How to add a diagonal split screen with Claude

Make a diagonal split screen from top.mp4 and bottom.mp4, the same size as top.mp4 (540x960), 30fps, 6 seconds. A straight line runs from the left edge at 64% of the height (y=614) to the right edge at 36% of the height (y=346). Above the line show top.mp4 moved up by 18% of the height (173 px), so the frame shows its rows 173 to 960; below the line show bottom.mp4 (fitted to the same size, cropped from the center) moved down by 173 px. Draw an 8-pixel pure white (255,255,255) line along the dividing edge, anti-aliased, with nothing else white on screen. Draw the mask and line as images with Pillow (not ffmpeg's drawtext or geq) and composite them with ffmpeg. Keep top.mp4's audio, encode H.264 yuv420p and save it as output.mp4.

#!/usr/bin/env python3
"""Diagonal split screen: two clips share the frame, divided by a slanted white line that drops from right to left.

Usage: python3 diagonal-split-screen.py top.mp4 bottom.mp4 output.mp4
Needs ffmpeg and Pillow (pip install pillow). The output keeps top.mp4's size and sound.
"""
import json
import subprocess
import sys
import tempfile
from pathlib import Path

from PIL import Image, ImageDraw

LEFT_Y, RIGHT_Y = 0.64, 0.36   # where the dividing line meets the left and right edges (0-1 down the frame)
SHIFT = 0.18                   # each clip slides this share of the height away from the line, so both subjects stay in view
DIVIDER = 8                    # white line width in pixels (0 for none)
FPS = 30
SS = 4                         # supersampling for smooth edges on the mask and line


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]))
    s = info["streams"][0]
    return s["width"], s["height"], float(info["format"]["duration"])


def line():
    """The dividing line's two ends, at supersampled size."""
    return [(0, LEFT_Y * h * SS), (w * SS, RIGHT_Y * h * SS)]


def mask():
    """White where the bottom clip shows: everything below the line."""
    img = Image.new("L", (w * SS, h * SS), 0)
    (x0, y0), (x1, y1) = line()
    ImageDraw.Draw(img).polygon([(x0, y0), (x1, y1), (x1, h * SS), (x0, h * SS)], fill=255)
    return img.resize((w, h), Image.LANCZOS)


def divider():
    img = Image.new("RGBA", (w * SS, h * SS))
    if DIVIDER:
        ImageDraw.Draw(img).line(line(), fill="white", width=DIVIDER * SS)
    return img.resize((w, h), Image.LANCZOS)


top, bottom, out = sys.argv[1:4]
w, h, duration = probe(top)
s = round(h * SHIFT)
with tempfile.TemporaryDirectory() as tmp:
    mask().save(f"{tmp}/mask.png")
    divider().save(f"{tmp}/divider.png")
    fit = f"scale={w}:{h}:force_original_aspect_ratio=increase,crop={w}:{h},fps={FPS},setsar=1"
    subprocess.run(["ffmpeg", "-y", "-v", "error", "-i", top, "-i", bottom,
                    "-loop", "1", "-t", str(duration), "-i", f"{tmp}/mask.png",
                    "-loop", "1", "-t", str(duration), "-i", f"{tmp}/divider.png",
                    "-filter_complex",
                    # top clip moves up and bottom clip moves down by the shift, then the mask cuts the bottom one
                    f"[0:v]{fit},crop={w}:{h - s}:0:{s},pad={w}:{h}:0:0[t];"
                    f"[1:v]{fit},crop={w}:{h - s}:0:0,pad={w}:{h}:0:{s}[b];"
                    f"[2:v]fps={FPS},format=gray[m];[b][m]alphamerge[bm];"
                    f"[3:v]fps={FPS},format=rgba[d];"
                    f"[t][bm]overlay[tb];[tb][d]overlay,format=yuv420p[v]",
                    "-map", "[v]", "-map", "0:a?", "-t", str(duration),
                    "-c:v", "libx264", "-crf", "18", "-c:a", "copy", out], check=True)