I forgot to mention that my TIFF image experiment showed some additional quirky
behavior in POV-Ray (in addition to the main problem.)  Actually, I never use
TIFF images for this fade-out trick-- in fact, I rarely make TIFF images at all!
But it seemed like an interesting experiment to try...

I used an animation to check it out, with 'transmit all' based on the clock
value. (My TIFF image_map has a soft-edged alpha-channel matte.) The result-- as
'transmit all' goes from 0 to 1-- shows that POV-Ray treats the alpha-channel as
either 'on or off', depending on some threshold value as transmit changes. In
other words, it looks like a (changing-size) hard-edged matte. The other
strangeness is that final full-transparency of the 'opaque' parts of the image
do not occur until transmit reaches 1.0 (or very nearly so), at which point the
opaque-to-transparency shift is quite abrupt. (Actually, the 'final
transparency' seems to depend on the invisible background color of the TIFF
image(!), as evidenced by the black-to-white gradation that I added to the
original image_map.)

Funny thing: I looked up the TIFF file format on Wikipedia-- a long and detailed
technical discussion there-- but it fails to even mention that an alpha-channel
can be used with a TIFF image(!). I had to look elsewhere. I'm guessing that
TIFF encodes alpha-channel information in an entirely different way than a PNG
file.

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