How to add a slit-scan with Claude

Add a slit-scan (time displacement) effect to input.mp4. Every output row comes from an earlier frame than the row above it: row y (0 at the top, h-1 at the bottom) of output frame i is row y of input frame i - round(L × y / (h - 1)), where L is the lag in frames, clamped so it never reaches before frame 0. L = 2.5 seconds × 30 fps × s(t), where s(t) is 0 before 1.0 seconds and after 5.5 seconds, eases up to 1 between 1.0 and 2.0 seconds and back down to 0 between 4.5 and 5.5 seconds, with the smoothstep curve u²(3 − 2u) for both ramps. So the top row is always the current frame, the bottom row is up to 2.5 seconds behind, and before 1.0 seconds and after 5.5 seconds the video is unchanged. Use Python with numpy and ffmpeg, keep the size (540x960), 30 fps and the audio, and save it as output.mp4.

#!/usr/bin/env python3
"""Slit-scan: each row of the picture shows a slightly earlier moment than the row above, so anything moving bends and leans.

Usage: python3 slit-scan.py input.mp4 output.mp4
Needs ffmpeg and numpy (pip install numpy).
"""
import json
import subprocess
import sys

import numpy as np

START = 1.0    # second the bend starts
END = 5.5      # second it is fully gone again
RAMP = 1.0     # seconds it takes to ease in and to ease out
DELAY = 2.5    # seconds the bottom row lags behind the top row at full strength


def strength(t):
    """0 outside START-END, easing up to 1 over RAMP seconds and back down before END."""
    u = min(max(min(t - START, END - t) / RAMP, 0), 1)
    return u * u * (3 - 2 * u)


def slit_scan(history, lag):
    """Build a frame row by row: row y comes from the frame lag * y / (h - 1) frames back."""
    back = np.round(lag * np.arange(h) / (h - 1)).astype(int)
    back = np.minimum(back, len(history) - 1)
    frame = np.empty_like(history[-1])
    for k in np.unique(back):
        rows = back == k
        frame[rows] = history[-1 - k][rows]
    return frame


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)
keep = round(DELAY * fps) + 1  # frames of history the bottom row can reach back to
history, i = [], 0
while raw := reader.stdout.read(w * h * 3):
    history = (history + [np.frombuffer(raw, np.uint8).reshape(h, w, 3)])[-keep:]
    lag = DELAY * fps * strength(i / fps)
    writer.stdin.write((slit_scan(history, lag) if lag > 0 else history[-1]).tobytes())
    i += 1
writer.stdin.close()
if reader.wait() or writer.wait():
    sys.exit("ffmpeg failed")