Sometimes freehand drawing does not work in Segment editor

pieper · March 14, 2018, 8:09pm

Yes, this can be an issue when there are an even number of slices in the view direction (the default fit exactly centers).

In the old Editor all labeling tools (draw, paint, etc) have this in the constructor for that very reason:

github.com

Slicer/Slicer/blob/master/Modules/Scripted/EditorLib/LabelEffect.py#L193-L195


# make sure the slice plane does not lie on an index boundary
# - (to avoid rounding issues)
sliceLogic.SnapSliceOffsetToIJK()

It calls this code:

github.com

Slicer/Slicer/blob/f4ac8624e3ac55f4a5addfb68611639c370c353b/Libs/MRML/Logic/vtkMRMLSliceLogic.cxx#L2165-L2182


//----------------------------------------------------------------------------
void vtkMRMLSliceLogic::SnapSliceOffsetToIJK()
{
double offset, *spacing, bounds[6];
double oldOffset = this->GetSliceOffset();
spacing = this->GetLowestVolumeSliceSpacing();
this->GetLowestVolumeSliceBounds( bounds );


// number of slices along the offset dimension (depends on ijkToRAS and Transforms)
// - find the slice index corresponding to the current slice offset
// - move the offset to the middle of that slice
// - note that bounds[4] 'furthest' edge of the volume from the point of view of this slice
// - note also that spacing[2] may correspond to i, j, or k depending on ijkToRAS and sliceToRAS
double slice = (oldOffset - bounds[4]) / spacing[2];
int intSlice = static_cast<int> (slice);
offset = (intSlice + 0.5) * spacing[2] + bounds[4];
this->SetSliceOffset( offset );
}

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Sometimes freehand drawing does not work in Segment editor

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