[mythtv-theming] How would I create a vignette effect with fanart?

[David Asher]

I'm interested in creating a similar effect as the vignette version of  
the fanart on thetvdb.com (e.g. http://thetvdb.com/banners/fanart/vignette/73739-34.jpg) 
.  It seems like the best idea, however, is to download the full sized  
fanart and then use MythUI to create the vignette.  That way the theme  
can decide whether and what type of vignette to use.

Could it be done just with a single imagetype using a mask file?  Or  
is a mask file only useful for cropping?  The mask attribute  
description is:

mask allows a second image to be set to alpha-mask the original. This  
is a black and transparent image of identical size to the imagetype  
area, where black is visible and transparent will be masked. This  
allows for smooth edges and exotic shapes to your image. Masks cannot  
be resized or scaled.

The usage of "alpha-mask" implies I could get varying level of  
transparancy, but the later portion of the description seems to imply  
only black and transparent are relevant.