Transfer function editing - using the image picking

[aylward]

This is low priority - putting this here to record this "research" idea.

Instead of the user editing a transfer function in intensity/opacity/color space, I suggest that the user selects intensities of interest in the image by clicking on points in the image, the user then assigns each selection a color and opacity, and then the transfer function is generated from their selections. The GUI would be

The user can add / edit objects of interests as they want, and the transfer function is updated for each change.