I am working on a scene that will include some glasses with colored liquids
inside of them.  While working on the model for the liquid, I noticed that
something odd was happening when I used interior fading to color the liquid.

The image on the left is the shape of the liquid rendered with a solid texture:
pigment { color rgb 0.5 } interior { ior 1.34 }

The image on the right is the same shape rendered with a transparent texture and
fading:

pigment { color rgbf 1 } interior { ior 1.34 fade_color <1, 0, 0> fade_distance
1 }

From this viewpoint, in the solid version you can see the curve of the liquid
object bowing upwards as expected.  In the transparent version, there is a sharp
horizontal line where the fade_color ends, below that curve.  You can see the
interior is still applying to the space above that line, as the vertical green
cylinder is broken where the ior changes, but the fade color is completely
missing in that segment.

I would expect to see some variation in the color in that region due to light
traveling different distances based on the camera angle, but not a complete drop
out.

Any idea why this might be happening?

-- Chris R.

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