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

From clip to alpha overlay

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

Alpha is the point — here are the formats

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

Auto-transcribe — or bring your own

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

It is a caption-only video layer with a true alpha channel — everything except the styled captions is fully transparent. You place it over your own footage on a timeline, and the footage shows through wherever there is no text. No background to remove, no chroma key.

Keep exploring

Related caption rendering APIs

Subtitle API

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

Read more

Green-Screen Subtitles API

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

Read more

Burned-In Subtitles API

Permanent, styled captions baked into an MP4 for social and ads.

Read more

SRT to Burned-In Subtitles

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

Read more

Animated Captions API

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

Read more

Chinese Subtitles API

Per-language overlays with correct CJK line-breaking and font fallback.

Read more

For agencies

Pull styled caption overlays into your finishing pipeline as alpha layers.

Read more

vs Shotstack API

Caption-overlay rendering vs. general video automation timelines.

Read more

Render transparent caption overlays through the API

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

Create API keyRead API docs