--- title: "How to add an animated underline with Claude (tested prompt and script)" description: "A thick yellow line draws itself left to right under \"cleared\" the moment the voiceover says it, and stays under the caption. Install one skill and ask Claude Code, or run the Python script we tested yourself." url: https://www.recharm.com/effects/animated-underline --- # How to add an animated underline with Claude A thick yellow line draws itself left to right under "cleared" the moment the voiceover says it, and stays under the caption. Made by the script on this page, from our own test clip. ## Get this effect 1. ### Switch to Code in your [Claude Desktop App](https://claude.com/download) 2. ### Install the skill (once) ``` Install the Recharm video effects skill: download https://recharm-growth-staging.s3.us-east-2.amazonaws.com/website/downloads/recharm-video-effects.zip and unzip it into ~/.claude/skills ``` 3. ### Ask for the effect ``` Add an animated underline to input.mp4 using the word timings in words.json Key: cleared ``` ## Several videos, several effects, one ask Put your clips in the folder, mention them with @, and ask for everything at once. Claude runs one tested script after another on each clip, then joins them. Add an animated underline to **@clip1.mp4**, put a sale badge on **@clip2.mp4**, give **@clip3.mp4** a VHS look, then join all three with crossfades. ## Settings you can change Ask Claude to change any of these, or edit them at the top of the script. | Setting | Default | What it does | | ----------- | --------------- | ----------------------------------------------------------------- | | `KEY` | `cleared` | The word that gets underlined. | | `DRAW` | `0.4` | Seconds the line takes to draw, starting when the word is spoken. | | `LINE` | `3` | Words per caption line. | | `HOLD` | `0.5` | Seconds the caption stays after the last word. | | `SIZE` | `0.085` | Text height as a share of the frame width. | | `UNDERLINE` | `(255, 214, 0)` | Underline color. | ## Without the skill Don’t want to install the skill? Paste this prompt into Claude Code and Claude writes the script itself. ``` Add a caption with an animated underline to input.mp4 using the word timings in words.json. Show the whole spoken sentence in white bold text with a black outline, 46 px tall, three words per line, the block centered horizontally and 70% of the way down, from the first word's start until 0.5 seconds after the last word's end. When the word "cleared" starts being spoken, draw a yellow (255, 214, 0) rounded underline 7 px thick with a thin black outline just below that word, growing left to right across the word's width over 0.4 seconds, and keep it there until the caption goes. Nothing is underlined before that. Use Python with Pillow to draw it and ffmpeg to put it on the video (don't rely on ffmpeg's drawtext filter), keep the audio, and save it as output.mp4. ``` ## Run it without Claude This is the exact script we tested. Save it as `animated-underline.py` and run: ``` python3 animated-underline.py input.mp4 words.json output.mp4 ``` ``` #!/usr/bin/env python3 """Animated underline: a yellow line draws itself under one key word of the caption as it is spoken. Usage: python3 animated-underline.py input.mp4 words.json output.mp4 words.json holds word timings, for example from Whisper: [{"word": "This", "start": 0.30, "end": 0.69}, {"word": "serum", "start": 0.77, "end": 1.13}, ...] Needs ffmpeg and Pillow (pip install pillow). """ import json import math import subprocess import sys from pathlib import Path from PIL import Image, ImageDraw, ImageFont KEY = "cleared" # the word that gets underlined DRAW = 0.4 # seconds the line takes to draw, starting when the word is spoken LINE = 3 # words per caption line HOLD = 0.5 # seconds the caption stays after the last word SIZE = 0.085 # text height as a share of the frame width Y = 0.70 # where the caption's center sits (0 = top, 1 = bottom) UNDERLINE = (255, 214, 0) # underline color FONTS = ["/System/Library/Fonts/Supplemental/Arial Bold.ttf", # macOS "/usr/share/fonts/truetype/dejavu/DejaVuSans-Bold.ttf", # Linux "C:/Windows/Fonts/arialbd.ttf"] # Windows FONT_FILE = next((f for f in FONTS if Path(f).exists()), None) or sys.exit("No bold font found: add one to FONTS") def overlay(t): img = Image.new("RGBA", (w, h)) if not words[0]["start"] <= t < words[-1]["end"] + HOLD: return img draw = ImageDraw.Draw(img) names = [word["word"].strip() for word in words] lines = [names[i:i + LINE] for i in range(0, len(names), LINE)] step = font.size * 1.5 edge = round(font.size * 0.08) # black outline around the text and the line for row, line in enumerate(lines): text = " ".join(line) base = h * Y + (row - (len(lines) - 1) / 2) * step + font.size * 0.35 # this line's baseline draw.text((w / 2, base), text, font=font, anchor="ms", fill="white", stroke_width=edge, stroke_fill="black") if KEY in line: spoken = next(word["start"] for word in words if word["word"].strip() == KEY) p = min(1, (t - spoken) / DRAW) if p > 0: x = w / 2 - font.getlength(text) / 2 + font.getlength(" ".join(line[:line.index(KEY)] + [""])) top, thick = base + font.size * 0.16, font.size * 0.16 draw.rounded_rectangle([x, top, x + max(thick, font.getlength(KEY) * p), top + thick], radius=thick / 2, fill=UNDERLINE, outline="black", width=edge // 2) return img def probe(path): info = json.loads(subprocess.check_output(["ffprobe", "-v", "error", "-select_streams", "v:0", "-show_entries", "stream=width,height,r_frame_rate:format=duration", "-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), float(info["format"]["duration"]) def render(src, out): """Draw overlay(t) on every frame of src and save to out, keeping the audio.""" ff = subprocess.Popen(["ffmpeg", "-y", "-v", "error", "-i", src, "-f", "rawvideo", "-pix_fmt", "rgba", "-s", f"{w}x{h}", "-r", rate, "-i", "-", "-filter_complex", "[0:v][1:v]overlay=shortest=1,format=yuv420p[v]", "-map", "[v]", "-map", "0:a?", "-c:v", "libx264", "-crf", "18", "-c:a", "copy", out], stdin=subprocess.PIPE) for i in range(math.ceil(duration * fps)): ff.stdin.write(overlay(i / fps).tobytes()) ff.stdin.close() if ff.wait(): sys.exit("ffmpeg failed") src, words_file, out = sys.argv[1:4] words = json.loads(Path(words_file).read_text()) w, h, rate, fps, duration = probe(src) font = ImageFont.truetype(FONT_FILE, round(w * SIZE)) render(src, out) ``` ## More captions and text - [AI chat reply animation **AI chat reply animation**](https://www.recharm.com/effects/ai-chat-typing) - [Animated bar chart **Animated bar chart**](https://www.recharm.com/effects/animated-bar-chart) - [Animated checklist **Animated checklist**](https://www.recharm.com/effects/animated-checklist) - [Animated line graph **Animated line graph**](https://www.recharm.com/effects/animated-line-graph) - [Animated pie chart **Animated pie chart**](https://www.recharm.com/effects/animated-pie-chart) ## Find 3x _winning ads_ Recharm cuts your ad footage into tagged, searchable clips: hooks, testimonials, product shots. Find the shots that win, add the effect with Claude, and ship 3x more ad edits. [Start 14-day free trial](https://www.recharm.com/get-started)