set incorrectly.  Here are the things (I believe) have something to do with the

(1) an island in the distant background
(2) a sailboat in the foreground.
(3) the sails use a png file for the texture.  The png file has some totally
transparent areas along with some totally opaque areas.

#declare SailCloth = texture { uv_mapping pigment {
    image_map {png "SailCloth.png"
interpolate 2 map_type 0  transmit all 0 filter all 0 } } }

(4) I am using a fog effect

fog{C_SkyBottom fog_type 2 distance 1000 fog_alt 50 rotate <90, 0, 0> }

Looking through the sails at near objects (the boat say) or the sail casting
shadows on the boat appear to be correct.  However looking through the sails at
the distant background (where the fog effect comes into play) seems to nullify
the fog.  The two attached renderings show my problem.  In the circled area of
the fogged version, the left side is looking through two layers of the cloth.
This looks pretty much as I would expect it.  The right side is looking through
one layer.  I would expect it to be about the same brightness as the portion of
the beach that is un-blocked.  However, it is far brighter.  Its almost like a
polarizing filter effect on a real camera.

I hope you can point me to the error of my ways.

Thanks.

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Attachments:
Download 'fog.zip' (131 KB)