How to Resize a Video With FFmpeg (Keep the Aspect Ratio, Round to Even, Pad or Crop, and Test It With pytest)

ffmpeg -i in.mp4 -vf "scale=-2:720" -c:a copy out_720.mp4 

ffmpeg -i in.mp4 -vf "scale=-1:720" out.mp4 # [libx264] width not divisible by 2 (405x720) # Error while opening encoder - maybe incorrect parameters such as bit_rate, rate, width or height. # Nothing was written into output file, because at least one of its streams received no packets. 

ffmpeg -i portrait_540x960.mp4 -vf "scale=1280:720" stretched.mp4 

ffmpeg -i in.mp4 -vf "scale=-2:720" -c:v copy out.mp4 # Filtergraph 'scale=-2:720' was specified, but codec copy was selected. # Filtering and streamcopy cannot be used together. 

ffmpeg -i in.mp4 -vf "scale=1280:720:force_original_aspect_ratio=decrease:force_divisible_by=2:flags=lanczos,pad=1280:720:(ow-iw)/2:(oh-ih)/2:color=black,setsar=1" -c:v libx264 -crf 18 -c:a copy fit.mp4 

ffmpeg -i in.mp4 -vf "scale=1280:720:force_original_aspect_ratio=increase:flags=lanczos,crop=1280:720,setsar=1" -c:v libx264 -crf 18 -c:a copy fill.mp4 

"""Resize a video with FFmpeg without distorting it. Three modes: height - scale to a target height, width follows the aspect ratio (rounded to even) fit - fit inside WxH and pad the rest with black bars (letterbox / pillarbox) fill - cover WxH and crop the overflow (no bars, edges are lost) Audio is stream-copied, so it is bit-identical to the source. """ import json import subprocess import sys def probe(path): out = subprocess.run( ["ffprobe", "-v", "error", "-show_streams", "-show_format", "-of", "json", path], check=True, capture_output=True, text=True, ).stdout data = json.loads(out) video = next(s for s in data["streams"] if s["codec_type"] == "video") audio = [s for s in data["streams"] if s["codec_type"] == "audio"] return { "width": int(video["width"]), "height": int(video["height"]), "sar": video.get("sample_aspect_ratio", "N/A"), "duration": float(data["format"]["duration"]), "audio_codec": audio[0]["codec_name"] if audio else None, } def build_filter(mode, width=None, height=None): if mode == "height": return f"scale=-2:{height}:flags=lanczos,setsar=1" if mode == "fit": return ( f"scale={width}:{height}:force_original_aspect_ratio=decrease:force_divisible_by=2:flags=lanczos," f"pad={width}:{height}:(ow-iw)/2:(oh-ih)/2:color=black,setsar=1" ) if mode == "fill": return ( f"scale={width}:{height}:force_original_aspect_ratio=increase:flags=lanczos," f"crop={width}:{height},setsar=1" ) raise ValueError(f"unknown mode {mode!r}") def resize(src, dst, mode, width=None, height=None, crf=18): cmd = [ "ffmpeg", "-v", "error", "-y", "-i", src, "-vf", build_filter(mode, width, height), "-c:v", "libx264", "-crf", str(crf), "-preset", "medium", "-pix_fmt", "yuv420p", "-c:a", "copy", "-movflags", "+faststart", dst, ] subprocess.run(cmd, check=True, capture_output=True, text=True) return probe(dst) if __name__ == "__main__": src, dst, mode = sys.argv[1:4] dims = [int(x) for x in sys.argv[4:6]] if mode == "height": print(resize(src, dst, mode, height=dims[0])) else: print(resize(src, dst, mode, width=dims[0], height=dims[1])) 

import subprocess from pathlib import Path import pytest from resize import build_filter, probe, resize SRC = str(Path(__file__).parent / "fixtures" / "duet-9x16.mp4") # real Alors on Danse AI example, 540x960 def gray_column(path, x, t=5.0): """Return the gray values of pixel column x from the frame at t seconds.""" info = probe(path) raw = subprocess.run( ["ffmpeg", "-v", "error", "-ss", str(t), "-i", path, "-frames:v", "1", "-f", "rawvideo", "-pix_fmt", "gray", "-"], check=True, capture_output=True, ).stdout w, h = info["width"], info["height"] return [raw[row * w + x] for row in range(h)] def audio_md5(path): return subprocess.run( ["ffmpeg", "-v", "error", "-i", path, "-map", "0:a:0", "-c", "copy", "-f", "md5", "-"], check=True, capture_output=True, text=True, ).stdout.strip() def test_height_mode_rounds_width_to_even(tmp_path): out = resize(SRC, str(tmp_path / "h720.mp4"), "height", height=720) assert (out["width"], out["height"]) == (406, 720) # 540*720/960 = 405 -> 406 assert abs(out["width"] / out["height"] - 540 / 960) / (540 / 960) < 0.005 def test_minus_one_produces_odd_width_and_x264_refuses(tmp_path): r = subprocess.run( ["ffmpeg", "-v", "error", "-y", "-i", SRC, "-vf", "scale=-1:720", str(tmp_path / "odd.mp4")], capture_output=True, text=True, ) assert r.returncode != 0 assert "not divisible by 2" in r.stderr def test_fit_mode_pillarboxes_with_black_bars(tmp_path): path = str(tmp_path / "fit.mp4") out = resize(SRC, path, "fit", width=1280, height=720) assert (out["width"], out["height"], out["sar"]) == (1280, 720, "1:1") left_bar = gray_column(path, 10) centre = gray_column(path, 640) assert max(left_bar) <= 20 # pure black bar assert sum(centre) / len(centre) > 60 # picture, not bar def test_audio_is_bit_identical(tmp_path): path = str(tmp_path / "fit.mp4") out = resize(SRC, path, "fit", width=1280, height=720) assert out["audio_codec"] == "aac" assert abs(out["duration"] - probe(SRC)["duration"]) < 0.05 assert audio_md5(path) == audio_md5(SRC) def test_filter_and_stream_copy_cannot_mix(tmp_path): r = subprocess.run( ["ffmpeg", "-v", "error", "-y", "-i", SRC, "-vf", build_filter("height", height=720), "-c:v", "copy", str(tmp_path / "copy.mp4")], capture_output=True, text=True, ) assert r.returncode != 0 assert "Filtering and streamcopy cannot be used together" in r.stderr 

uvx --with pytest pytest -q test_resize.py # ..... [100%] # 5 passed in 1.15s