Hi Robert,
I’ll add this suggestion as a feature request. I’m assuming that your goal was to use the matte function in the pixelchooser to hold back the flare from areas in the image … is this correct? If so, you might want to try the obscuration map feature in the Lens Flare 3D. You can set this up to hold back the lens flare so that when the LF passes over rigid objects in the scene the lens flare will be obscured by the foreground object.
I hope this helps.
Peter.