Apply the clock offset to the experiment time

ledsa/data_extraction/init_functions.py

@@ -127,10 +127,10 @@ def _calc_experiment_and_real_time(build_type: str, config: ConfigData, tag: str
                             config['DEFAULT']['img_name_string'].format(int(img_number)))
     date_time_img = get_exif_datetime(img_path, tag)
 
-    experiment_time = date_time_img - config.get_datetime()
-    experiment_time = experiment_time.total_seconds()
     time_diff = config.getfloat(build_type, 'exif_time_infront_real_time')
     time = date_time_img - timedelta(seconds=time_diff)
+    # start_time is on the real clock, so the experiment time is measured from the corrected image time
+    experiment_time = (time - config.get_datetime()).total_seconds()
     return experiment_time, time
 
 

ledsa/tests/UnitTests/test_experiment_time_clock_offset.py

@@ -0,0 +1,66 @@
+"""Unit tests for the clock offset in the experiment time of the image infos files.
+
+start_time is on the real clock, the EXIF times of the images are on the camera clock. The
+experiment time used to be measured from the uncorrected EXIF time, so every experiment time
+was off by exif_time_infront_real_time, and cameras with different offsets stayed apart.
+"""
+import os
+from datetime import datetime, timedelta
+
+import pandas as pd
+
+from ledsa.core.ConfigData import ConfigData
+from ledsa.data_extraction.init_functions import generate_image_infos_csv
+
+CAMERA_AHEAD_OF_REAL_TIME = 30.0
+FIRST_REAL_TIME = datetime(2026, 10, 27, 12, 0, 0)
+IMG_INTERVAL = 10
+NUM_IMGS = 3
+
+
+def create_config(tmp_path, monkeypatch, time_diff, start_time=None) -> ConfigData:
+    monkeypatch.chdir(tmp_path)
+    ConfigData(load_config_file=False, img_directory='imgs/', img_name_string='img{:04d}.jpg',
+               date='2026:10:27', start_time=start_time, time_diff_to_image_time=time_diff,
+               first_img_experiment_id=1, last_img_experiment_id=NUM_IMGS,
+               first_img_analysis_id=1, last_img_analysis_id=NUM_IMGS,
+               ref_img_id=1, max_num_leds=10, num_arrays=1)
+    (tmp_path / 'analysis').mkdir()
+    return ConfigData()
+
+
+def patch_exif_times(monkeypatch, time_diff) -> None:
+    """Let image n be taken IMG_INTERVAL * (n - 1) seconds after FIRST_REAL_TIME on a camera clock running time_diff ahead."""
+    def fake_exif_datetime(path, tag):
+        img_id = int(os.path.basename(path)[3:7])
+        return FIRST_REAL_TIME + timedelta(seconds=time_diff + IMG_INTERVAL * (img_id - 1))
+
+    monkeypatch.setattr('ledsa.core.ConfigData.get_exif_datetime', fake_exif_datetime)
+    monkeypatch.setattr('ledsa.data_extraction.init_functions.get_exif_datetime', fake_exif_datetime)
+
+
+def experiment_times(tmp_path) -> list:
+    return list(pd.read_csv(tmp_path / 'image_infos.csv')['Experiment_Time[s]'])
+
+
+def test_the_first_image_is_at_zero_when_start_time_is_taken_from_it(tmp_path, monkeypatch):
+    config = create_config(tmp_path, monkeypatch, CAMERA_AHEAD_OF_REAL_TIME)
+    patch_exif_times(monkeypatch, CAMERA_AHEAD_OF_REAL_TIME)
+
+    generate_image_infos_csv(config, build_experiment_infos=True)
+
+    assert config['DEFAULT']['start_time'] == '12:00:00'
+    assert experiment_times(tmp_path) == [0.0, 10.0, 20.0]
+
+
+def test_cameras_with_different_offsets_share_the_time_axis(tmp_path, monkeypatch):
+    times = []
+    for time_diff in (CAMERA_AHEAD_OF_REAL_TIME, -5.0):
+        camera_path = tmp_path / f'camera_{time_diff}'
+        camera_path.mkdir()
+        config = create_config(camera_path, monkeypatch, time_diff, start_time='12:00:00')
+        patch_exif_times(monkeypatch, time_diff)
+        generate_image_infos_csv(config, build_experiment_infos=True)
+        times.append(experiment_times(camera_path))
+
+    assert times[0] == times[1] == [0.0, 10.0, 20.0]