wjs-overlaying-video

Post-production for a video clip: cover, captions, illustrations, CTA, custom motion graphics — all composed in ONE HyperFrames project and rendered in a SINGLE final encode. No cascade of decodes/re-encodes (each cascade pass degrades quality and burns time).

When to use

• Downstream of /wjs-segmenting-video — the segmentation skill hands you cropped clips + per-clip SRTs; this skill turns them into upload-ready MP4s with cover/captions/illustrations/CTA.
• User has a finished video and wants to dress it up with motion graphics: opening hook, key-quote callout, closing slogan, chapter cards, AI-generated cover as first frame.
• User wants HTML/CSS-quality captions on a video (kinetic word-by-word highlighting, custom fonts, large outlined text, seekable per cue).
• User wants illustration overlays at specific hook moments — diagrams, big text emphasis, flow charts.

Don't use for:

• Splitting one long video into clips → use /wjs-segmenting-video.
• Creating the source SRT → use /wjs-transcribing-audio (then /wjs-translating-subtitles if you need a different language).
• Full HyperFrames productions where the source isn't a fixed video → use hyperframes directly.
• 微信视频号 / 抖音 upload (no public API for those) → this skill produces the MP4; upload is manual.

What this skill IS — and IS NOT

The pipeline

clip.mp4 + clip.zh-CN.burn.srt   (from /wjs-segmenting-video hand-off)
   ↓
1. (Optional) Generate AI cover via gpt-image-2
   make_cover.py --segments S.json --out output/ --size 1024x1792
   cover_NN_slug.png

2. Scaffold a HyperFrames project per clip
   hf_clip_NN/1080/{index.html, clip.mp4, cover.png, captions.json}

3. Compose: cover scene + body video + caption track + chapter chip
            + 1-2 illustrations at hook moments + CTA scene

4. npm run check (lint + validate + visual inspect)
   npm run render → upload-ready MP4

A 2-minute vertical 1080×1920 composition renders in ~2-3 min on M-series Mac.

Standard overlay types (the 6 building blocks)

Every clip's final composition is built from some combination of these. The agent picks the right ones per clip — typically all 6 for a podcast highlight, or just 1-2 for a single annotation overlay.

1. cover — full-frame AI image as first frame

The cover IS the first frame (no animation, no zoom) so platforms that auto-pick the first frame as the thumbnail get your designed cover by default. Always verify with ffmpeg -ss 0 -vframes 1 — frame 0 must NOT be black or platform thumbnails will be black.

HTML:

\x3Cdiv id="cover" class="clip" data-start="0" data-duration="1.6"
     data-track-index="1" data-layout-allow-overflow>
  \x3Cimg src="cover.png" alt="" data-layout-allow-overflow />
\x3C/div>

CSS:

#cover { position: absolute; inset: 0; background: #0c0d10; overflow: hidden; }
#cover img { position: absolute; inset: 0; width: 100%; height: 100%; object-fit: cover; }

Generation: use /wjs-segmenting-video/scripts/make_cover.py (wraps gpt-image-2 images edit with the midpoint frame as ref):

# For 1080×1920 vertical output (视频号 / 抖音):
make_cover.py --segments S.json --out output/ --size 1024x1792 [--single N]

# For 1920×1080 horizontal output (YouTube / B站):
make_cover.py --segments S.json --out output/ --size 1536x1024

Aspect must match output frame. --size 1024x1536 (2:3, the script default) gets letterboxed or cropped on 9:16 output — always pass 1024x1792 for vertical. The cover image's aspect is what the viewer sees full-frame, so mismatch is visible. Re-roll one with --single N; codex provider can transient-fail mid-batch.

Codex auth required: the script calls codex CLI via gpt-image-2-skill. If ~/.codex/auth.json is missing, the script errors. See gpt-image-2-skill for setup.

2. caption — outlined HTML/CSS captions synced to SRT

White text with thick black stroke, no bubble background, vertically centered in a fixed zone (so 1-line vs 2-line captions don't make the visual center jump up and down).

HTML:

\x3Cdiv id="caption" class="clip" data-start="{body_start}"
     data-duration="{body_dur}" data-track-index="4">\x3C/div>

CSS (vertical 1080×1920):

#caption {
  position: absolute; left: 0; right: 0; bottom: 240px;
  height: 240px; z-index: 10; overflow: visible;
}
#caption .bubble {
  position: absolute; top: 50%; left: 50%;
  display: inline-block;
  padding: 0 24px;
  font-size: 56px; line-height: 1.18; font-weight: 900;
  color: #ffffff; max-width: 1020px; text-align: center;
  -webkit-text-stroke: 5px #000;
  paint-order: stroke fill;
  text-shadow: 0 6px 12px rgba(0,0,0,0.55), 0 0 4px rgba(0,0,0,0.6);
  letter-spacing: 0.01em;
}

JS (one bubble per cue + GSAP fade in/out, all centered at container midpoint):

// SRT cues are loaded as inline JSON. Each cue's start/end is offset
// by the cover-scene duration (e.g., 1.5s) so the timing aligns with
// the composition timeline (not the body's own t=0).
const captionEl = document.getElementById("caption");
const groups = JSON.parse(document.getElementById("captions-data").textContent);
const bubbles = groups.map((g, i) => {
  const b = document.createElement("span");
  b.className = "bubble"; b.id = "cap-" + i;
  b.textContent = g.text; b.style.opacity = "0";
  captionEl.appendChild(b);
  return b;
});
// GSAP xPercent/yPercent for centering (CSS transform would get
// overwritten the moment we tween y).
gsap.set(bubbles, { xPercent: -50, yPercent: -50 });
groups.forEach((g, i) => {
  const el = bubbles[i];
  tl.fromTo(el, { opacity: 0, y: 12 }, { opacity: 1, y: 0, duration: 0.18, ease: "power2.out" }, g.start);
  const exitStart = Math.max(g.start + 0.18, g.end - 0.12);
  tl.to(el, { opacity: 0, duration: 0.12, ease: "power2.in" }, exitStart);
  tl.set(el, { opacity: 0 }, g.end);
});

Source SRT — slice + shift before inlining. Take clip_NN.zh-CN.burn.srt from segmentation, parse each cue, add the cover duration to every start/end, and inline as JSON in a \x3Cscript id="captions-data" type="application/json"> block.

MarginV / position notes:

• Vertical (1080×1920): bottom: 240px keeps captions clear of the 视频号/抖音 bottom UI overlay (likes/comments/share buttons).
• Horizontal (1920×1080): bottom: 100px, font-size: 48px, -webkit-text-stroke: 4px is a reasonable default.

Caption length cap. If a single cue exceeds ~18 Chinese chars on 1080-wide at 56px, it wraps to 2 lines awkwardly. This is upstream discipline — /wjs-translating-subtitles should cap cues at ~18 chars using word-gap split + punctuation split. If you receive longer cues, either reduce font-size to 48px or accept the wrap.

3. chapter — top-left chapter chip (4s reveal then fade)

A subtle badge identifying the segment. Enters at body start, fades after a few seconds so it doesn't compete with the rest of the composition.

HTML:

\x3Cdiv id="chapter" class="clip" data-start="{body_start}"
     data-duration="{body_dur}" data-track-index="3">
  \x3Cspan class="dot">\x3C/span>
  \x3Cspan class="text">第一段 · 自然语言才是新代码\x3C/span>
\x3C/div>

CSS:

#chapter {
  position: absolute; top: 80px; left: 60px; z-index: 9;
  display: inline-flex; align-items: center; gap: 12px;
  padding: 12px 20px;
  background: rgba(12,13,16,0.78);
  border: 1px solid rgba(199,150,85,0.4);
  border-radius: 999px;
}
#chapter .dot { width: 10px; height: 10px; border-radius: 999px; background: #e8b063; }
#chapter .text {
  font-size: 24px; color: #f4f4f5; letter-spacing: 0.04em; font-weight: 600;
}

GSAP:

tl.from("#chapter", { x: -40, opacity: 0, duration: 0.5, ease: "expo.out" }, body_start + 0.4);
tl.to("#chapter", { opacity: 0, duration: 0.4, ease: "power2.in" }, body_start + 4.0);

4. stack illustration — top-right vertical list card

A list of items (e.g., language hierarchy, workflow steps, levels) in a dark card at the top-right. One item can be accented in amber to highlight the relevant level/step.

Use for: showing a hierarchy or list while the speaker explains it. Card stays visible 8-50s.

HTML:

\x3Cdiv id="ill-stack" class="clip" data-start="{start}" data-duration="{dur}" data-track-index="5">
  \x3Cdiv class="ill-card">
    \x3Cdiv class="ill-card-label">我们写的层级\x3C/div>
    \x3Cdiv class="ill-row">\x3Cspan class="ill-tag accent">自然语言\x3C/span>\x3C/div>
    \x3Cdiv class="ill-row">\x3Cspan class="ill-tag">Python\x3C/span>\x3C/div>
    \x3Cdiv class="ill-row">\x3Cspan class="ill-tag">C\x3C/span>\x3C/div>
    \x3Cdiv class="ill-row">\x3Cspan class="ill-tag">Assembly\x3C/span>\x3C/div>
  \x3C/div>
\x3C/div>

CSS: (see references/illustration_patterns.md for the full canonical CSS — copy verbatim)

GSAP — slide in from right + stagger rows:

tl.fromTo("#ill-stack", { x: 360, opacity: 0 }, { x: 0, opacity: 1, duration: 0.6, ease: "expo.out" }, start + 0.2);
tl.from("#ill-stack .ill-row", { y: 20, opacity: 0, duration: 0.4, stagger: 0.12, ease: "power2.out" }, start + 0.4);
tl.to("#ill-stack", { x: 360, opacity: 0, duration: 0.5, ease: "power2.in" }, end - 0.5);

5. hammer illustration — center-frame big equation/text overlay

A BIG center-frame text/equation that visually "hammers" a key claim. Best for the single most quotable moment in a clip (e.g., "LLM = 编译器", "Token = 新 GDP", "AI ≠ 更快的轿子"). Visible 4–8s.

HTML:

\x3Cdiv id="ill-hammer" class="clip" data-start="{start}" data-duration="{dur}" data-track-index="6">
  \x3Cdiv class="ill-h-content">
    \x3Cdiv class="ill-h-eq">
      \x3Cspan class="ill-h-left">LLM\x3C/span>
      \x3Cspan class="ill-h-equals">=\x3C/span>
      \x3Cspan class="ill-h-right">新编译器\x3C/span>
    \x3C/div>
    \x3Cdiv class="ill-h-foot">自然语言 → Python → 汇编\x3C/div>
  \x3C/div>
\x3C/div>

GSAP — scale-pop entrance + stagger each piece + scale-fade exit:

tl.fromTo("#ill-hammer", { scale: 0.85, opacity: 0 },
  { scale: 1.0, opacity: 1, duration: 0.45, ease: "back.out(1.6)" }, start);
tl.from("#ill-hammer .ill-h-left", { x: -40, opacity: 0, duration: 0.4, ease: "expo.out" }, start + 0.2);
tl.from("#ill-hammer .ill-h-equals", { scale: 0, opacity: 0, duration: 0.4, ease: "back.out(2)" }, start + 0.4);
tl.from("#ill-hammer .ill-h-right", { x: 40, opacity: 0, duration: 0.4, ease: "expo.out" }, start + 0.6);
tl.from("#ill-hammer .ill-h-foot", { y: 20, opacity: 0, duration: 0.4, ease: "power2.out" }, start + 0.8);
tl.to("#ill-hammer", { scale: 1.05, opacity: 0, duration: 0.45, ease: "power2.in" }, end - 0.45);

(see references/illustration_patterns.md for full canonical CSS)

6. cta — end-card with channel CTA

A branded outro for the final 3 seconds. Use 王建硕 as the channel name (per global instructions) — never put a guest's name in the CTA slot.

HTML:

\x3Cdiv id="cta" class="clip" data-start="{cta_start}" data-duration="3.24" data-track-index="1">
  \x3Cdiv class="cta-line-1">关注王建硕\x3C/div>
  \x3Cdiv class="arrow">↓\x3C/div>
  \x3Cdiv class="cta-line-2">微信公众号 · 视频号\x3C/div>
  \x3Cdiv class="cta-foot">AI 炼金术 · 持续更新\x3C/div>
\x3C/div>

CSS / GSAP: see references/illustration_patterns.md.

Legacy types (for one-off overlays on a single video)

The spec.json + scaffold.py workflow also supports these older overlay types — useful when you want to dress up ONE existing video without going through the full post-production workflow above:

• quote — full-width kinetic typography, top or bottom gradient. Best for opening hooks and key-quote callouts.
• slogan — alias for quote with position: bottom and larger type. Best for closing slogans.
• callout — small annotation panel in a corner. Best for chapter labels, lower-thirds, "as seen in" notes.
• custom — escape hatch. Claude writes the overlay's HTML/CSS/GSAP inside an overlays/\x3Cname>.html fragment file. See references/custom_overlay_recipes.md.

Workflow A — Post-segmentation preset (most common)

Use this when you're coming directly from /wjs-segmenting-video and want the standard cover + caption + chapter + illustrations + CTA treatment for each clip.

Step 1 — Generate AI covers at the right aspect

# For vertical 9:16 output (视频号 / 抖音):
python3 ~/.claude/skills/wjs-segmenting-video/scripts/make_cover.py \
    --segments segments.json --out output/ --size 1024x1792 --single 1
# Verify segment 1's cover; then batch:
python3 ~/.claude/skills/wjs-segmenting-video/scripts/make_cover.py \
    --segments segments.json --out output/ --size 1024x1792

Step 2 — For each clip, scaffold a HyperFrames project

hf_clip_NN/1080/ with:

• index.html — the composition (from template; see references/post_segmentation_template.html)
• clip.mp4 — copied from output/clip_NN_slug.mp4
• cover.png — copied from output/cover_NN_slug.png
• captions.json — generated from output/clip_NN_slug.zh-CN.burn.srt with every cue's start/end shifted by +cover_duration (so cues align with the composition timeline, not the body's own clock)

The build script at references/build_hf_clips.py does this for all segments in one pass. It reads segments.json + an ILLUSTRATIONS dict (illustrations per clip, see Step 3) + the template, and emits 5 ready-to-render projects.

Step 3 — Define illustrations per clip

For each clip, identify 1-2 hook moments and pick stack or hammer:

ILLUSTRATIONS = {
    1: [
        # The language hierarchy as a stack card during the opening
        {"key": "stack", "pattern": "stack", "body_start": 0.3, "body_end": 9.0,
         "label": "我们写的层级",
         "rows": [
             {"text": "自然语言", "accent": True},
             {"text": "Python",   "accent": False},
             {"text": "C",        "accent": False},
             {"text": "Assembly", "accent": False},
         ]},
        # The hammer at the most quotable moment
        {"key": "hammer", "pattern": "hammer", "body_start": 10.8, "body_end": 14.6,
         "left": "LLM", "equals": "=", "right": "新编译器",
         "foot": "自然语言 → Python → 汇编"},
    ],
    # ... clips 2-5
}

Timestamps are body-relative (after the cover-scene duration); the build script adds the cover offset when emitting GSAP positions.

Step 4 — Build + render

python3 references/build_hf_clips.py    # scaffolds all projects
for n in 01 02 03 04 05; do
  cd "hf_clip_$n/1080"
  npx hyperframes lint
  npx hyperframes validate
  npx hyperframes render
  cd ../..
done

A 2:30 clip renders in ~3 min. Output: hf_clip_NN/1080/renders/*.mp4.

Workflow B — Custom overlays on a single video (legacy spec.json)

Use this when you have ONE existing video and want to add a few ad-hoc overlays (title cards, annotations, lower-thirds).

spec.json schema

{
  "source_video": "../path/to/source.mp4",
  "duration": 135.4,
  "size": "1920x1080",
  "name": "clip_01_animated",
  "overlays": [
    {"id": "o1", "type": "quote", "start": 8.0, "duration": 6.0,
     "position": "top", "lines": ["代码不存在错误", "只存在意图错配"],
     "accent": [false, true]},
    {"id": "o2", "type": "callout", "start": 30.0, "duration": 5.0,
     "anchor": "top-right", "text": "FRP 概念"},
    {"id": "o3", "type": "slogan", "start": 122.0, "duration": 13.4,
     "lines": ["改 prompt", "不改 AI 生成的代码"], "accent": [false, true]}
  ]
}

Scaffold + render

python3 ~/.claude/skills/wjs-overlaying-video/scripts/scaffold.py spec.json
cd \x3Cname> && npm run check && npm run render

Output checklist

Before considering a clip done:

• Frame 0 is the cover (not black) — ffmpeg -ss 0 -vframes 1 out.mp4
• Captions are synced with audio (lint a few seconds with audio playback)
• All illustrations enter and exit at the speech moments they support
• CTA renders correctly (关注王建硕, not a guest's name)
• npx hyperframes lint && npx hyperframes validate both pass
• npx hyperframes inspect shows no layout overflow
• Total duration matches the source clip + cover + CTA durations

Common mistakes

• Cover aspect ≠ output aspect. 1024x1536 (the default make_cover.py size) is 2:3 and gets letterboxed or cropped on 9:16 output. Always pass --size 1024x1792 for vertical.
• Caption alignment jumps with line count. Anchor by CENTER (translate(-50%, -50%)) inside a fixed-height container so 1-line vs 2-line cues share the same visual midline. NOT anchored from bottom (causes growth-upward).
• GSAP overwrites CSS transform centering. If you set transform: translate(-50%, -50%) in CSS and then tween y, GSAP replaces the transform and centering breaks. Use gsap.set(el, { xPercent: -50, yPercent: -50 }) instead so xPercent/yPercent compose with subsequent y/x tweens.
• Burning libass subs on top of HTML/CSS captions. Pick ONE caption system per output video. If you're using this skill's HTML/CSS captions, do NOT also burn subs in /wjs-segmenting-video — request the raw clip via the hand-off package.
• Frame 0 is black. If your cover scene has an opacity fade-in starting from 0, the literal first frame is black and the platform thumbnail will be black. Place the cover statically (no opacity tween) and verify with ffmpeg -ss 0 -vframes 1.
• Channel name in CTA = guest's name. Always use 王建硕. Guests belong in description text inside the metadata, not in the on-screen CTA.
• Cover image cropped because of object-fit: cover on mismatched aspect. Either regenerate the cover at the right aspect (see Step
  1. or letterbox with object-fit: contain + dark background.

Integration with other skills

• /wjs-segmenting-video — the typical upstream. After it cuts
  • crops + slices SRTs, this skill picks up. The hand-off package is clip_NN.mp4 + clip_NN.zh-CN.burn.srt + segments.json.
• /wjs-transcribing-audio + /wjs-translating-subtitles — if no SRT exists, run them first. The word-level Whisper or Volcano/豆包 ASR output is preferred for accurate cue timing.
• hyperframes — the underlying composition framework. This skill is a thin wrapper that encodes the proven post-production patterns; everything in the hyperframes skill applies (preview, render, transitions, audio-reactive, etc.). Read it whenever you write custom overlays.
• hyperframes-cli — the CLI commands the project uses (init, lint, validate, inspect, render).
• gpt-image-2-skill — the cover generator. make_cover.py invokes it via the codex CLI; the codex auth in ~/.codex/auth.json is required.
• /wjs-uploading-video — the next downstream after this skill produces an MP4. Uploads the renders to YouTube with title / description / tags from a metadata file.

Files & references

• scripts/scaffold.py — Workflow B scaffolder (legacy spec.json for ad-hoc overlays)
• references/post_segmentation_template.html — Workflow A template: the canonical cover + caption + chapter + illustration + CTA composition shape, with placeholder substitutions
• references/build_hf_clips.py — Workflow A multi-clip builder. Reads segments.json + per-clip illustrations dict, scaffolds and populates one project per clip
• references/illustration_patterns.md — canonical CSS / GSAP for the stack and hammer illustration patterns
• references/custom_overlay_recipes.md — reusable custom overlay recipes (terminal demo, layer-stack diagram, callout with arrow)
• references/example_spec.json — Workflow B example