ENH: Add RGBGibbsPriorFilter accessor invariants GTest

Cover NumberOfClasses / MaximumNumberOfIterations default values,
Set/Get round-trip via both subclass and MRFImageFilter base pointer,
MTime advancement on change, MTime invariance on identical assignment,
and confirm MRFImageFilter::MaximumNumberOfIterations default of 50
differs from the subclass default of 10.

Establishes the API contract pre-cleanup so a follow-up that removes
RGBGibbsPriorFilter's shadow declarations of m_NumberOfClasses and
m_MaximumNumberOfIterations can be verified non-regressive.

Author: hjmjohnson

Modules/Segmentation/MarkovRandomFieldsClassifiers/test/CMakeLists.txt

@@ -21,3 +21,11 @@ itk_add_test(
     ITKMarkovRandomFieldsClassifiersTestDriver
     itkRGBGibbsPriorFilterTest
 )
+
+set(ITKMarkovRandomFieldsClassifiersGTests itkRGBGibbsPriorFilterGTest.cxx)
+
+creategoogletestdriver(
+  ITKMarkovRandomFieldsClassifiers
+  "${ITKMarkovRandomFieldsClassifiers-Test_LIBRARIES}"
+  "${ITKMarkovRandomFieldsClassifiersGTests}"
+)

Modules/Segmentation/MarkovRandomFieldsClassifiers/test/itkRGBGibbsPriorFilterGTest.cxx

@@ -0,0 +1,104 @@
+/*=========================================================================
+ *
+ *  Copyright NumFOCUS
+ *
+ *  Licensed under the Apache License, Version 2.0 (the "License");
+ *  you may not use this file except in compliance with the License.
+ *  You may obtain a copy of the License at
+ *
+ *         https://www.apache.org/licenses/LICENSE-2.0.txt
+ *
+ *  Unless required by applicable law or agreed to in writing, software
+ *  distributed under the License is distributed on an "AS IS" BASIS,
+ *  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ *  See the License for the specific language governing permissions and
+ *  limitations under the License.
+ *
+ *=========================================================================*/
+
+#include "itkGTest.h"
+#include "itkMRFImageFilter.h"
+#include "itkRGBGibbsPriorFilter.h"
+#include "itkImage.h"
+#include "itkVector.h"
+
+namespace
+{
+constexpr unsigned int NumberOfBands{ 1 };
+constexpr unsigned int ImageDimension{ 3 };
+using PixelType = itk::Vector<unsigned short, NumberOfBands>;
+using InputImageType = itk::Image<PixelType, ImageDimension>;
+using LabelImageType = itk::Image<unsigned short, ImageDimension>;
+using BaseFilterType = itk::MRFImageFilter<InputImageType, LabelImageType>;
+using GibbsFilterType = itk::RGBGibbsPriorFilter<InputImageType, LabelImageType>;
+} // namespace
+
+TEST(RGBGibbsPriorFilter, DefaultNumberOfClassesIsZero)
+{
+  auto filter = GibbsFilterType::New();
+  EXPECT_EQ(filter->GetNumberOfClasses(), 0u);
+}
+
+TEST(RGBGibbsPriorFilter, DefaultMaximumNumberOfIterationsIsTen)
+{
+  auto filter = GibbsFilterType::New();
+  EXPECT_EQ(filter->GetMaximumNumberOfIterations(), 10u);
+}
+
+TEST(MRFImageFilter, DefaultMaximumNumberOfIterationsIsFifty)
+{
+  auto filter = BaseFilterType::New();
+  EXPECT_EQ(filter->GetMaximumNumberOfIterations(), 50u);
+}
+
+TEST(RGBGibbsPriorFilter, SetGetNumberOfClassesViaSubclass)
+{
+  auto filter = GibbsFilterType::New();
+  filter->SetNumberOfClasses(7u);
+  EXPECT_EQ(filter->GetNumberOfClasses(), 7u);
+}
+
+TEST(RGBGibbsPriorFilter, SetGetMaximumNumberOfIterationsViaSubclass)
+{
+  auto filter = GibbsFilterType::New();
+  filter->SetMaximumNumberOfIterations(25u);
+  EXPECT_EQ(filter->GetMaximumNumberOfIterations(), 25u);
+}
+
+TEST(RGBGibbsPriorFilter, SetGetNumberOfClassesViaBasePointer)
+{
+  auto                   gibbs = GibbsFilterType::New();
+  const BaseFilterType * basePtr = gibbs;
+  BaseFilterType *       baseMutable = gibbs;
+  baseMutable->SetNumberOfClasses(5u);
+  EXPECT_EQ(basePtr->GetNumberOfClasses(), 5u);
+  EXPECT_EQ(gibbs->GetNumberOfClasses(), 5u);
+}
+
+TEST(RGBGibbsPriorFilter, SetGetMaximumNumberOfIterationsViaBasePointer)
+{
+  auto                   gibbs = GibbsFilterType::New();
+  const BaseFilterType * basePtr = gibbs;
+  BaseFilterType *       baseMutable = gibbs;
+  baseMutable->SetMaximumNumberOfIterations(33u);
+  EXPECT_EQ(basePtr->GetMaximumNumberOfIterations(), 33u);
+  EXPECT_EQ(gibbs->GetMaximumNumberOfIterations(), 33u);
+}
+
+TEST(RGBGibbsPriorFilter, ModifiedTimeAdvancesOnNumberOfClassesChange)
+{
+  auto filter = GibbsFilterType::New();
+  filter->SetNumberOfClasses(3u);
+  const auto t1 = filter->GetMTime();
+  filter->SetNumberOfClasses(4u);
+  EXPECT_GT(filter->GetMTime(), t1);
+}
+
+TEST(RGBGibbsPriorFilter, ModifiedTimeUnchangedOnIdenticalAssignment)
+{
+  auto filter = GibbsFilterType::New();
+  filter->SetNumberOfClasses(3u);
+  const auto t1 = filter->GetMTime();
+  filter->SetNumberOfClasses(3u);
+  EXPECT_EQ(filter->GetMTime(), t1);
+}