Correct definition of is_fixed in BundleShotPoses (#1115)

Summary:
This is a PR for Issue #1083

As discussed there, the definition of “is_fixed” in BundleShotPoses is probably incorrect, leading to unnecessary runs of the Ceres optimizer. Although the impact is rather minor, a correct definition is preferable.

Pull Request resolved: #1115

Differential Revision: D92679317

Pulled By: fabianschenk

fbshipit-source-id: d2fd3c75c58ec078b272730353222181cb4bbfdb

Author: kielnino

opensfm/src/sfm/src/ba_helpers.cc

@@ -492,8 +492,9 @@ py::dict BAHelpers::BundleShotPoses(
       auto& shot = map.GetShot(shot_id);
       shot_cameras[shot_id] = shot.GetCamera()->id;
       shot_rig_cameras[shot_id] = shot_n_rig_camera.second->id;
-
-      const auto is_fixed = shot_ids.find(shot_id) != shot_ids.end();
+      // If the shot_id is in the shot_ids we aim to optimize this shot, so
+      // don't fix it
+      const auto is_fixed = shot_ids.find(shot_id) == shot_ids.end();
       if (!is_fixed) {
         if (config["bundle_use_gps"].cast<bool>()) {
           const auto pos = shot.GetShotMeasurements().gps_position_;