ENH: Add support for volume rendering in fast picking

Point picking (getting 3D position from 2D screen position, for example when using Shift+MouseMove) in a 3D view is slow for large meshes if vtkMRMLCrosshairNode::FastPick3D is disabled.
Fast picking is disabled by default because it cannot pick volume rendering and semi-transparent surfaces. This commit adds fast picking for volume rendering.

Fast picking is still disabled by default because semi-transparent surfaces are still ignored. A potential solution for that could be to run accurate picking selectively, just for semi-transparent
surfaces. Then picking would only be slow if there are large semi-transparent models.

Libs/MRML/DisplayableManager/vtkMRMLThreeDViewInteractorStyle.cxx

@@ -32,6 +32,7 @@
 #include <vtkPoints.h>
 #include <vtkRenderWindowInteractor.h>
 #include <vtkRenderer.h>
+#include <vtkVolumePicker.h>
 #include <vtkWorldPointPicker.h>
 
 vtkStandardNewMacro(vtkMRMLThreeDViewInteractorStyle);
@@ -43,6 +44,7 @@ vtkMRMLThreeDViewInteractorStyle::vtkMRMLThreeDViewInteractorStyle()
   this->AccuratePicker = vtkSmartPointer<vtkCellPicker>::New();
   this->AccuratePicker->SetTolerance( .005 );
   this->QuickPicker = vtkSmartPointer<vtkWorldPointPicker>::New();
+  this->QuickVolumePicker = vtkSmartPointer<vtkVolumePicker>::New();
 }
 
 //----------------------------------------------------------------------------
@@ -126,9 +128,26 @@ bool vtkMRMLThreeDViewInteractorStyle::QuickPick(int x, int y, double pickPoint[
     return false;
   }
 
-  this->QuickPicker->Pick(x, y, 0, this->CurrentRenderer);
-
+  bool quickPicked = (this->QuickPicker->Pick(x, y, 0, this->CurrentRenderer) > 0);
   this->QuickPicker->GetPickPosition(pickPoint);
 
+  // QuickPicker ignores volume-rendered images, do a volume picking, too.
+  if (this->CameraNode && this->QuickVolumePicker->Pick(x, y, 0, this->CurrentRenderer))
+    {
+    double volumePickPoint[3] = { 0.0, 0.0, 0.0 };
+    this->QuickVolumePicker->GetPickPosition(volumePickPoint);
+    double* cameraPosition = this->CameraNode->GetPosition();
+    // Use QuickVolumePicker result instead of QuickPicker result if picked volume point
+    // is closer to the camera (or QuickPicker did not find anything).
+    if (!quickPicked
+      || vtkMath::Distance2BetweenPoints(volumePickPoint, cameraPosition)
+      < vtkMath::Distance2BetweenPoints(pickPoint, cameraPosition))
+      {
+      pickPoint[0] = volumePickPoint[0];
+      pickPoint[1] = volumePickPoint[1];
+      pickPoint[2] = volumePickPoint[2];
+      }
+    }
+
   return true;
 }

Libs/MRML/DisplayableManager/vtkMRMLThreeDViewInteractorStyle.h

@@ -27,6 +27,7 @@
 #include "vtkSmartPointer.h"
 
 class vtkCellPicker;
+class vtkVolumePicker;
 class vtkWorldPointPicker;
 
 /// \brief Interactive manipulation of the camera.
@@ -82,9 +83,13 @@ class VTK_MRML_DISPLAYABLEMANAGER_EXPORT vtkMRMLThreeDViewInteractorStyle :
 
   vtkMRMLCameraNode *CameraNode;
 
-  /// For jump to slice feature (when mouse is moved while shift key is pressed)
+  // For jump to slice feature (when mouse is moved while shift key is pressed)
+  // Slow but can pick anything (volumes and semi-transparent surfaces)
   vtkSmartPointer<vtkCellPicker> AccuratePicker;
+  // Picker that uses Z buffer. Fast but ignores volumes and semi-transparent surfaces.
   vtkSmartPointer<vtkWorldPointPicker> QuickPicker;
+  // Picker for volume-rendered images. Fast, as it only computes a single ray.
+  vtkSmartPointer<vtkVolumePicker> QuickVolumePicker;
 
 private:
   vtkMRMLThreeDViewInteractorStyle(const vtkMRMLThreeDViewInteractorStyle&) = delete;