The old default SliceToRAS coordinate systems were left-handed for no particular reason. We use right-handed coordinate system everywhere else. Using left-handed slice coordinate system turns models inside out and can introduce other small inconsistencies when we transform objects to the slice plane, so I would consider it as an undesirable behavior.
Comparison of left/right-handedness of coordinate systems between preview and stable release
Using Slicer-4.11.20110226: two left handed, one right-handed
>>> import numpy as np
>>> np.linalg.det(arrayFromVTKMatrix(getNode('vtkMRMLSliceNodeRed').GetSliceToRAS()))
-1.0
>>> np.linalg.det(arrayFromVTKMatrix(getNode('vtkMRMLSliceNodeGreen').GetSliceToRAS()))
1.0
>>> np.linalg.det(arrayFromVTKMatrix(getNode('vtkMRMLSliceNodeYellow').GetSliceToRAS()))
-1.0
Using Slicer-4.11.20110327: all right-handed
>>> import numpy as np
>>> np.linalg.det(arrayFromVTKMatrix(getNode('vtkMRMLSliceNodeRed').GetSliceToRAS()))
1.0
>>> np.linalg.det(arrayFromVTKMatrix(getNode('vtkMRMLSliceNodeGreen').GetSliceToRAS()))
1.0
>>> np.linalg.det(arrayFromVTKMatrix(getNode('vtkMRMLSliceNodeYellow').GetSliceToRAS()))
1.0
With the new slice axis directions, the arrow buttons work more intuitively with the default 3D orientation, too:
Slicer-4.11.20110226:
- red: arrow right moves the slice up → as expected
- green: arrow right moves the slice backward → I would not expect this
- yellow: arrow right moves the slice left → inconsistent
Slicer-4.11.20110327:
- red: arrow right moves the slice up → as expected
- green: arrow right moves the slice forward → as expected
- yellow: arrow right moves the slice right → as expected
So, overall, the new behavior is more consistent at all levels, expect it is different from the old behavior.
I just read the top post and indeed behavior of the mouse wheel is opposite of what is expected, I’ll invert that.