fix: Fix Kalman filter extrapolation after smoothing (#2470)

KF and CKF are extrapolating from the last track state to the perigee. This can decrease the resolution drastically if that last state is a material state or hole (state without measurement). Here I exclude these states and propagate from the last measurement to the perigee. **old** ![old](https://github.com/acts-project/acts/assets/487211/3977b191-c2e2-4f74-a2f8-36b94cd11635) **new** ![new](https://github.com/acts-project/acts/assets/487211/4f929459-a334-4e5c-8352-602deafe1290)
acts-project · Sep 23, 2023 · e34660a · e34660a
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diff --git a/CI/physmon/reference/performance_amvf_truth_smeared_hist.root b/CI/physmon/reference/performance_amvf_truth_smeared_hist.root
diff --git a/CI/physmon/reference/performance_ivf_truth_smeared_hist.root b/CI/physmon/reference/performance_ivf_truth_smeared_hist.root
diff --git a/CI/physmon/reference/performance_truth_tracking.root b/CI/physmon/reference/performance_truth_tracking.root
diff --git a/CI/physmon/reference/tracksummary_ckf_orthogonal_hist.root b/CI/physmon/reference/tracksummary_ckf_orthogonal_hist.root
diff --git a/CI/physmon/reference/tracksummary_ckf_seeded_hist.root b/CI/physmon/reference/tracksummary_ckf_seeded_hist.root
diff --git a/CI/physmon/reference/tracksummary_ckf_truth_estimated_hist.root b/CI/physmon/reference/tracksummary_ckf_truth_estimated_hist.root
diff --git a/CI/physmon/reference/tracksummary_ckf_truth_smeared_hist.root b/CI/physmon/reference/tracksummary_ckf_truth_smeared_hist.root
diff --git a/Core/include/Acts/TrackFinding/CombinatorialKalmanFilter.hpp b/Core/include/Acts/TrackFinding/CombinatorialKalmanFilter.hpp
@@ -1150,8 +1150,11 @@ class CombinatorialKalmanFilter {
       result.fittedStates->applyBackwards(lastMeasurementIndex, [&](auto st) {
         bool isMeasurement =
             st.typeFlags().test(TrackStateFlag::MeasurementFlag);
-        bool isMaterial = st.typeFlags().test(TrackStateFlag::MaterialFlag);
-        if (isMeasurement || isMaterial) {
+        bool isOutlier = st.typeFlags().test(TrackStateFlag::OutlierFlag);
+        // We are excluding non measurement states and outlier here. Those can
+        // decrease resolution because only the smoothing corrected the very
+        // first prediction as filtering is not possible.
+        if (isMeasurement && !isOutlier) {
           firstStateIndex = st.index();
         }
         nStates++;

diff --git a/Core/include/Acts/TrackFitting/KalmanFitter.hpp b/Core/include/Acts/TrackFitting/KalmanFitter.hpp
@@ -893,8 +893,11 @@ class KalmanFitter {
           result.lastMeasurementIndex, [&](auto st) {
             bool isMeasurement =
                 st.typeFlags().test(TrackStateFlag::MeasurementFlag);
-            bool isMaterial = st.typeFlags().test(TrackStateFlag::MaterialFlag);
-            if (isMeasurement || isMaterial) {
+            bool isOutlier = st.typeFlags().test(TrackStateFlag::OutlierFlag);
+            // We are excluding non measurement states and outlier here. Those
+            // can decrease resolution because only the smoothing corrected the
+            // very first prediction as filtering is not possible.
+            if (isMeasurement && !isOutlier) {
               firstStateIndex = st.index();
             }
             nStates++;

diff --git a/Examples/Python/tests/root_file_hashes.txt b/Examples/Python/tests/root_file_hashes.txt
@@ -23,13 +23,13 @@ test_itk_seeding__particles_initial.root: 88315e93ed4cb5d40a8721502048a9d1fc100e
 test_propagation__propagation_steps.root: 174301b25784dbb881196b658f2d7f99c0a2ea688a0129e6110fc19aa5cf8e54
 test_material_recording__geant4_material_tracks.root: e411152d370775463c22b19a351dfc7bfe40b51985e10a7c1a010aebde80715d
 test_truth_tracking_kalman[generic-0.0]__trackstates_fitter.root: 0cdfeb13eaff45df38451820fcc15a7dfb40e55dab220e6e1d4d6f027ee8d4c8
-test_truth_tracking_kalman[generic-0.0]__tracksummary_fitter.root: c893152e43fc76247d767292979a7acc61d693539620a7b6d3de5e9ce7945414
+test_truth_tracking_kalman[generic-0.0]__tracksummary_fitter.root: 135ce6fbbe1789c56d42a834e148e6ce395c7425373989f8c03e6c32c989fe94
 test_truth_tracking_kalman[generic-0.0]__performance_track_finder.root: ada9cda2ec3c648b144bdaa081d6eff923c80f3d484c4196006e847428cf67a8
 test_truth_tracking_kalman[generic-1000.0]__trackstates_fitter.root: 5d54954ad21b8153b1788ccc296680a50969c761924f1c5db0600033d4577a3c
 test_truth_tracking_kalman[generic-1000.0]__tracksummary_fitter.root: 3ecd6851e1633103b9ae20140aa8f6a5a27aecc777177cd83078ed292544d9ae
 test_truth_tracking_kalman[generic-1000.0]__performance_track_finder.root: ada9cda2ec3c648b144bdaa081d6eff923c80f3d484c4196006e847428cf67a8
 test_truth_tracking_kalman[odd-0.0]__trackstates_fitter.root: 7afa54c9f2c989e11236861b672e822f881443c5976dfcb65cebbab155280a83
-test_truth_tracking_kalman[odd-0.0]__tracksummary_fitter.root: 08ed2bc87b5c8e6fa92104d4e1322f5c76755ada6a20899eed293dc5814b5c94
+test_truth_tracking_kalman[odd-0.0]__tracksummary_fitter.root: 85cdf6aced8c7f7eb1c156d912f6be6a7ab1370b4c72955c0bd7b09095df9a60
 test_truth_tracking_kalman[odd-0.0]__performance_track_finder.root: 39aec6316cceb90e314e16b02947faa691c18f57c3a851a25e547a8fc05a4593
 test_truth_tracking_kalman[odd-1000.0]__trackstates_fitter.root: 04ec5361f32eae9151497c54d89c2a0b03a088050be7a37f521f65cd3abf85c8
 test_truth_tracking_kalman[odd-1000.0]__tracksummary_fitter.root: 2a6de6dedd20acae66eef29649a0966f0b83ea00cee9475fbaaca696058b2525
@@ -56,15 +56,15 @@ test_ckf_tracks_example[generic-truth_estimated]__trackstates_ckf.root: 7dd654c2
 test_ckf_tracks_example[generic-truth_estimated]__tracksummary_ckf.root: e9886b61af4934c1b2a825719ea69ec3b69b245927cb22d8cca5071612609fe5
 test_ckf_tracks_example[generic-truth_estimated]__performance_seeding.root: 1facb05c066221f6361b61f015cdf0918e94d9f3fce2269ec7b6a4dffeb2bc7e
 test_ckf_tracks_example[generic-truth_smeared]__trackstates_ckf.root: 0351e6e87bbda82c666e3580f8437fa9736f9d95ec4dd8658129bc73bb3fbf6c
-test_ckf_tracks_example[generic-truth_smeared]__tracksummary_ckf.root: e72c400c9e7e9ca5f3612005254b6fa1cae070e86c533fc5487776e4120968a5
+test_ckf_tracks_example[generic-truth_smeared]__tracksummary_ckf.root: 21eb776b8b6974543c0bfed6cdc18773a9f20629bed5d99154f2ded79a4297cd
 test_ckf_tracks_example[odd-full_seeding]__trackstates_ckf.root: bf564fc788eed6e72791d4eac639bd439ba2e167554cee4edb6a6162cec22089
 test_ckf_tracks_example[odd-full_seeding]__tracksummary_ckf.root: f107b73de07e5fb035c2a3e50b2d2928df77d7ce6579341bc5a227485972c878
 test_ckf_tracks_example[odd-full_seeding]__performance_seeding_trees.root: 43c58577aafe07645e5660c4f43904efadf91d8cda45c5c04c248bbe0f59814f
 test_ckf_tracks_example[odd-truth_estimated]__trackstates_ckf.root: 3defdfb49c4942e53e5cda8f2cd01475ce2bfe1b92b6745d83ea17d9c2ad535f
 test_ckf_tracks_example[odd-truth_estimated]__tracksummary_ckf.root: b9a5e794e3d8c3c64426efb4abe2ce38e338de96f3de4f751b238b5ae1997189
 test_ckf_tracks_example[odd-truth_estimated]__performance_seeding.root: 1a36b7017e59f1c08602ef3c2cb0483c51df248f112e3780c66594110719c575
 test_ckf_tracks_example[odd-truth_smeared]__trackstates_ckf.root: d4b47a6c9621847be84a17a21fb929a050c49d0c7b3d944bf14fee365478609d
-test_ckf_tracks_example[odd-truth_smeared]__tracksummary_ckf.root: 2c94035575fc222eff022368052b98c84d54f1b75e94681e794bd30282cc7146
+test_ckf_tracks_example[odd-truth_smeared]__tracksummary_ckf.root: e01769e412c89fe73dac106465f2d6b13744da2d8e1b6d73edb8f91830024cd6
 test_vertex_fitting_reading[Truth-False-100]__performance_vertexing.root: 76ef6084d758dfdfc0151ddec2170e12d73394424e3dac4ffe46f0f339ec8293
 test_vertex_fitting_reading[Iterative-False-100]__performance_vertexing.root: 60372210c830a04f95ceb78c6c68a9b0de217746ff59e8e73053750c837b57eb
 test_vertex_fitting_reading[Iterative-True-100]__performance_vertexing.root: e34f217d524a5051dbb04a811d3407df3ebe2cc4bb7f54f6bda0847dbd7b52c3