The documentation at     3.2.4.1 Output File Type   says,
"Note: In version 3.7 alpha handling for image file OUTPUT has changed.
Effectively, the background *now requires* a filter or transmit value in order
for alpha transparency to work properly...The philosophy of the alpha channel
feature in POV-Ray is that the background color should not be present in the
color of the image when the alpha channel is used..."

So even for quick tests of using Output_Alpha (or +ua) to make a .png
alpha-channel render, I have always dutifully added at least a
background{rgbt 1} statement to a scene-- because I thought at least *something*
was required there.

But if there is no background *at all* in a scene, Output_Alpha can be used by
itself and still work correctly. Duh.

I must have been mis-reading the docs all this time...although the 'requirement'
statement is a bit misleading, IMO. Or a little tidbit of clarifying info was
overlooked.

You would think that, at *sometime* in the past, I would have made a mistake
writing even a simple scene and discovered this little fact by chance. But no.
:-(

Anyway, I learned something new...today!

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