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Digging deeper, it turns out that "clip" means something else to VTK than what I'd assume.
Starting from the Slicer coordinate system documentation:
http://www.slicer.org/slicerWiki/index.php/Coordinate_systems#Anatomical_coordinate_system
Which says:
the axial plane is parallel to the ground and separates the head (Superior) from the feet (Inferior)
the coronal plane is perpendicular to the ground and separates the front from (Anterior) the back (Posterior)
the sagittal plane separates the Left from the Right
And checking the models module clipping documentation:
http://www.slicer.org/slicerWiki/index.php/Documentation/Nightly/Modules/Models
which says:
Red Slice Clipping: Control on which side of the red slice plane is clipped on the model
Yellow Slice Clipping: Control on which side of the yellow slice plane is clipped on the model
Green Slice Clipping: Control on which side of the green slice plane is clipped on the model
I added this tool tip to the Red Slice Clipping checkbox:
Use the positive or negative space defined by the Red slice plane to clip the model. Positive side is toward the Superior, negative is toward the Inferior
And this on the positive radio buttons:
Use the positive space, toward the Superior (head), defined by the Red slice plane to clip the model.
But when I turn on positive clipping on the red slice, the part of the model on the Superior side of the red slice plane is visible, and the part of the model on the Inferior side of the red slice plane is invisible.
This seems that what the application is defining as clipping in the positive space is:
Hide the part of the model that is in the negative space defined by the slice plane and show what's on the positive side.
I was assuming that picking the positive option would clip away the positive side of the model and leave the negative side visible.
Digging into the code, the clipping widget slot updateNodeRedClipState calls:
this->setRedSliceClipState(this->redSliceClipState());
redSliceClipState will return ClipPositiveSpace if the red slice box is checked and the positive radio button is checked.
setRedSliceClipState will call SetRedSliceClipState on the MRMLClipNode
which is a vtkMRMLClipModelsNode, with the clip state defined as 0=off, 1=positive, 2=negative
In vtkMRMLModelDisplayableManager sets the plane direction by checking against the clip negative space enum (if the slice clip state is clip negative, set the plane direction to -1, otherwise to 1).
vtkClipPolyData documentation says:
"... returning everything inside of the specified implicit function"
which I think does imply that it's going to return things on the side you say, rather than the other side...
So if I pick to clip on the positive side, that's defined as "inside" my function, and it's going to return the part of the model on the positive side of the plane.
This isn't quite what I'd expect as a user from the word clip, so the next question is does this make sense as an updated tool tip:
Use the positive or negative space defined by the Red slice plane to clip the model. The part of the model in the selected space is kept, parts outside of it are clipped away. Positive side is toward the Superior, negative is toward the Inferior. |
