[Bug] Blending mode for PET/CT

[salimkanoun]

Describe the Bug

In the fused viewport the Opacity is not applied as a linear scale, the problem is visible as we need to reach opacity >0.75 to see the PET component of the viewport. see https://www.cornerstonejs.org/live-examples/changecolormap

The problems lies in the blending algorithm and has been discussed there Kitware/vtk-js#2093 and more info in this response : Kitware/vtk-js#2093 (comment)

The current blending algorithm is using a opacity expressed /mm which made it sensitive of slab thickness, we can experiment that in OHIF, setting an Opacity at 0.8 and modifying the slab thickess induce a change of opacity.

For blending PET and CT both chanel should be applied in a linear scale and with not effect of the slab thickness.

Similar issue has been found and reported to VolView Kitware/VolView#355 and a PR has been proposed here : Kitware/VolView#449 (comment)
The fusion algorithm of volview seems now good (but don't know yet if it is insensitive of slab thickness), i wonder if a similar fix could be ported / tested in cornerstone 3D

Steps to Reproduce

View a fused viewport and change opacity of the PET layer

The current behavior

Fusion PET/CT is not linear, opacity of PET only shows for value > 0.9 and depends on slab thickness

The expected behavior

Opacity should be linear from 0 to 1 and stay stable even if change slab thickness

OS

Linux

Node version

18

Browser

Chrome lastest