[Python] Unreal Python script for adding pre-roll

Im trying to create a python script that adds pre-roll to my tracks in Unreal 5.4.

Currently it correctly sets the CameraCuts, but fails to correctly update Subsequences and CameraComponents > Spawned + Transform to Pre Roll Frames 0100, instead setting it to 0000* which is odd. I have tried saving and re-opening to see if its a visual bug but it doesn't seem to update and I really need this to reflect the change.

here is my script

Any help is greatly appreciated!

import unreal

# ----------------------------- CONFIG ---------------------------------
SUBSEQ_PREROLL_FRAMES = 100
CAMERA_PREROLL_FRAMES = 100
CAMERA_CUT_HEAD_EXTEND_FRAMES = SUBSEQ_PREROLL_FRAMES - 12 # 88 frames
# ----------------------------------------------------------------------

# ----------------------------- Helpers ---------------------------------
def _get_or_open_target_sequence():
seq = unreal.LevelSequenceEditorBlueprintLibrary.get_current_level_sequence()
if seq:
return seq
# Fallback: exactly one selected LevelSequence asset
try:
selected = unreal.EditorUtilityLibrary.get_selected_assets()
except Exception:
selected = []
level_sequences = [a for a in selected if isinstance(a, unreal.LevelSequence)]
if len(level_sequences) == 1:
seq = level_sequences[0]
try:
unreal.LevelSequenceEditorBlueprintLibrary.open_level_sequence(seq)
except Exception:
pass
return seq
return None

def _get_sections(track):
try:
return list(track.get_sections())
except Exception:
return []

def _get_section_start_end(section):
try:
start = int(section.get_start_frame())
end = int(section.get_end_frame())
return start, end
except Exception:
pass
try:
rng = section.get_range()
start = int(rng.lower_bound.value)
end = int(rng.upper_bound.value)
return start, end
except Exception:
return None, None

def _set_section_start_end(section, new_start, end):
try:
section.set_start_frame(int(new_start))
section.set_end_frame(int(end))
return True
except Exception:
pass
try:
section.set_range(int(new_start), int(end))
return True
except Exception:
return False

# ----------------------------- Debugging ---------------------------------
def debug_tracks(seq):
unreal.log(f"=== Debug Tracks for sequence '{seq.get_name()}' ===")
try:
tracks = unreal.MovieSceneSequenceExtensions.get_tracks(seq)
except Exception:
tracks = []
unreal.log(f"Master tracks found: {len(tracks)}")
for t in tracks:
try:
sections = t.get_sections()
unreal.log(f"Track: {t.get_display_name()} ({t.get_class().get_name()}), sections: {len(sections)}")
for sec in sections:
start, end = _get_section_start_end(sec)
pre_roll = sec.get_pre_roll_frames() if hasattr(sec, "get_pre_roll_frames") else "N/A"
unreal.log(f" Section: {sec.get_name()}, start={start}, end={end}, pre_roll={pre_roll}")
except Exception as e:
unreal.log_warning(f"Error reading track: {str(e)}")

# ----------------------------- Main logic ---------------------------------
def _process_subsequence_tracks(seq):
modified = 0
try:
tracks = unreal.MovieSceneSequenceExtensions.get_tracks(seq)
except Exception:
tracks = []
for t in tracks:
if isinstance(t, unreal.MovieSceneSubTrack):
for sec in _get_sections(t):
start, end = _get_section_start_end(sec)
if start is not None and end is not None:
try:
sec.set_pre_roll_frames(SUBSEQ_PREROLL_FRAMES)
modified += 1
unreal.log(f"[PreRoll] Subsequence section '{sec.get_name()}' set to {SUBSEQ_PREROLL_FRAMES}")
except Exception as e:
unreal.log_warning(f"[PreRoll] Failed to set subsequence section '{sec.get_name()}': {e}")
else:
unreal.log_warning(f"[PreRoll] Skipping subsequence section '{sec.get_name()}' (invalid start/end)")
return modified

def _process_camera_tracks(seq):
modified = 0
try:
bindings = unreal.MovieSceneSequenceExtensions.get_bindings(seq)
except Exception:
bindings = []
for b in bindings:
try:
tracks = unreal.MovieSceneBindingExtensions.get_tracks(b)
except Exception:
tracks = []
for tr in tracks:
sections = _get_sections(tr)
track_name = tr.get_display_name() if hasattr(tr, "get_display_name") else "UnnamedTrack"
track_class = tr.get_class().get_name() if hasattr(tr, "get_class") else "UnknownClass"
for sec in sections:
try:
sec_name = sec.get_name()
start, end = _get_section_start_end(sec)
pre_roll_before = sec.get_pre_roll_frames() if hasattr(sec, "get_pre_roll_frames") else "N/A"
unreal.log(f"[DEBUG] Track '{track_name}' ({track_class}) Section '{sec_name}', start={start}, end={end}, pre_roll={pre_roll_before}")

# Determine if we should set pre-roll
valid_types_for_preroll = (unreal.MovieSceneSpawnTrack, unreal.MovieScene3DTransformTrack)
if isinstance(tr, valid_types_for_preroll):
sec.set_pre_roll_frames(CAMERA_PREROLL_FRAMES)
modified += 1
unreal.log(f"[PreRoll] Camera section '{sec_name}' set to {CAMERA_PREROLL_FRAMES}")
else:
# Only skip tracks that are not supported (e.g., Material, Visibility, Float)
if start is None or end is None:
unreal.log_warning(f"[PreRoll] Skipping unsupported section '{sec_name}'")
except Exception as e:
unreal.log_warning(f"[PreRoll] Failed to process camera section '{sec_name}': {e}")
return modified

def _process_camera_cuts(seq):
modified = 0
try:
tracks = unreal.MovieSceneSequenceExtensions.get_tracks(seq)
except Exception:
tracks = []
for t in tracks:
if not isinstance(t, unreal.MovieSceneCameraCutTrack):
continue
sections = _get_sections(t)
if not sections:
continue
sec = sections[0] # only first CameraCut
try:
start, end = _get_section_start_end(sec)
if start is None or end is None:
continue
current_marker = sec.get_pre_roll_frames() or 0
if current_marker >= CAMERA_CUT_HEAD_EXTEND_FRAMES:
continue
delta = CAMERA_CUT_HEAD_EXTEND_FRAMES - current_marker
new_start = start - int(delta)
if _set_section_start_end(sec, new_start, end):
sec.set_pre_roll_frames(CAMERA_CUT_HEAD_EXTEND_FRAMES)
modified += 1
unreal.log(f"[PreRoll] CameraCut '{sec.get_name()}' start moved from {start} to {new_start}")
except Exception as e:
unreal.log_warning(f"[PreRoll] Failed to process CameraCut '{sec.get_name()}': {e}")
return modified

def add_preroll_to_sequence_safely():
seq = _get_or_open_target_sequence()
if not seq:
unreal.log_error("[PreRoll] No Level Sequence active or selected. Open one or select exactly one LevelSequence asset.")
return False

unreal.log("=== DEBUG BEFORE MODIFICATION ===")
debug_tracks(seq)

total_modified = 0
with unreal.ScopedEditorTransaction("Add Sequencer Pre-roll (All Sections)"):
total_modified += _process_subsequence_tracks(seq)
total_modified += _process_camera_tracks(seq)
total_modified += _process_camera_cuts(seq)

try:
unreal.EditorAssetLibrary.save_asset(seq.get_path_name())
except Exception:
pass

if total_modified > 0:
unreal.log(f"[PreRoll] Applied pre-roll updates to '{seq.get_name()}'. Sections modified: {total_modified}")
else:
unreal.log(f"[PreRoll] '{seq.get_name()}' already compliant. No changes applied.")

unreal.log("=== DEBUG AFTER MODIFICATION ===")
debug_tracks(seq)
return True

# ----------------------------- RUN ------------------------------------
add_preroll_to_sequence_safely()

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