Improve bundle adjustment configuration by ensuring constant camera pose is set only for valid registered images.

models/SpaTrackV2/models/tracker3D/spatrack_modules/ba.py

@@ -502,13 +502,27 @@ def ba_pycolmap(world_tracks, intrs, c2w_traj, visb, tracks2d, image_size, cam_t
         ba_options.refine_extra_params = False
         ba_config = pycolmap.BundleAdjustmentConfig()
         reg_image_ids = rec.reg_image_ids()
-        for image_id in reg_image_ids:
-            ba_config.add_image(image_id)
+
         # Fix frame 0 (or the first available frame), i.e, the end frame of the last window
         if len(reg_image_ids) == 0:
             return c2w_traj_glob, world_tracks_refine, intrs
-        first_image_id = min(reg_image_ids) if 0 not in reg_image_ids else 0
-        ba_config.set_constant_cam_pose(first_image_id)
+
+        # Add all registered images to bundle adjustment config
+        for image_id in reg_image_ids:
+            ba_config.add_image(image_id)
+
+        # Select the first image to fix - prefer 0 if available, otherwise use the minimum available id
+        first_image_id = 0 if 0 in reg_image_ids else min(reg_image_ids)
+
+        # Only set constant pose if the image was successfully added
+        if ba_config.has_image(first_image_id):
+            ba_config.set_constant_cam_pose(first_image_id)
+        elif len(reg_image_ids) > 0:
+            # Fallback: try with the first registered image
+            for img_id in sorted(reg_image_ids):
+                if ba_config.has_image(img_id):
+                    ba_config.set_constant_cam_pose(img_id)
+                    break
 
         # fix the 3d points
         for point3D_id in rec.points3D: