diff --git a/instantsfm/processors/view_graph_calibration.py b/instantsfm/processors/view_graph_calibration.py
index bd2f807..e133be5 100644
--- a/instantsfm/processors/view_graph_calibration.py
+++ b/instantsfm/processors/view_graph_calibration.py
@@ -15,30 +15,39 @@
 from bae.optim import LM
 from bae.autograd.function import TrackingTensor
 
-def SolveViewGraphCalibration(view_graph:ViewGraph, cameras, images, VIEW_GRAPH_CALIBRATOR_OPTIONS):
-    valid_image_pairs = {pair_id: image_pair for pair_id, image_pair in view_graph.image_pairs.items()
-                         if image_pair.is_valid and image_pair.config in [ConfigurationType.CALIBRATED, ConfigurationType.UNCALIBRATED]}
-    focals = np.array([np.mean(cam.focal_length) for cam in cameras])
-
-    problem = pyceres.Problem()
-    options = pyceres.SolverOptions()
-    loss_function = pyceres.CauchyLoss(VIEW_GRAPH_CALIBRATOR_OPTIONS['thres_loss_function'])
+def SolveViewGraphCalibration(view_graph:ViewGraph, cameras, images, VIEW_GRAPH_CALIBRATOR_OPTIONS):
+    valid_image_pairs = {pair_id: image_pair for pair_id, image_pair in view_graph.image_pairs.items()
+                         if image_pair.is_valid and image_pair.config in [ConfigurationType.CALIBRATED, ConfigurationType.UNCALIBRATED]}
+    if len(valid_image_pairs) == 0:
+        print('No valid image pairs for view graph calibration; skipping.')
+        return
+    focals = np.array([np.mean(cam.focal_length) for cam in cameras])
+
+    problem = pyceres.Problem()
+    options = pyceres.SolverOptions()
+    loss_function = pyceres.CauchyLoss(VIEW_GRAPH_CALIBRATOR_OPTIONS['thres_loss_function'])
     if len(cameras) < 50:
         options.linear_solver_type = pyceres.LinearSolverType.DENSE_NORMAL_CHOLESKY
     else:
         options.linear_solver_type = pyceres.LinearSolverType.SPARSE_NORMAL_CHOLESKY
     
-    for image_pair in valid_image_pairs.values():
-        image1, image2 = images[image_pair.image_id1], images[image_pair.image_id2]
-        cam1, cam2 = cameras[image1.cam_id], cameras[image2.cam_id]
-        cam_id1, cam_id2 = image1.cam_id, image2.cam_id
-        if cam_id1 == cam_id2:
-            cost_function = FetzerFocalLengthSameCameraCostFunction(image_pair.F, cam1.principal_point)
-            problem.add_residual_block(cost_function, loss_function, [focals[cam_id1:cam_id1+1]])
-        else:
-            cost_function = FetzerFocalLengthCostFunction(image_pair.F, cam1.principal_point, cam2.principal_point)
-            problem.add_residual_block(cost_function, loss_function, [focals[cam_id1:cam_id1+1], focals[cam_id2:cam_id2+1]])
-    problem.set_parameter_lower_bound(focals, 0, 1e-3)
+    bounded_cam_ids = set()
+    for image_pair in valid_image_pairs.values():
+        image1, image2 = images[image_pair.image_id1], images[image_pair.image_id2]
+        cam1, cam2 = cameras[image1.cam_id], cameras[image2.cam_id]
+        cam_id1, cam_id2 = image1.cam_id, image2.cam_id
+        if cam_id1 == cam_id2:
+            cost_function = FetzerFocalLengthSameCameraCostFunction(image_pair.F, cam1.principal_point)
+            problem.add_residual_block(cost_function, loss_function, [focals[cam_id1:cam_id1+1]])
+        else:
+            cost_function = FetzerFocalLengthCostFunction(image_pair.F, cam1.principal_point, cam2.principal_point)
+            problem.add_residual_block(cost_function, loss_function, [focals[cam_id1:cam_id1+1], focals[cam_id2:cam_id2+1]])
+        if cam_id1 not in bounded_cam_ids:
+            problem.set_parameter_lower_bound(focals[cam_id1:cam_id1+1], 0, 1e-3)
+            bounded_cam_ids.add(cam_id1)
+        if cam_id2 not in bounded_cam_ids:
+            problem.set_parameter_lower_bound(focals[cam_id2:cam_id2+1], 0, 1e-3)
+            bounded_cam_ids.add(cam_id2)
 
     options.max_num_iterations = VIEW_GRAPH_CALIBRATOR_OPTIONS['max_num_iterations']
     options.function_tolerance = VIEW_GRAPH_CALIBRATOR_OPTIONS['function_tolerance']
@@ -179,4 +188,4 @@ def forward(self, ds, camera_indices1, camera_indices2):
             if loss_sq[idx*2] > thres_two_view_error_sq:
                 invalid_counter += 1
                 view_graph.image_pairs[pair_id].is_valid = False
-        print(f'invalid / total number of two view geometry: {invalid_counter} / {len(valid_image_pairs)}')
\ No newline at end of file
+        print(f'invalid / total number of two view geometry: {invalid_counter} / {len(valid_image_pairs)}')
diff --git a/instantsfm/scripts/feat.py b/instantsfm/scripts/feat.py
index fc3718c..264d2b7 100644
--- a/instantsfm/scripts/feat.py
+++ b/instantsfm/scripts/feat.py
@@ -1,3 +1,4 @@
+import os
 import time
 from argparse import ArgumentParser
 
@@ -19,9 +20,12 @@ def run_feature_handler():
     if not path_info:
         print('Invalid data path, please check the provided path')
         return
-    if path_info.database_exists:
-        print('Database path already exists')
-        return
+    force_rebuild = os.environ.get("INSTANTSFM_FORCE_REBUILD_DB", "1").lower() in ("1", "true", "yes")
+    if path_info.database_exists and not force_rebuild:
+        if os.path.getsize(path_info.database_path) > 0:
+            print('Database already exists; skipping feature extraction/matching.')
+            return
+        print('Database file exists but is empty; rebuilding.')
 
     start_time = time.time()
     config = Config(handler_args.feature_handler, handler_args.manual_config_name)