, change it from Composite to Alpha Channel.


In the Program monitor, everything that is black is transparent in the shot, and everything that is solid white is opaque. In this example, note that some white or grey fragments are visible around the subject. These bright spots need to be removed otherwise they will become partially transparent in these parts of the picture. 

 


Use the Matte settings to adjust the highlights to remove bright spots. The key should be much cleaner. Each setting has a different purpose:

 

      ·      Matte Generation – after choosing the key colour, Matte Generation adjusts the way the colour is interpreted.

      ·      Matte Cleanup – these controls will help clean the matte once it’s been defined. Choke shrinks the matte, but don’t use too much of it otherwise you might lose some edging detail. Soften applies a blur, which helps improve the “blending” of the foreground and background. Contrast improves the black and white image to help define the key more clearly.

      ·      Spill Suppression – compensates for the key colour bouncing off the edges of your subject. Spill suppression adds colour, opposite on a colour wheel, to the key colour. For example, when using a green screen, spill suppression will add some magenta to the edges of the subject. This neutralises the “spill.”

      ·      Colour Correction – these controls adjust the foreground subject to help blend it with the background. These adjustments are applied after the key so it won’t pose problems by adjusting colours.



If you need Adobe Premiere to try harder at removing the colour, the settings in the Ultra Key filter has an option to be more Aggressive. Or you can select Relaxed where the computer doesn’t have to try as hard to remove the colour.



At the bottom of the Ultra Key filter is an option for Colour Correction. You can tweak the colour in the subject to make the foreground subject appear as if it belongs in front of the background.