Green-Screen Subtitles API

Captions on a green canvas — key them in anywhere

When your tool cannot read an alpha channel, get styled captions back on a solid green backdrop instead. ZapCap renders a caption layer over #04F404 that you chroma-key in OBS, a hardware or software switcher, or any NLE — perfect for live overlays and tools without alpha support.

Create API keyRead API docs
Solid #04F404 canvas · key out in OBS / switchers · live-overlay friendly
WorkflowStylingOutput modesTranscriptsLanguagesPricingProofFAQ
When alpha is not an option

Why a green canvas instead of transparency

Plenty of live and broadcast tools can't ingest a real alpha channel — but they can all chroma-key. A green-screen caption layer is the universal fallback for those pipelines.

OPTION 01

Transparent overlay
ProRes 4444 · VP9 alpha
  • Cleanest edges
  • Best when alpha is supported
  • Won't load in alpha-less tools
  • No good for many switchers

OPTION 02

Burned-in MP4
Flattened render
  • Final, self-contained clip
  • Captions locked to footage
  • Can't overlay on a live feed
  • Re-render to change

OPTION 03 · YOU ARE HERE

ZapCap · green-screen layer
Captions on #04F404
  • Works in any chroma-key tool
  • Live overlays in OBS / switchers
  • Standard MP4, ingests anywhere
  • Universal alpha-less fallback

OPTION 04

Subtitle file
SRT / VTT
  • Timed text only
  • No styled layer to key
  • No animation
  • Not a compositing source
API workflow

From clip to green-screen layer

Upload the clip, create a task in green-screen output mode, then poll or get a webhook. ZapCap renders the styled captions over a #04F404 canvas and returns a standard MP4 you key in downstream.

  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 with your chosen template. 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

    Burned-in subtitles, served from a global CDN. No watermark. MP4 ready for any social platform.

    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 captions

Leave the canvas keyable.

The layer is your styled captions over a flat green field. Send a templateId for a complete look, or override individual renderOptions. Pick caption colors that contrast with #04F404 so the key stays clean.

  • Templates — Beast, Hormozi, Tracy, Devin, plus 25 more (29 presets) — the full styled look on a green canvas.
  • Animation — word-by-word pops, karaoke fill, fades — rendered over the green field.
  • Keyword emphasis — flag punchwords to color / scale / box for live emphasis.
  • Layout — font, color, stroke, shadow, words per cue, vertical position with safe-zone math.
  • Contrast tip — avoid green-on-green — choose caption fills that key out cleanly against #04F404.
Try a style
Render options
{
  "templateId": "46d20d67-255c-4c6a-b971-31fddcfea7f0",
  "outputMode": "greenScreen",
  "renderOptions": {
    "subsOptions": {
      "emphasizeKeywords": true,
      "animation": true,
      "displayWords": 3
    },
    "styleOptions": {
      "fontUppercase": true,
      "fontShadow": "m"
    }
  }
}
Output modes

Green-screen layer — and the alpha options it falls back from

This API

Green-screen layer

Styled captions over a solid #04F404 canvas, returned as a standard MP4. Chroma-key it in OBS, a switcher, or your NLE — works in any tool that can key a color, including live pipelines.

.mp4 · #04F404 backdrop
Cleaner edges

Transparent overlay

When your tool can read alpha, prefer a transparent overlay — ProRes 4444 or VP9 with a real alpha channel for cleaner edges and no keying step.

.mov ProRes 4444 · .webm VP9 alpha
Self-contained

Burned-in MP4

If you just need a finished clip rather than a layer to composite, render the captions burned into the frames instead.

.mp4 · h264

Use green-screen when your tool cannot ingest alpha. The canvas is #04F404 — choose caption colors that contrast with it so the key is clean and there is no green fringing on the type.

Two transcript paths

Auto-transcribe — or bring your own

Auto-transcribe

ZapCap transcribes and times the captions, then renders them onto the green canvas. Edit cues first when the broadcast or stream calls for it.

  • Inspect / edit the transcript via PUT /videos/:id/task/:taskId/transcript
  • Re-render the green-screen layer after a transcript edit
  • Render one transcript into multiple styled green-screen layers

Bring your own transcript

Send approved cues — from a show script or localization vendor — and ZapCap renders the green-screen layer without retranscribing.

  • Supported via the SRT-to-burned-in workflow
  • Preserve approved names, claims, and disclaimers
  • Produce a keyable layer per language from approved files
Multilingual layers

A keyable caption layer per language

Render a separate green-screen caption layer per target language and switch them in your live or post pipeline. CJK and Thai use language-aware line-breaking so wrapped lines key cleanly.

  • Swap caption-language layers in OBS or a switcher mid-stream
  • A brand-term dictionary keeps product names accurate in each layer
  • Language-aware layout for Chinese, Japanese, Thai
Key it out
抠掉绿色
คีย์สีเขียวออก
Simple API credits

Per-minute, usage-based credits

Pay for the minutes you render. The green-screen layer is a standard MP4 render — see pricing for the full multiplier 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. Final pricing depends on render mode and output format. API access requires a Pro plan plus credits.

View full pricingTalk to us about volume
Customer · Anonymized

A live-streaming team overlaid styled captions on their broadcast by keying ZapCap green-screen layers in their switcher — no alpha support required

Caption layers render over #04F404 and load as standard MP4s into a switcher that cannot ingest alpha. The team keys out the green and composites captions over the live feed, swapping layers per segment.

Read case study
any tool
Keys in alpha-less switchers
live
Captions over a broadcast feed
standard MP4
Ingests anywhere, no special codec
swappable
Layers changed per segment

About the Green-Screen Subtitles API

They are styled captions rendered over a solid green canvas (#04F404) instead of a transparent background. You chroma-key out the green in your editor or live tool to composite the captions over your footage — the universal fallback for pipelines that cannot read an alpha channel.

Keep exploring

Related caption rendering APIs

Subtitle API

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

Read more

Transparent Subtitle Overlays

Real alpha-channel caption layers when your tool supports alpha.

Read more

Burned-In Subtitles API

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

Read more

Animated Captions API

TikTok / Reels / Shorts animated caption styles, rendered into the layer.

Read more

Webhook Video Captioning

Signed render callbacks, retries, and eventId dedupe for the async caption jobs behind this layer.

Read more

For agencies

Composite styled caption layers into live and post pipelines.

Read more

vs Shotstack API

Keyable caption-layer rendering vs. general video automation timelines.

Read more

Render green-screen caption layers through the API

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

Create API keyRead API docs