Apple Shake 4 Tutorials - Manual (Page 216)

216 Chapter 7 Tracking and Stabilization
2 Toggle applyTransform to active.
If an image appears brighter in an Over operation (in effect adding the two images),
you should immediately suspect the absence of a mask in the foreground. Because the
sign image is a JPEG and does not support alpha channels, you must add an alpha
channel to the image.
3 In the sign node parameters (the FileIn node), enable autoAlpha.
The autoAlpha parameter examines the input image for an alpha channel. If no alpha
channel is found, it sets the entire alpha channel to 1. If an alpha channel is present, it is
left untouched by autoAlpha.
This is an improvement. The four corners of the foreground image are snapped to the
four corners of the trackers. If the image is twisted around, you have done something
such as put track 3 in track 4’s corner. You can unravel that in the next step.
Adjust the Sign Position
The sign may not fit exactly into the background’s sign limits. A little bit peeks out from
behind.