BUG: Fix issue #1950, ImageMaskSpatialObject access outside image buffer

Fixed issue #1950, "ImageMaskSpatialObject tries to access pixels
outside the image buffer (segfault)".

In the original code, `TransformPhysicalPointToContinuousIndex` did
estimate whether the specified point was inside the image buffer, but
then `GetInterpolator()->EvaluateAtContinuousIndex(index)` did in some
cases still try to access a pixel outside the image buffer.

This fix avoids using an interpolator. It only accesses a pixel when its
index is inside the buffered region.

The use of an interpolator appears less relevant for an image mask than
for other spatial objects, as for each mask pixel value, it is usually
only interesting to know whether it is zero or non-zero.

Added ImageMaskSpatialObject.CornerPointIsNotInsideMaskOfZeroValues unit
test.

Modules/Core/SpatialObjects/include/itkImageMaskSpatialObject.hxx

@@ -40,20 +40,12 @@ template <unsigned int TDimension, typename TPixel>
 bool
 ImageMaskSpatialObject<TDimension, TPixel>::IsInsideInObjectSpace(const PointType & point) const
 {
-  typename Superclass::InterpolatorType::ContinuousIndexType index;
-  if (this->GetImage()->TransformPhysicalPointToContinuousIndex(point, index))
-  {
-    using InterpolatorOutputType = typename InterpolatorType::OutputType;
-    bool insideMask = (Math::NotExactlyEquals(DefaultConvertPixelTraits<InterpolatorOutputType>::GetScalarValue(
-                                                this->GetInterpolator()->EvaluateAtContinuousIndex(index)),
-                                              NumericTraits<PixelType>::ZeroValue()));
-    if (insideMask)
-    {
-      return true;
-    }
-  }
+  const ImageType * const image = this->GetImage();
+
+  const IndexType index = image->TransformPhysicalPointToIndex(point);
 
-  return false;
+  return image->GetBufferedRegion().IsInside(index) &&
+         Math::NotExactlyEquals(image->GetPixel(index), NumericTraits<PixelType>::ZeroValue());
 }
 
 

Modules/Core/SpatialObjects/test/itkImageMaskSpatialObjectGTest.cxx

@@ -306,3 +306,20 @@ TEST(ImageMaskSpatialObject, IsInsideIndependentOfDistantPixels)
 
 
 #endif
+
+
+// Tests that IsInsideInObjectSpace returns false for a corner point, when the
+// mask image is filled with zero values. This test would sometimes fail on
+// ITK v5.0.1 and v5.1.0
+TEST(ImageMaskSpatialObject, CornerPointIsNotInsideMaskOfZeroValues)
+{
+  // Create a mask image, and fill the image with zero vales.
+  const auto image = itk::Image<unsigned char>::New();
+  image->SetRegions(itk::Size<>{ { 2, 2 } });
+  image->Allocate(true);
+
+  const auto imageMaskSpatialObject = itk::ImageMaskSpatialObject<2>::New();
+  imageMaskSpatialObject->SetImage(image);
+  const double cornerPoint[] = { 1.5, 1.5 };
+  ASSERT_FALSE(imageMaskSpatialObject->IsInsideInObjectSpace(cornerPoint));
+}