How to add a zoom blur with Claude

In input.mp4, add a radial zoom blur centered on the middle of the frame (x 50%, y 50%) from 1.0 to 3.9 seconds; before 1.0s and after 3.9s the video must be untouched. Make each blurred frame with Python, OpenCV and numpy by averaging 20 copies of the frame scaled up around the center by factors spread evenly from 1.0 to 1 + 0.25 x amount (cv2.warpAffine, edges reflected), so the center stays sharp and the edges streak outward. The amount rises from 0 to 1 between 1.0s and 1.4s with a smoothstep curve, stays at 1 until 3.5s and falls back to 0 by 3.9s with the same curve. Pipe frames in and out with ffmpeg. Keep the size (540x960), 30fps, 6 seconds and the audio. Encode H.264 yuv420p and save it as output.mp4. There is no text to draw; if you add any, draw it with Pillow, not ffmpeg's drawtext.

#!/usr/bin/env python3
"""Zoom blur: from 1s to 3.5s streaks burst outward from the center of the frame, as if the camera rushes forward.

Usage: python3 zoom-blur.py input.mp4 output.mp4
Needs ffmpeg, OpenCV and numpy (pip install opencv-python-headless numpy).
Each frame is averaged with copies of itself scaled up around the center, so the middle stays sharp
and the edges smear along lines pointing out from it.
"""
import json
import subprocess
import sys

import cv2
import numpy as np

START = 1.0      # second the streaks start (before it the video is untouched)
END = 3.5        # second they start fading back out
RAMP = 0.4       # seconds to build up and to fade out
STRENGTH = 0.25  # how far the streaks reach: the largest copy is 25% bigger than the frame
CENTER = (0.5, 0.5)  # point the streaks burst out from (0-1 across and down the frame)
SAMPLES = 20     # copies averaged per frame; more gives smoother streaks


def amount(t):
    """0 before START, rising to 1 over RAMP, held until END, back to 0 over RAMP (smoothstep at both ends)."""
    p = min(max((t - START) / RAMP, 0), 1) * min(max((END + RAMP - t) / RAMP, 0), 1)
    return p * p * (3 - 2 * p)


def zoom_blur(img, strength):
    cx, cy = CENTER[0] * w, CENTER[1] * h
    acc = np.zeros((h, w, 3), np.float32)
    for k in range(SAMPLES):
        s = 1 + strength * k / (SAMPLES - 1)
        m = np.float32([[s, 0, cx * (1 - s)], [0, s, cy * (1 - s)]])
        acc += cv2.warpAffine(img, m, (w, h), flags=cv2.INTER_LINEAR, borderMode=cv2.BORDER_REFLECT)
    return (acc / SAMPLES).astype(np.uint8)


def probe(path):
    info = json.loads(subprocess.check_output(["ffprobe", "-v", "error", "-select_streams", "v:0", "-show_entries",
                                               "stream=width,height,r_frame_rate", "-of", "json", path]))
    s = info["streams"][0]
    num, den = s["r_frame_rate"].split("/")
    return s["width"], s["height"], s["r_frame_rate"], float(num) / float(den)


src, out = sys.argv[1:3]
w, h, rate, fps = probe(src)
reader = subprocess.Popen(["ffmpeg", "-v", "error", "-i", src, "-f", "rawvideo", "-pix_fmt", "rgb24", "-"],
                          stdout=subprocess.PIPE)
writer = subprocess.Popen(["ffmpeg", "-y", "-v", "error", "-f", "rawvideo", "-pix_fmt", "rgb24", "-s", f"{w}x{h}",
                           "-r", rate, "-i", "-", "-i", src, "-map", "0:v", "-map", "1:a?", "-c:v", "libx264",
                           "-crf", "18", "-pix_fmt", "yuv420p", "-c:a", "copy", "-shortest", out],
                          stdin=subprocess.PIPE)
i = 0
while True:
    buf = reader.stdout.read(w * h * 3)
    if len(buf) < w * h * 3:
        break
    img = np.frombuffer(buf, np.uint8).reshape(h, w, 3)
    a = amount(i / fps)
    writer.stdin.write((zoom_blur(img, STRENGTH * a) if a > 0 else img).tobytes())
    i += 1
writer.stdin.close()
if reader.wait() or writer.wait():
    sys.exit("ffmpeg failed")