動画中の描画領域を特定して，Cropした動画を作成，その後，そのCropした動画を縦長(1080x1920)サイズ中にセンタリングして配置した動画を作成する．

.py

# %%
import cv2
import numpy as np

# ffmpeg -i input.mov -c copy input.mp4
cap = cv2.VideoCapture('input.mp4')
fps = cap.get(cv2.CAP_PROP_FPS)
width = int(cap.get(cv2.CAP_PROP_FRAME_WIDTH))
height = int(cap.get(cv2.CAP_PROP_FRAME_HEIGHT))

fourcc = cv2.VideoWriter_fourcc(*'mp4v')
out = cv2.VideoWriter('output_cropped.mp4', fourcc, fps, (0, 0))  # 後でサイズ更新

padding = 10  # 少し余白を持たせる（ピクセル）

min_x, min_y, max_x, max_y = width, height, 0, 0
frames = []

while True:
    ret, frame = cap.read()
    if not ret:
        break

    # 白背景以外をマスク（RGBすべて255に近いものを除外）
    threshold = 250
    mask = cv2.inRange(frame, (0, 0, 0), (threshold, threshold, threshold))  # 白を黒に

    # 輪郭検出
    contours, _ = cv2.findContours(mask, cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_SIMPLE)

    if contours:
        # すべての輪郭を囲む最小矩形
        x, y, w, h = cv2.boundingRect(np.concatenate(contours))
        min_x = min(min_x, x)
        min_y = min(min_y, y)
        max_x = max(max_x, x + w)
        max_y = max(max_y, y + h)

    frames.append(frame)

cap.release()

# 全体の最小矩形 + padding
crop_x = max(0, min_x - padding)
crop_y = max(0, min_y - padding)
crop_w = min(width - crop_x, max_x - min_x + 2*padding)
crop_h = min(height - crop_y, max_y - min_y + 2*padding)

# 再エンコード
out = cv2.VideoWriter('output_cropped.mp4', fourcc, fps, (crop_w, crop_h))

for frame in frames:
    cropped = frame[crop_y:crop_y+crop_h, crop_x:crop_x+crop_w]
    out.write(cropped)

out.release()
print(f"Cropped to: {crop_w}x{crop_h} at offset ({crop_x}, {crop_y})")

# %%
# FFmpeg一発版（参考）
# ffmpeg -i output_cropped.mp4 -vf "scale=1080:-1,pad=1080:1920:(ow-iw)/2:(oh-ih)/2:color=black" -c:v libx264 -crf 18 output_shorts.mp4
# ffmpeg -i output_cropped.mp4 -vf "scale=1080:-1,pad=1080:1920:(ow-iw)/2:(oh-ih)/2:color=white" -c:v libx264 -crf 18 output_shorts.mp4
# ====================== Shorts用に上下余白追加 ======================
target_width = 1080
target_height = 1920

cap = cv2.VideoCapture('output_cropped.mp4')
fps = cap.get(cv2.CAP_PROP_FPS)
fourcc = cv2.VideoWriter_fourcc(*'mp4v')

out_shorts = cv2.VideoWriter('output_shorts_py.mp4', fourcc, fps, (target_width, target_height))

while True:
    ret, frame = cap.read()
    if not ret:
        break

    h, w = frame.shape[:2]  # 現在のフレームサイズ（1720x1288など）

    # 横幅を1080にスケール（アスペクト比を維持）
    scale = target_width / w
    new_h = int(h * scale)

    resized = cv2.resize(frame, (target_width, new_h))

    # 黒背景（上下余白）のキャンバス作成
    # canvas = np.zeros((target_height, target_width, 3), dtype=np.uint8)   # 黒背景

    # 白背景にしたい場合は → np.full((target_height, target_width, 3), 255, dtype=np.uint8)
    canvas = np.full((target_height, target_width, 3), 255, dtype=np.uint8)

    # 中央に配置
    y_offset = (target_height - new_h) // 2
    canvas[y_offset:y_offset+new_h, 0:target_width] = resized

    out_shorts.write(canvas)

cap.release()
out_shorts.release()
print("縦長動画を作成しました: output_shorts_py.mp4")

# %%