[vtkusers] Set the opacity or thresholding of a VTKVolume

[Michael K]

Hi David,

I don't know much about ITK, but you should be able to achieve the same with
a well defined opacity function by setting the opacity to 0 for the density
values representing the flesh.

So if the opacity transfer function starting at a density value of 84 (like
your example) still shows the tissue, and you want to show only the more
dense bones, just start at a higher density value.

B.T.W. you defined the opacity value for 8 bit data (range up to 255), but
CTs often have 12 bit, so defining a transfer function between 0 and 4095
could also be an issue.

Defining the proper transfer functions is sometimes a little bit tricky. To
get an idea on a nice transfer function for opacity I simply use two sliders
defining a lower and and an upper threshold for the density values, so
creating an opacity transfer function building up a linear ramp from (lower
threshold, 0) up to (upper threshold, 1) (with an opacity of 0 for densities
below lower threshold and 1 for densities above upper threshold when using
the default clamping behaviour of a VTK piecewise function).  Instead of 1 I
often use a value around 0.8 as the maximum opacity resulting in a deeper
penetration of the dense areas.
Fiddling around with the sliders for this threshold provides most
information needed for defining a proper opacity transfer function,
depending on the data and the parts I want to be shown.

Bye,
Michael

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