Skill Video Caption Overlay

Render TikTok-style animated pill captions onto short-form videos using MoviePy + PIL. Takes a base MP4, a captions JSON, and optional background audio — outputs a final video with fade-in/out pill overlays. Fixes the PIL textbbox y-offset bug that causes text to sit outside pill boundaries. Use for TikTok ads, Reels, YouTube Shorts.

Video Caption Overlay

Animated pill-style caption overlays for short-form video. No Premiere, no CapCut — pure Python.

Usage

bash

uv run --with moviepy --with pillow scripts/overlay.py \
  --video base.mp4 \
  --output final.mp4 \
  --captions scripts/example_captions.json \
  --audio music.mp3 \
  --audio-start 8 \
  --audio-vol 0.5

No --audio if you want to keep the original video audio.

Custom fonts

bash

--font-black /path/to/Montserrat-Black.ttf \
--font-bold  /path/to/Montserrat-Bold.ttf

Falls back to Montserrat from ~/.local/share/fonts/ if not specified.

captions.json format

Array of phases — each phase is a time window with one or more pill lines stacked vertically.

json

[
  {
    "start": 0,
    "end": 3.2,
    "y_frac": 0.06,
    "lines": [
      {
        "text": "POV:",
        "size": 28,
        "bold": true,
        "bg": [0, 195, 255],
        "fg": [0, 0, 0],
        "bg_opacity": 0.9,
        "px": 20, "py": 9, "r": 12
      },
      {
        "text": "drink more water",
        "size": 50,
        "bg": [255, 255, 255],
        "fg": [0, 0, 0]
      }
    ]
  }
]

Field	Type	Default	Description
start	float	required	Phase start time (seconds)
end	float	required	Phase end time (seconds)
y_frac	float	0.06	Vertical position as fraction of video height
lines[].text	string	required	Caption text
lines[].size	int	50	Font size (px)
lines[].bold	bool	false	Use bold font (vs black/heavy)
lines[].bg	[R,G,B]	[255,255,255]	Pill background color
lines[].fg	[R,G,B]	[0,0,0]	Text color
lines[].bg_opacity	float	0.93	Pill background opacity (0–1)
lines[].px	int	26	Horizontal padding
lines[].py	int	13	Vertical padding
lines[].r	int	18	Border radius

PIL textbbox fix

PIL's textbbox((0,0), text, font) returns (x0, y0, x1, y1) where y0 is a non-zero offset (typically 7–15px depending on font size). Drawing text at (x, y) without compensating for this offset causes text to appear below the pill's visual center.

Fix implemented in pill():

python

bb    = draw.textbbox((0, 0), text, font=font)
x_off, y_off = bb[0], bb[1]
vis_w = bb[2] - bb[0]   # actual visual width
vis_h = bb[3] - bb[1]   # actual visual height

# Compensate offsets when drawing text
tx = cx - vis_w // 2 - x_off
ty = y - y_off
draw.text((tx, ty), text, font=font, fill=fg)

Emoji note

NotoColorEmoji.ttf fails with PIL at arbitrary sizes (bitmap font with limited supported sizes). Use text alternatives ("Free delivery" instead of "Free delivery 🚚") for reliable rendering.

Example output

See scripts/example_captions.json for the full 3-phase TikTok ad structure:

• Phase 1 (0–3.2s): Hook — top-screen pill stack
• Phase 2 (2.8–5.8s): Product claim — overlapping fade
• Phase 3 (5.3–8.0s): CTA — bottom-screen price + delivery + bio link