[Gimp-user] alpha channel computation

[Nerius Landys]

Hello.  I have a TGA image that is fully opaque.  I want to introduce
translucency to the image, making each pixel translucent according to how
far away from black it is.  If a pixel is originally black, it becomes fully
opaque.  If a pixel is originally white, it becomes fully transparent.  If a
pixel is 0x7f7f7f (gray) or 0xff007f (approximately halfway in brighness
between dark and light), it becomes somewhere around half transparent
(computed using diagonal of color cube).

Is there an easy way to do this in GIMP?  i can imagine writing a quickie
computer program to do this for me, but I'm sure the easier way would be to
convince GIMP to do this for me.

Thanks.
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