Chroma Key Tutorial: Green Screening and Motion Tracking
This chroma key tutorial explains green screening and motion tracking while using this feature. Chroma Keying allows users to remove the background from objects and then combine two images. This is done via green or blue screen technology. Once the background has been scrubbed, motion tracking can be applied to that image, which allows another image or layer to have the same track as the original object.
Step 1: Using Chroma Keying
Chroma keying can be most often seen in weather reports. A meteorologist will be standing in front of a blue or green screen, but it will appear that he is standing in front of a radar map. The chroma keying basically allows the user to cut out the image of the person and superimpose it onto another image.
For example, if a photograph was taken against a background with unwanted objects, chroma keying can be used to remove that background and change it into a blue or green screen. In Apple’s Aperture 2.1, this is done using dvGarage’s dpMatte plug-in, but this effect can also be done in other programs.
Step 2: Removing the Background in Aperture
To remove the background in Aperture, editors will use an 8-point garbage matte. This will essentially paint over the current background and turn it into a blue or green screen. From there, it’s time to manipulate the background even further.
Step 3: Using the Unspill Process
If the background came out as purple, the unspill process can be used to turn the background into the appropriate color. The background colors can be further manipulated by using the Blur, Black Point or White Point amount sliders. The compositing tools can also be used to adjust the background and add the picture to a different background.
Step 4: Motion Tracking
Motion tracking allows users to track the motion of an object, and this will essentially create a map of the motion. This map can then be added to another object or layer so that it will mimic the movement of the original image. This can be most effectively done using Adobe After Effects, but this same effect can be done in other programs.
The first thing that needs to be done is to open up the composite shot from the chroma keying. Pick an object that can be easily tracked. View the entire sequence to determine whether or not the image can be tracked throughout the shot.
Step 5: Track Type
Now, select the Track Type menu, and this will create track points in the Layer panel. Next, users should add additional track points if necessary. Move the attach point to the object or layer that will be tracked. The object that is going to be tracked should be completely enveloped by the attach point box.
Step 6: Analyze
To actually track the motion, the Analyze button will have to be selected, and After Effects will find the object in each frame. For more intricate shots, the user may actually want to personally analyze each shot. Check to make sure that the correct object was tracked, and then hit Apply. The map of the tracked object will now be created.