Use Hounsfield scale for transfer functions

[mlavik1]

Changes:

• Transfer function now use absolute data values by default.
• When applying old TFs to a volume, we convert to absolute values on the fly.
• Added a new transfer function upgrader tool window, for upgrading old TFs.

Details:

Currently there's a big limitation related to the transfer functions: They use relative data values (relative to the min/max data values of a dataset), and the same TF can therefore not be applied to different transfer functions. In practice we often deal with datasets that use the same (Hounsfield) scale, such as DICOM and NRRD. So this has become a limitation for many users.

It was implemented like this long before DICOM support was added. The idea was to have TFs that can work across different scales (we only supported RAW datasets in the beginning, and they can often have very different scales). However, we should be able to support both by specifying the scale in the saved TF.

TODO:

• Add a "use relative scale" checkbox to the TF editor???
• Support 2D transfer functions?

[mlavik1]

TF upgrader tool: