Transparent Subtitle Overlays API for Premiere, After Effects, and DaVinci

A caption layer with real alpha for editing timelines

Get styled captions back as a transparent overlay, not a baked-in MP4. ZapCap renders a caption-only layer with a true alpha channel — ProRes 4444 or VP9 — so you can lay it over your edit in Premiere Pro, After Effects, DaVinci Resolve, Final Cut, or CapCut. No chroma key, no transcoding your source.

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Real alpha channel · ProRes 4444 or VP9 · no chroma key · editor-ready layer
WorkflowStylingOutput modesTranscriptsLanguagesPricingProofFAQ
Layer, not a flattened render

How editors get captions onto a cut

When captions have to sit on top of footage you're still editing in Premiere, After Effects, DaVinci, or another NLE, you don't want a flattened MP4 — you want a transparent layer you can place, move, and re-time on your own timeline.

OPTION 01

Burned-in MP4
Flattened render
  • Great for final social posts
  • Captions locked to that cut
  • Re-render to change anything
  • Hard to composite further

OPTION 02

Green-screen layer
Solid-color backdrop
  • Works without alpha support
  • You must key out the green
  • Edge fringing on fine type
  • Extra keying step

OPTION 03 · YOU ARE HERE

ZapCap · transparent overlay
Real alpha caption layer
  • True alpha channel, clean edges
  • ProRes 4444 or VP9
  • No chroma key needed
  • Drop straight onto a timeline

OPTION 04

Subtitle file
SRT / VTT into the NLE
  • Imports as editor titles
  • NLE styling, not yours
  • No animation
  • Look varies per editor
API workflow

How the Transparent Subtitle Overlays API works

Upload the clip, create a task that requests a transparent output codec, then poll or get a webhook. ZapCap renders a caption-only layer with alpha preserved and hands you the file to import.

  1. 1

    Upload your video

    POST the file to /videos. We stream it to storage and hand you back a videoId.

    POSTPOST /videos
  2. 2

    Create the captioning task

    One POST starts transcription, styling, and rendering — set the output codec to ProRes 4444 or VP9 so the result is an alpha layer, not a flattened MP4. Add a notification webhook to skip polling.

    POSTPOST /videos/:id/task
  3. 3

    Receive the webhook

    We POST status updates to your endpoint as the render moves through transcribing → rendering → completed.

    HOOKPOST → your URL
  4. 4

    Download the finished render

    Pull the caption-only overlay from a global CDN — a ProRes 4444 .mov or VP9 .webm with a real alpha channel. Drop it onto a timeline over your footage; no chroma key, no source transcode.

    GETGET renderUrl
Step 1 / 4·~2s
1import { readFileSync } from "node:fs";
2
3const form = new FormData();
4form.append(
5 "file",
6 new Blob([readFileSync("clip.mp4")]),
7 "clip.mp4",
8);
9
10const { id: videoId } = await fetch(
11 "https://api.zapcap.ai/videos",
12 {
13 method: "POST",
14 headers: { "x-api-key": process.env.ZAPCAP_KEY! },
15 body: form,
16 },
17).then(r => r.json());

POST /videos·Upload your video

Styling as an API primitive

Style the layer

Keep the background clear.

The overlay carries only your styled captions over transparency. Send a templateId for a complete look, or override individual renderOptions — the styling renders into the layer while everything else stays fully transparent.

  • Templates — Beast, Hormozi, Tracy, Devin, plus 25 more (29 presets) — the full styled look on a clear background.
  • Animation — word-by-word pops, karaoke fill, fades — rendered into the alpha layer.
  • Keyword emphasis — flag punchwords to color / scale / box, all over transparency.
  • Layout — font, color, stroke, shadow, words per cue, vertical position with safe-zone math.
  • Aspect ratios — render the overlay at 9:16, 1:1, or 16:9 to match your sequence.
Try a style
Render options
{
  "templateId": "21327a45-df89-46bc-8d56-34b8d29d3a0e",
  "outputCodec": "prores4444",
  "renderOptions": {
    "subsOptions": {
      "emphasizeKeywords": true,
      "animation": true,
      "displayWords": 3
    },
    "styleOptions": {
      "fontUppercase": true,
      "fontShadow": "m"
    }
  }
}
Output modes

Transparent Subtitle Overlays API output modes

ProRes

ProRes 4444 overlay

A caption-only layer with a real alpha channel, the reliable choice for Premiere Pro, After Effects, DaVinci Resolve, and Final Cut. Drop it on a track over your footage — no key, no source transcode.

.mov · ProRes 4444
Smaller alpha file
WebM

VP9 alpha overlay

The same transparent caption layer as a lighter VP9 file with real alpha, ideal for web players and lightweight compositing.

.webm · VP9 alpha
No-alpha fallback

Green-screen layer

When a tool can't read alpha at all. Captions on a #04F404 canvas you key out — use the alpha overlays whenever you can for cleaner edges.

.mp4 · #04F404 backdrop

ProRes 4444 (.mov) is the safe choice for professional NLEs; VP9 (.webm) gives a smaller real-alpha file for web compositing. If your downstream tool cannot read alpha at all, use the green-screen layer instead.

Two transcript paths

Transparent Subtitle Overlays API transcript options

Auto-transcribe

ZapCap transcribes and times the captions, then renders them onto the transparent layer. Edit cues first if the edit calls for it.

  • Inspect / edit the transcript via PUT /videos/:id/task/:taskId/transcript
  • Re-render the overlay after a transcript edit
  • Render one transcript into multiple styled overlays

Bring your own transcript

Send approved cues — from your edit script or localization vendor — and ZapCap renders the overlay without retranscribing.

  • Supported via the SRT-to-burned-in workflow
  • Preserve approved names, claims, and disclaimers
  • Generate alpha overlays per language from approved files
Multilingual overlays

One sequence, an overlay per language

Render a separate transparent caption layer per target language and swap them on your timeline. CJK and Thai use language-aware line-breaking so wrapped lines composite cleanly over your footage.

  • Swap caption-language layers without re-cutting the sequence
  • A brand-term dictionary keeps product names accurate in each overlay
  • Language-aware layout for Chinese, Japanese, Thai
Alpha preserved
保留透明通道
คงช่องอัลฟา
Simple API credits

Per-minute, usage-based credits

Pay for the minutes you render. Transparent (alpha) outputs use a different multiplier than burned-in MP4 — see pricing for the full table.

  • Top up credits to keep renders flowing in production
  • Volume credits available at scale
  • No per-seat fee — pay for renders, not users
$0.10 / min

Indicative starting rate. Alpha and high-resolution outputs carry different multipliers. API access requires a Pro plan plus credits.

View full pricingTalk to us about volume
Customer · Anonymized

A post-production team pulled styled caption overlays into their NLE as alpha layers, dropping the chroma-key step from their finishing workflow

Editors request a ProRes 4444 overlay per clip and place it over the cut on a track. Clean alpha edges mean no keying artifacts on fine type, and re-timing captions is a timeline operation, not a re-render.

Read case study
real alpha
No chroma key in the pipeline
clean edges
No fringing on fine type
on a track
Captions placed, moved, re-timed
no transcode
Source footage left untouched

About the Transparent Subtitle Overlays API

Keep exploring

Related caption rendering APIs

Subtitle API

The umbrella styled subtitle rendering API: video in, captioned video out.

Green-Screen Subtitles API

Caption layer on a green canvas for tools that cannot read alpha.

SRT to Burned-In Subtitles API

Render an existing, approved SRT into a styled output — no retranscription.

Animated Captions API

Word-pop, karaoke fill, and keyword emphasis — the same styles, rendered into an alpha overlay.

Webhook Video Captioning API

Async caption workflows with signed events, retries, and eventId-based dedupe.

Render transparent caption overlays through the API

Create a key on a Pro plan and buy credits in the dashboard.

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