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SynthEyes can use an alpha channel painted into your shot to determine which image areas correspond to which object or the camera, or whether or not to include any given pixel in the planar tracking process (see the Planar Tracking Manual for setup details).
The alpha channel is a fourth channel (in addition to Red, Green, and Blue) for each pixel in your image. You will need external program, typically a compositor, to create such an alpha channel. Plus, you will need to store the shot as sequenced DPX, OpenEXR, SGI, TARGA, or TIFF images, as these formats accommodate an alpha channel.
Or, you can store the alpha channel in separate files, named appropriately. See Separate Alpha Channels. In this case, the original files are left with no alpha data, and the alpha is written in separate files, typically as gray-scale PNG files.
Suppose you wish to have a camera track ignore a portion of the images with a “garbage matte.” Create the matte with the alpha value of 255 (1.0, white) for the areas to be tracked, and 0 (0.0, black) for the areas to be ignored. You’ll need to do this for every frame in the shot, which is why the features of a good compositing program can be helpful. [Note: if a shot lacks an alpha channel, SynthEyes creates a default channel that is black(0) for all hard black pixels (R=G=B=0), and white(255) for all other pixels.]
You can make sure the incoming alpha channel is correct in SynthEyes after you open the shot by temporarily changing the Camera View Type on the Advanced Feature Control dialog (launched from the Feature Panel) to Alpha, or using the Alpha channel selection in the Image Preprocessor subsystem.
Next, on the Rotoscoping panel, delete or ignore the default full-size-rectangular spline. This is very important, because otherwise this spline will assign all blips to its designated object. The alpha channel is used only when a blip is not contained in any spline!
Change the Shot Alpha Levels spinner to 2, because there are two potential values: zero and one. This setting affects the shot (and consequently all the objects and the camera attached to it).
Tip: keep Shot Alpha Levels=0 (off!) unless you’re really using alpha channel input to guide tracking!
Change the Object Alpha Value spinner to 255. Any blip in an area with this alpha value will be assigned to the camera; other blips will be ignored. This spinner sets the alpha value for the currently-active object only.
Tip: When you add a separate alpha channel to a shot, you’ll be asked if you want SynthEyes to make those three adjustments for you.
Note: Shot Alpha Levels controls the tolerance in the alpha level when making an assignment; other nearby alpha values are rounded towards the evenly-spaced values 255*i/(levels-1) for i an integer from 0 to levels-1. To always match alpha values exactly, set Shot Alpha Levels to 256.
If you are tracking the camera and a moving object along with a garbage matte simultaneously, you would create the alpha channel with three levels, for example 0=garbage; 128=camera; 255=object. Note that these values aren’t important, only consistency. You could use a renderer’s object ID# channel, with 0, 1, 2 values, if you want, though you should then set Shot Alpha Levels to 256.
After creating the matte, you would set the Shot Alpha Levels to 3. Then switch to the Camera object on the Shot menu and set the Object Alpha Value to 128. Finally, switch to the moving object on the Shot menu, and set the Object Alpha Value to 255 (or whichever values you have chosen).
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