Advanced Video Camera and Editing


 
Chroma Keying



The process of electronically removing the colour from a shot is called chroma keying. The most common form involves what television stations do during the weathercast when the meteorologist stands in front of the weather map. Actually, the meteorologist is standing in front of a green or blue screen, which is “keyed” out of the shot and replaced with the weather map. 


Removing the colour is a way of “cutting out” the subject in the shot, allowing you to place the subject in front of any background you choose. Whether the subject is a meteorologist or a spacecraft miniature from a Star Wars film, chroma keying allows the imagination to run free with possibilities!

Chroma Keying in Final Cut Pro is not quite an exact science. You can’t get the precise settings you want without loads of trial and error. However, there are some basic procedures that will get you results. Success depends on how evenly lit your background colour happens to be, how close your subject is to the background, the quality of the camera and tape format, how well you understand the tools, and how patient you are when using them. Some efforts are easy while others are more difficult. While the results aren’t always perfect, the following techniques will help you create mostly solid keys. 

 


Chroma Key Tips

The chroma key will often depend on the quality of the source material; the higher the quality of the video, the better the chroma key. Good video comes from good cameras as well as tape formats.

 

Tape formats:

Although you can use miniDV in your projects, better results will come from using DVCPro, digital Betacam, or HDTV.

 

Background lighting:

The background should be evenly and flatly lit as much as possible. Try to remove any creases and shadows in the backdrop. Additionally, make sure to turn off fluorescent light sources. One thing that can diminish the success of a good chroma key is fluorescent light flicker. Local AC powers at a certain number of cycles and this can cause changes in the fluorescent light’s illumination. Although the human eye can’t see it, the camera captures this illumination cycle, which is also referred to as a “colour roll.” As the cycle is not constant, such colour rolls can ruin a basic chroma key. This problem can be avoided by using incandescent lighting.


Keep some distance between background and subject:

Make sure to move your subject as far from the background as possible so to avoid any “spill” of colour that may appear on shoulder areas, hair, or reflective surfaces, such as certain types of jewellery. Make sure the subject is well-lit and that they aren’t wearing the same colour in their clothing that you’re trying to key out of the shot.

 


Preparing the footage for keying

Place the chroma key clip in V2 track:

In Final Cut Pro, capture the video as always. Place the clip in V2 track. That way, V1 can be used for the new background footage. You can start editing the chroma key with the new background, or you can leave it black for now. 


However, you can change the black settings in favour of using the checkerboard background. The choices for backgrounds can be found in the View pop up menu at the top of the Canvas.



The Pop Up menus are located at the top of the Canvas. On the far left is the Size pop up and the far right is the View. 


The View Pop Up Menu 


 

Size the Canvas at 100%:

To better see the quality of the work you’re doing, make sure the Canvas is set to 100%. You can set the size in the Size pop up menu at the top of the Canvas. 


Applying the Filters


Step 1:  Adding the Colour Smoothing Filter:

Before adding the chroma key filter, start by applying the Chroma Smoothing Filter. Make sure the clip is double-clicked, and then

Go to Effects > Video Filters > Key > Color Smoothing – 4:1:1.

The 4:1:1 colour smoothing filter is necessary because of the limited colourspace of 25 Megabit miniDV or DVCam. This filter “pre-blurs” the colour in the image slightly, allowing you to get smoother edges out of your key. Use 4:1:1 Colour Smoothing with NTSC or PAL DV-25 video sources. Use 4:2:2 Colour Smoothing for DVCPRO50 and 8- and 19-bit uncompressed video.

 

Step 2: Apply the Chroma Key filter:

Go to Effects > Video Filters > Key > Chroma Key.

There are two primary interfaces with this filter. One is found in the Viewer’s Filter tab that allows you a numeric interface. The other is found in the Chroma Key tab at the top of the Viewer, which has a visual interface. Both interfaces work the same way, but users might find it easier to start off with the visual interface first, which allows you to use the Select Colour Eyedropper. 



 
Numeric Interface

 
Visual Interface

 

You can switch back and forth between interfaces easily.

From the Visual interface, click on Numeric. From the Numeric interface, click on Visual.


The Visual Interface

  

  • The top row shows the Colour Control Strip.
  • The second row shows the Saturation Control Strip.
  • The third row controls the Luma Control Strip.
  • There are control handles in each of the strips. The top portion of the handles are the Minimum Width controls; the bottom portion of the handles allow you to soften the effect.
  • The bottom portion of the interface has the controls for Edge Thin, Softness and Enhance. On the side, the three buttons operate the eye dropper, the View Final Matte-Source button, and the Invert Selection button.   

Step 3: Crop the footage

One objective to good chroma keying is to reduce as much of the surrounding colour as much as possible. If your subject doesn’t travel in these areas, then why bother keying them out? It’s far easier to key out areas where there are fewer differences between shades. Check your footage and evaluate how much you can crop out of the shot without affecting your subject’s motion.

There are 2 ways to crop:

  1. Double-click the clip in the Timeline and open to the Motion tab in the Viewer. Under the Motion properties, open Crop and change the left, right, top, and bottom parameters.
  2. Or you can add an 8-point garbage matte: Double-click the clip to modify. Then go to Effects > Video Filters > Matte > 8-point Garbage Matte.

 
Crop controls found in the Viewer's Motion tab


 

 


Use the controls in the Viewer’s Filter tab, clicking on the cross hairs for each of the 8-points and drag them in the Canvas until you have the desired matte around your subject. 



You can round out the edges of the matte by turning up the Smooth parameters in the 8-point Garbage Matte controls. 


You can even use keyframes for the matte if the subject moves out of the matte area.  If you use the 8-point Garbage Matte it must appear below the Chroma Key filter in the Viewer’s Filter tab stacking order.  In other words, apply the chroma key filter first.



Remove the 8-point numbers by selecting Final in the View Mode. 



Step 4: Remove the primary colour

Begin by removing the primary colour: Select the eyedropper in the Chroma Key Visual Interface. Click the eyedropper in the Canvas on a portion of the colour next to your subject. When you click, a good portion of the colour should disappear. 


If there are remaining sections of colour, try selecting the eye dropper again, hold the Shift key down while clicking in the Canvas on any remaining chunks of colours. Continue this process until most of the colour is knocked out. 



Step 5: Fine-tuning the matte

There may still be some edging around your subject. You can fine-tune around the edges by adjusting the controls in the Colour Control Strip, Saturation Control Strip, and Luma Control Strip. The matte should begin to define the shape of your subject. 


Start in the Colour control strip by clicking and dragging the top  part of the handles a bit left or right. Both handles should move in unison. This allows the selection of a few more shades of the colour that you’re trying to key out. Then click and drag on the bottom handles, to soften the colour tolerance, so that other shades of green can be introduced to the matte gradually.

Now go to the Saturation control strip. Make adjustments with each of the handles, noting the differences in your picture. Do the same in the Luma controls. Stretch out the bottom handles a bit more than the ones at the top. 




  

Remember that the area around your subject should be black for 100% transparency and the subject should be white for 100% opacity. Use the Saturation to eliminate any grey areas. 



Adjusting the edges

Even after adjusting the controls, your subject might still have some green or blue around the edge.


Begin by sliding the Edge Thin controls. Also try adding some Softening, but both Edge Thin and Softening should be used gently. The Enhance slider will help adjust for any colour spill on your subject. But for now, stick with the Edge controls.


Edge and Soften controls:

These controls will “choke” the matte, either expanding or contracting the edge of the matte. This control will actually shrink the size of the matte, but you need to be careful not to eliminate detail like fingers or other detail in the hair or face. Softening will smooth out the edges of the matte, to help blend the subject in the background. Once you have the edge figured out, try clicking on the far right arrow of the Enhance slider until any spill disappears. However, if the edge turns into a magenta halo, you’ve gone too far. 


If you still see fringing around your subject, then try applying the Matte Choker Filter.

Go to Effects > Video Filters > Matte > Matte Choker Filter.

This filter allows for additional touch ups to eliminate any faintly remaining blue or green fringing or pixels that may surround the edge of your subject.

 

Step 6: Apply the background clip

Now you can position your background clip in the V1 track.



 

Other Tips:

For subjects that happen to be closer to the camera, you might consider giving depth to your shot by blurring the background slightly.

Go to Effects > Video Filters > Blur > Gaussian.  


Source/Matte/Final button

You can monitor the matte by clicking on the Source/Matte/Final button, which is the one that has a key on it (on the right of the Visual Interface). The red key monitors the image in its final state. The black key monitors the matte itself. Black denotes 100% transparency, white shows as 100% opacity, while grey areas illustrate semi-transparent areas. Finally, the blue key represents the source image, which is nice to have if you need a reference for colour correction.