I think POV-Ray antialiases pixels in an undesirable way when the alpha
channel is used.

POV-Ray Team member Thorsten Froehlich believes it is a bug and encouraged
me to report it here.

I have posted an image to povray.binaries.images in the message entitled
"Alpha Channel and Antialiasing" that shows the problem.

I believe the problem is caused by the fact that POV-Ray, when calculating
antialiased pixels, includes the background colour in the colour average.
When the alpha channel is used, the background colour should not be included
in the colour average since the background is already taken into account in
the alpha channel.

A side effect (but not a wrong one) of the fix is that the blend between
objects and the background will seem not to be antialiased when viewed in
programs that does not have alpha channel support (such as POV-Ray's own
viewer).

Rune

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