[vtkusers] opacity property behaves discontinuously -> 1

[Liam Kurmos]

if i render a fully opaque object in front of a semi transparent one,
i appears with full color. If i render it  behind such an object their
colors are combined giving the appearance of being behind the object,
as desired.
However if rather than being opaque the first object is opacity = 0.99
then the color rendered is the same combined color whether it is in
front of or behind the semi-transparent object.

How can i achieve the affect that a semi transparent object appears in
front of another semi transparent object when it is in front and
behind when it is behind?

best regards

Liam