How to add a luma fade with Claude

Join clipA.mp4 and clipB.mp4 with a 1.0 second luma fade that starts 1.0 seconds before A ends, so the result is 11 seconds. On each transition frame, take A's brightness (grayscale), blur it with a 6 pixel Gaussian blur, and stretch it so A's darkest 2% is 0 and brightest 2% is 1. With progress p going from 0 to 1 across the 30 frames, set threshold = 1 - p * 1.25 and show B where the mask clip((brightness - threshold) / 0.25, 0, 1) is 1, A where it is 0, and a mix in between, so B appears through A's brightest areas first and A's darkest areas go last. Before the transition show only A, after it only B. Fit B to A's size (scale to cover, center crop) at 30fps. Do the per-pixel mixing in Python with numpy and Pillow, piping frames to and from ffmpeg. Crossfade the audio over the same 1.0 second. Encode H.264 yuv420p and save it as output.mp4.

#!/usr/bin/env python3
"""Luma fade: clip B shows through the brightest parts of clip A first, then spreads down into the shadows.

Usage: python3 luma-fade.py clipA.mp4 clipB.mp4 output.mp4  (both clips need an audio track)
Needs ffmpeg, Pillow and numpy (pip install pillow numpy).
"""
import json
import subprocess
import sys

import numpy as np
from PIL import Image, ImageFilter

D = 1.0          # transition length in seconds
SOFT = 0.25      # width of the soft edge between the clips, as a share of the brightness range
BLUR = 6         # blur radius (pixels) on the brightness map, so B spreads in smooth shapes, not specks
FPS = 30


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


def frames(path):
    """Every frame of a clip, fitted to A's size at 30fps, as an RGB array."""
    fit = f"scale={w}:{h}:force_original_aspect_ratio=increase,crop={w}:{h},fps={FPS}"
    ff = subprocess.Popen(["ffmpeg", "-v", "error", "-i", path, "-vf", fit, "-f", "rawvideo", "-pix_fmt", "rgb24", "-"],
                          stdout=subprocess.PIPE)
    while chunk := ff.stdout.read(w * h * 3):
        yield np.frombuffer(chunk, np.uint8).reshape(h, w, 3)


def brightness(img):
    """A's brightness, blurred and stretched so its darkest 2% is 0 and brightest 2% is 1."""
    lum = np.asarray(Image.fromarray(img).convert("L").filter(ImageFilter.GaussianBlur(BLUR)), np.float32)
    lo, hi = np.percentile(lum, [2, 98])
    return np.clip((lum - lo) / max(hi - lo, 1), 0, 1)


def luma_fade(a, b, p):
    """Mix of A and B at progress p (0-1): a pixel turns to B once the falling threshold passes its brightness."""
    threshold = 1 - p * (1 + SOFT)
    mask = np.clip((brightness(a) - threshold) / SOFT, 0, 1)[..., None]
    return (a * (1 - mask) + b * mask).astype(np.uint8)


a_path, b_path, out = sys.argv[1:4]
w, h, a_len = probe(a_path)
start = round((a_len - D) * FPS)    # first frame of the transition
ff = subprocess.Popen(["ffmpeg", "-y", "-v", "error", "-f", "rawvideo", "-pix_fmt", "rgb24", "-s", f"{w}x{h}",
                       "-r", str(FPS), "-i", "-", "-i", a_path, "-i", b_path,
                       "-filter_complex", f"[1:a][2:a]acrossfade=d={D}[au]", "-map", "0:v", "-map", "[au]",
                       "-c:v", "libx264", "-crf", "18", "-pix_fmt", "yuv420p", "-c:a", "aac", out], stdin=subprocess.PIPE)
b_frames = frames(b_path)
for i, a in enumerate(frames(a_path)):
    if i >= start:
        a = luma_fade(a, next(b_frames), (i - start + 1) / (round(D * FPS) + 1))
    ff.stdin.write(a.tobytes())
for b in b_frames:
    ff.stdin.write(b.tobytes())
ff.stdin.close()
if ff.wait():
    sys.exit("ffmpeg failed")