Fix exact PMF height threshold units

segmentation/include/pcl/segmentation/impl/progressive_morphological_filter.hpp

@@ -84,7 +84,9 @@ pcl::ProgressiveMorphologicalFilter<PointT>::extract (Indices& ground)
     if (iteration == 0)
       height_threshold = initial_distance_;
     else
-      height_threshold = slope_ * (window_size - window_sizes[iteration-1]) * cell_size_ + initial_distance_;
+      // Exact PMF window sizes are already in input coordinate units.
+      height_threshold =
+          slope_ * (window_size - window_sizes[iteration - 1]) + initial_distance_;
 
     // Enforce max distance on height threshold
     if (height_threshold > max_distance_)

test/segmentation/test_progressive_morphological_filter.cpp

@@ -13,6 +13,36 @@
 
 #include <type_traits>
 
+namespace {
+pcl::Indices
+classifyScaledCloud(float scale)
+{
+  auto cloud = pcl::make_shared<pcl::PointCloud<pcl::PointXYZ>>();
+  cloud->reserve(13);
+  // The raised seven-point plateau survives the smaller openings and is
+  // evaluated against the height threshold at the final window size.
+  for (int i = -6; i <= 6; ++i) {
+    const float x = 0.25f * static_cast<float>(i) * scale;
+    const float z = (i >= -3 && i <= 3) ? 0.8f * scale : 0.0f;
+    cloud->emplace_back(x, 0.0f, z);
+  }
+
+  pcl::ProgressiveMorphologicalFilter<pcl::PointXYZ> filter;
+  filter.setInputCloud(cloud);
+  filter.setCellSize(0.25f * scale);
+  filter.setMaxWindowSize(2.25f * scale);
+  filter.setBase(2.0f);
+  filter.setExponential(true);
+  filter.setSlope(1.0f);
+  filter.setInitialDistance(0.1f * scale);
+  filter.setMaxDistance(100.0f * scale);
+
+  pcl::Indices ground;
+  filter.extract(ground);
+  return ground;
+}
+} // namespace
+
 TEST(ProgressiveMorphologicalFilter, FractionalMaximumWindowSize)
 {
   pcl::ProgressiveMorphologicalFilter<pcl::PointXYZ> filter;
@@ -23,6 +53,15 @@ TEST(ProgressiveMorphologicalFilter, FractionalMaximumWindowSize)
   EXPECT_FLOAT_EQ(filter.getMaxWindowSize(), 6.25f);
 }
 
+TEST(ProgressiveMorphologicalFilter, ClassificationIsInvariantToCoordinateScale)
+{
+  // Scaling the coordinates and every distance parameter must not change which
+  // points are classified as ground.
+  const auto ground = classifyScaledCloud(1.0f);
+  EXPECT_EQ(ground.size(), 13u);
+  EXPECT_EQ(ground, classifyScaledCloud(4.0f));
+}
+
 /* ---[ */
 int
 main(int argc, char** argv)