Merge pull request #711 from lsst/tickets/DM-41012

DM-41012: Check for empty spanset prior to trying to transform it

src/geom/SpanSet.cc

@@ -848,6 +848,12 @@ std::shared_ptr<SpanSet> SpanSet::transformedBy(lsst::geom::AffineTransform cons
 }
 
 std::shared_ptr<SpanSet> SpanSet::transformedBy(TransformPoint2ToPoint2 const& t) const {
+    // If the SpanSet is empty, its bounding box corners are not
+    // meaningful and cannot be transformed.
+    if (_spanVector.empty()) {
+        return std::make_shared<SpanSet>();
+    }
+
     // Transform points in SpanSet by Transform<Point2Endpoint, Point2Endpoint>
     // Transform the original bounding box
     lsst::geom::Box2D newBBoxD;
@@ -857,6 +863,18 @@ std::shared_ptr<SpanSet> SpanSet::transformedBy(TransformPoint2ToPoint2 const& t
         fromCorners.emplace_back(lsst::geom::Point2D(fc));
     }
     auto toPoints = t.applyForward(fromCorners);
+
+    // Check that this was a valid transformation.
+    // Inverses should be good to floating point precision, this is just
+    // a very rough check.
+    auto fromToPoints = t.applyInverse(toPoints);
+    for (int i = 0; i < 4; ++i) {
+        if ((std::abs(fromToPoints[i].getX() - fromCorners[i].getX()) > 1.0) ||
+            (std::abs(fromToPoints[i].getY() - fromCorners[i].getY()) > 1.0)) {
+            return std::make_shared<SpanSet>();
+        }
+    }
+
     for (auto const& tc : toPoints) {
         newBBoxD.include(tc);
     }