Key Filters


Key filters are used to remove backgrounds of elements, people, or objects to isolate these elements from their original backgrounds and composite them over a different or new background. An everyday example of a key is the one used in weather segments of newscasts.

The weatherman is not standing in front of a moving and ever-changing background. In reality, he is being shot in front of a green or blue screen most of the time. The sometimes-animated background of a storm front is supplied by an animated graphic from a computer system. To see what's behind him onscreen in the composite that viewers see at home, the weatherman watches this composite on monitors placed where he can see them. (Watch his eyesit's sometimes a dead giveaway!) That's how he pinpoints certain things like the temperature of the area he is reporting on, even though his pointer is actually touching a green or blue screen.

Achieving Quality Keys When the Foreground was Shot on Tape

To achieve a good key, it's important to use footage that has been exposed properly and shot in as high-quality a video format as possible. Lighting a background evenly helps, and separating the foreground image as far as possible from the background helps too. Be sure to add a backlight to the foreground image to completely separate it from the background and to avoid color spill from a very saturated background. It's not a good idea for talent to wear the same color as the key color (such as a blue shirt shot against a blue screen background), because you are likely to key both the shirt and the background. (However, this technique could be useful for creating a key of just a head!) Keeping colors very distinct from each other in the first place makes keying easier in the postproduction process. It's a good idea to check out a key during the shooting process. This can point out an error you can avoid by simply changing a light or two and checking the key again. Many people create the key live for this same reason.

It's typically very difficult to use material shot with DV cameras to create clean and perfect color or chroma keys. This is because the color space that they work in does not record as much color information in the picture as a format such as Betacam does. In fact, only about half the color information about any given pixel is recorded in DV when compared to what a Motion JPEG compressed video signal contains, such as digital files created from Betacam SP or Digibeta. Typically, Betacam (or better) material is digitized or captured into this format with capture cards such as those from Aurora, Pinnacle, Digital Voodoo, and AJA. Unfortunately, using one of these cards to capture material shot with a DV camera does not add the color information that wasn't acquired by the format in the first place. You need to have shot Betacam SP or better (HD) to have the easiest time creating a key. This is because the algorithms used to create a key such as a color or chroma key determine what is to be kept (the foreground) and what is to be thrown away (the background) by very specific and detailed color information about each pixel. DV compression usually doesn't record enough color information to create a clean color key. The error occurs around the edges of the foreground as it composites over the new background. If you will do any keying work with NTSC DV source, use the 4:1:1 Color Smoothing filter firstevery time.

That said, luminance keys work much better in DV. This is because DV records as much luminance information as possible about a clip. The techniques used to set up a good luminance key (called the luma key in Final Cut Pro) are the same. Lighting only the foreground image and keeping the background absolutely black is the way to start. Remember that if the source footage is lit and exposed properly in the first place, it creates a fine key quickly in Final Cut Pro. Usually, if you have a lot of trouble keying, it's because it wasn't shot with the proper techniques in the first place. It isn't the fault of the software.

Even when you use a higher-quality acquisition format, compressing this signal in your computer when you digitize or capture it results in lower-quality keys. It's best to use the highest-quality digital image you can create to achieve the highest-quality results. The old saying appliesgarbage in, garbage out. Rarely can you satisfactorily fix things in post.

When applied to an element that is shot against a blue or green background, Blue and Green Screen keys out the color background, allowing you to key the foreground over a different background. The View pop-up menu allows you to look at the source of the clip (the original clip), the matte created by the filter, the final matted image, or a special composite of the source, matte, and final image for reference. The Key Mode pop-up menu allows you to select blue, green, or a blue/green difference as the key color. The Color Level slider allows you to select the amount of blue or green in your clip to key out, and the Color Tolerance slider allows you to expand the key into adjacent areas containing other shades of the key color. The Edge Thin slider allows you to expand or contract the matte area to try to eliminate fringing, and the Edge Feather slider lets you blur out the edges of the matte to create a softer edge. Before you use these sliders, try using a Matte Choker filter. The Invert check box allows you to invert the matte, making what was masked solid and what was solid masked. I strongly suggest that you use the Chroma Keyer for your chroma effects instead of using the Blue and Green Screen filter. It is a legacy filter. It's still included in FCP because if you've created an old projectsay, in FCP 1you need this filter to translate your files into FCP 4 files.

Chroma Keyer is a more sophisticated filter that keys out any color, including blue and green. You can even use it as a luma keyer if you disable the color and saturation. When you perform a key with this filter, adjust the Color Range and Saturation controls to fine-tune the effect. The Chroma Keyer has the same settings as the Color Correctors Limit Effect controls. It also has spill suppression built in. Start with this keyer when creating chroma keys. It's much better than the earlier iterations of filters for this effect. The Blue and Green Screen filter is a legacy filter, included in FCP so that you can open previously affected clips from earlier versions of Final Cut Pro.

Color Key is similar to the Blue and Green Screen filter. It allows you to select any color for keying. The Color Picker control allows you to select which color you'd like to key out. It's great if what you are trying to key is blue or green, so using these colors as the background in preparation for a key doesn't work well.

Color Smoothing - 4:1:1 is new to FCP 4. You should apply this filter before you process chroma keys. The 4:1:1 version should be used with all DV25 formats (video captured in the 4:1:1 color space).

Color Smoothing - 4:2:2 also is new to FCP 4. You should apply this filter before you process chroma keys. It's the same as the 4:1:1 filter. However, you use this one if your source material is DVCPRO50 or 8- and 10-bit uncompressed video (video captured in the 4:2:2 color space).

Difference Matte compares two clips and keys out areas that are the same. The View pop-up menu allows you to look at the source of the clip (with no key applied), the matte created by the filter, the final matted image, or a special composite of the source, matte, and final image for reference. The Clip control allows you to specify another clip to compare to the current image for keying. The Threshold and Tolerance sliders allow you to adjust the key to try to isolate the parts of your image you want to keep. For example, consider video footage of a man walking next to a wall. If you use a video clip of the wall by itself shot before the man walked by as the specified difference layer, you can adjust the Threshold and Tolerance sliders to isolate the man as he walks, because he's the only element that's different.

Luma Key is similar to Color Key, except that Luma Key creates a matte based on the brightest or darkest areas of an image. Keying out a luminance value works best when the image has a wide variance between what you want to key out and what you want to preserve. For example, if you want to keep something that is bright, you should shoot it against a totally black background. The View pop-up menu allows you to look at the clip's source (with no key applied), the matte created by the filter, the final matted image, or a composite of the source, matte, and final image for reference. The Key Mode pop-up menu allows you to specify whether this filter keys out brighter, darker , similar, or dissimilar areas of the image. The Matte pop-up menu allows you to create alpha channel information for that clip. You also can create a high-contrast matte image applied to your clip's color channels, based on the matte created by this filter.

When you use the Blue and Green Screen key to key out the blue in a clip, there might be a bit of unwanted blue color around the edges of the key (called spill ). The Spill Suppressor, Blue filter removes this spill by desaturating the edges where it appears. Use this filter after a Color Key in the Filters list that appears in the Viewer's Filters tab. It might have a slight effect on your image's color balance.

Spill Suppressor, Green works just like the Spill Suppressor, Blue, but it works on green edges instead of blue ones.

Jerry Hofmann on Final Cut Pro 4
Jerry Hofmann on Final Cut Pro 4
ISBN: 735712816
Year: 2005
Pages: 189 © 2008-2017.
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