How to add a pixel stretch with Claude

Add a pixel stretch effect to input.mp4 using Python with numpy, rembg (session "u2net") and ffmpeg, frame by frame at 30fps. From 1.0 to 5.0 seconds, cut the product out of each frame with rembg, find the middle of its cut-out (the average x of the matte, weighted by its values) and copy that one column of pixels over every pixel to its right, so everything right of the product's middle becomes flat horizontal streaks of that column. The streaks shoot out from the column to the right edge over 1.0 to 1.5 seconds (ease out cubic: reach = 1 - (1 - p)^3) and pull back to the column over 4.5 to 5.0 seconds the same way. Leave frames before 1.0 and from 5.0 seconds on untouched, and leave everything left of the column untouched. There is no text to draw (if you add any, draw it with Pillow, not ffmpeg's drawtext). Keep the audio and save it as output.mp4 (H.264, yuv420p).

#!/usr/bin/env python3
"""Pixel stretch: the one column of pixels running down the product's middle is dragged out to the right edge in long streaks.

Usage: python3 pixel-stretch.py input.mp4 output.mp4
Needs ffmpeg, numpy, Pillow and rembg to find the product in every frame
(pip install numpy pillow "rembg[cpu]"; rembg downloads its u2net model on first use).
"""
import json
import subprocess
import sys

import numpy as np
from PIL import Image
from rembg import new_session

START, END = 1.0, 5.0    # seconds the stretch is on screen
GROW = 0.5               # seconds the streaks take to shoot out to the edge, and to pull back at the end
OFFSET = 0.0             # where the stretched column sits, as a share of the product's width right of its middle
FPS = 30                 # output frame rate


def reach(t):
    """How far the streaks reach, 0 (none) to 1 (the frame's right edge), easing out and back in."""
    if not START <= t < END:
        return 0.0
    p = min(t - START, END - t, GROW) / GROW
    return 1 - (1 - p) ** 3


def stretch_column(rgb):
    """The x of the column to stretch: the middle of the product's cut-out (rembg), weighted by its matte."""
    matte = np.asarray(session.predict(Image.fromarray(rgb))[0], np.float32)
    weight = matte.sum(axis=0)
    xs = np.nonzero(weight > weight.max() * 0.05)[0]
    middle = (weight * np.arange(w)).sum() / weight.sum()
    return round(middle + OFFSET * (xs[-1] - xs[0]))


def pixel_stretch(rgb, t):
    """Copy the product's middle column over every pixel from it to the streaks' front."""
    x = stretch_column(rgb)
    front = x + 1 + round((w - x - 1) * reach(t))
    img = rgb.copy()
    img[:, x:front] = rgb[:, x:x + 1]
    return img


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


def render(src, out):
    """Read src frame by frame at FPS, stretch the frames inside START-END and save to out, keeping the audio."""
    reader = subprocess.Popen(["ffmpeg", "-v", "error", "-i", src, "-vf", f"fps={FPS}",
                               "-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", str(FPS), "-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 len(raw := reader.stdout.read(w * h * 3)) == w * h * 3:
        rgb = np.frombuffer(raw, np.uint8).reshape(h, w, 3)
        t = i / FPS
        writer.stdin.write((pixel_stretch(rgb, t) if reach(t) > 0 else rgb).tobytes())
        i += 1
    writer.stdin.close()
    if reader.wait() or writer.wait():
        sys.exit("ffmpeg failed")


src, out = sys.argv[1:3]
w, h = probe(src)
session = new_session("u2net")
render(src, out)