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# SPDX-FileCopyrightText: Copyright (c) 2025 NVIDIA CORPORATION & AFFILIATES. All rights reserved.
# SPDX-License-Identifier: Apache-2.0
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
# http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
import os
import argparse
import subprocess
import tempfile
import shutil
from fractions import Fraction
def ffprobe_fps(path):
"""Return float FPS for the first video stream, preferring avg_frame_rate."""
def probe(field):
cmd = [
"ffprobe", "-v", "error",
"-select_streams", "v:0",
"-show_entries", f"stream={field}",
"-of", "default=nw=1:nk=1",
path
]
out = subprocess.check_output(cmd, text=True).strip()
return out
val = probe("avg_frame_rate")
if val in ("0/0", "", "N/A"):
val = probe("r_frame_rate")
try:
return float(Fraction(val))
except Exception:
return 30.0 # fallback default FPS
def escape_concat_path(p):
"""Quote a path for ffmpeg concat list file."""
return "'" + p.replace("'", r"'\''") + "'"
def subsample_one(input_path, stride, temp_path, target_fps):
"""
Keep every Nth frame, normalize timing, and set CFR to target_fps.
Audio is dropped to avoid desync; can be adapted to speed audio if needed.
"""
vf = f"select='not(mod(n\\,{stride}))',setpts=N/FRAME_RATE/TB,fps=fps={target_fps}"
cmd = [
"ffmpeg", "-y",
"-i", input_path,
"-vf", vf,
"-an", # remove audio for sync safety
"-pix_fmt", "yuv420p",
"-c:v", "libx264", "-crf", "18", "-preset", "fast",
"-movflags", "+faststart",
temp_path
]
subprocess.run(cmd, check=True)
def process_and_concat(input_dirs, strides, output_dir):
if len(strides) != len(input_dirs):
raise ValueError("Number of strides must match number of input directories.")
os.makedirs(output_dir, exist_ok=True)
# Get common MP4 filenames across all dirs
def mp4s(d):
return set(f for f in os.listdir(d)
if os.path.isfile(os.path.join(d, f)) and f.lower().endswith(".mp4"))
common_files = set.intersection(*(mp4s(d) for d in input_dirs))
if not common_files:
print("No common .mp4 filenames across the provided directories.")
return
temp_root = tempfile.mkdtemp(prefix="stride_concat_")
try:
for filename in sorted(common_files):
# Get FPS from the first directory’s file
ref_input = os.path.join(input_dirs[0], filename)
fps = ffprobe_fps(ref_input)
processed = []
for i, (d, stride) in enumerate(zip(input_dirs, strides)):
inp = os.path.join(d, filename)
seg = os.path.join(
temp_root,
f"{i:02d}__{os.path.basename(os.path.normpath(d))}__{filename}"
)
print(f"[{filename}] {d} stride={stride} → {seg}")
subsample_one(inp, stride, seg, fps)
processed.append(seg)
# Create concat list file
list_path = os.path.join(temp_root, f"{filename}.txt")
with open(list_path, "w", encoding="utf-8") as f:
for p in processed:
f.write(f"file {escape_concat_path(os.path.abspath(p))}\n")
# Final concat output
out_path = os.path.join(output_dir, filename)
print(f"[{filename}] concatenating {len(processed)} segments → {out_path}")
concat_cmd = [
"ffmpeg", "-y",
"-f", "concat", "-safe", "0",
"-i", list_path,
"-c", "copy",
out_path
]
subprocess.run(concat_cmd, check=True)
finally:
shutil.rmtree(temp_root, ignore_errors=True)
def parse_args():
p = argparse.ArgumentParser(
description="Temporally concat matching MP4s from multiple directories, "
"after per-directory frame subsampling (stride)."
)
p.add_argument("input_dirs", nargs="+",
help="Input directories (order = concat order). Each must contain matching MP4 filenames.")
p.add_argument("-s", "--strides", nargs="+", type=int, required=True,
help="Stride per directory (e.g. -s 2 3 5). Must match number of input dirs.")
p.add_argument("-o", "--output_dir", required=True,
help="Directory to write concatenated outputs (filenames preserved).")
return p.parse_args()
def main():
args = parse_args()
process_and_concat(args.input_dirs, args.strides, args.output_dir)
if __name__ == "__main__":
main()
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