Fix renderer incorrectly filtering out negative scale factor (#974)

CHANGELOG.md

@@ -7,6 +7,8 @@ on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 
 - The application now has a MacOS-specific logo that more closely follows Apple's icon
   guidelines.
+- Negative scale factors are now supported by the 3D renderer, which can be handy for
+  mirroring meshes (thanks @carlosedubarreto, #974)
 
 ## [0.5.18] - 2025/01/16
 

libOpenSimCreator/Graphics/SimTKDecorationGenerator.cpp

@@ -19,6 +19,7 @@
 #include <SimTKcommon/internal/PolygonalMesh.h>
 #include <SimTKcommon/internal/State.h>
 
+#include <cmath>
 #include <cstddef>
 #include <filesystem>
 
@@ -37,7 +38,9 @@ namespace
         SimTK::Vec3 sf = geom.getScaleFactors();
 
         for (int i = 0; i < 3; ++i) {
-            sf[i] = sf[i] <= 0.0 ? 1.0 : sf[i];
+            // filter out NaNs, but keep negative values, because some
+            // users use negative scales to mimic mirror imaging (#974)
+            sf[i] = not std::isnan(sf[i]) ? sf[i] : 0.0;
         }
 
         return to<Vec3>(sf);

libOpenSimCreator/Graphics/SimTKDecorationGenerator.tests.cpp

@@ -53,3 +53,26 @@ TEST(SimTKDecorationGenerator, PropagatesHiddenRepresentation)
     });
     ASSERT_EQ(ncalls, 1) << "should only emit one is_wireframe sphere";
 }
+
+// ensure that the `SimTKDecorationGenerator` propagates negative scale factors,
+// because some users use them to mirror-image geometry (#974)
+TEST(SimTKDecorationGenerator, PropagatesNegativeScaleFactors)
+{
+    SceneCache cache;
+
+    SimTK::MultibodySystem sys;
+    SimTK::SimbodyMatterSubsystem matter{sys};
+    SimTK::State state = sys.realizeTopology();
+    sys.realize(state);
+
+    SimTK::DecorativeSphere sphere;
+    sphere.setBodyId(0);
+    sphere.setRepresentation(SimTK::DecorativeGeometry::Hide);
+    sphere.setRadius(1.0);
+    sphere.setScaleFactors(SimTK::Vec3(1.0, -1.0, 1.0));  // note: negative
+
+    osc::GenerateDecorations(cache, matter, state, sphere, 1.0f, [&](const SceneDecoration& dec)
+    {
+        ASSERT_EQ(dec.transform.scale.y, -1.0f);
+    });
+}