Burn SRT subtitles into a video - Rendi - FFmpeg API

Render an SRT subtitle file permanently onto a video (burned-in captions). Useful when the target player doesn’t support subtitle tracks or you need guaranteed rendering.

Code

Node.js
Python
cURL

const API_KEY = process.env.RENDI_API_KEY;

const submit = await fetch("https://api.rendi.dev/v1/run-ffmpeg-command", {
  method: "POST",
  headers: {
    "Content-Type": "application/json",
    "X-API-KEY": API_KEY,
  },
  body: JSON.stringify({
    input_files: {
      in_video: "https://storage.rendi.dev/sample/sample.avi",
      in_srt: "https://storage.rendi.dev/sample/subtitles.srt",
    },
    output_files: {
      out_1: "subtitled.mp4",
    },
    ffmpeg_command:
      "-i {{in_video}} -ss 00:00 -to 01:00 -vf subtitles={{in_srt}} -c:v libx264 -crf 20 -c:a aac -b:a 192k {{out_1}}",
  }),
});
const { command_id } = await submit.json();

while (true) {
  const res = await fetch(`https://api.rendi.dev/v1/commands/${command_id}`, {
    headers: { "X-API-KEY": API_KEY },
  });
  const data = await res.json();
  if (data.status === "SUCCESS") {
    console.log("Subtitled URL:", data.output_files.out_1.storage_url);
    break;
  }
  if (data.status === "FAILED") throw new Error("Command failed");
  await new Promise((r) => setTimeout(r, 2000));
}

import os
import time
import requests

API_KEY = os.environ["RENDI_API_KEY"]
BASE = "https://api.rendi.dev/v1"
headers = {"X-API-KEY": API_KEY}

submit = requests.post(
    f"{BASE}/run-ffmpeg-command",
    headers=headers,
    json={
        "input_files": {
            "in_video": "https://storage.rendi.dev/sample/sample.avi",
            "in_srt": "https://storage.rendi.dev/sample/subtitles.srt",
        },
        "output_files": {"out_1": "subtitled.mp4"},
        "ffmpeg_command": "-i {{in_video}} -ss 00:00 -to 01:00 -vf subtitles={{in_srt}} -c:v libx264 -crf 20 -c:a aac -b:a 192k {{out_1}}",
    },
)
command_id = submit.json()["command_id"]

while True:
    res = requests.get(f"{BASE}/commands/{command_id}", headers=headers).json()
    if res["status"] == "SUCCESS":
        print("Subtitled URL:", res["output_files"]["out_1"]["storage_url"])
        break
    if res["status"] == "FAILED":
        raise RuntimeError("Command failed")
    time.sleep(2)

curl --request POST \
  --url https://api.rendi.dev/v1/run-ffmpeg-command \
  --header 'Content-Type: application/json' \
  --header 'X-API-KEY: <api-key>' \
  --data '{
    "input_files": {
      "in_video": "https://storage.rendi.dev/sample/sample.avi",
      "in_srt": "https://storage.rendi.dev/sample/subtitles.srt"
    },
    "output_files": {"out_1": "subtitled.mp4"},
    "ffmpeg_command": "-i {{in_video}} -ss 00:00 -to 01:00 -vf subtitles={{in_srt}} -c:v libx264 -crf 20 -c:a aac -b:a 192k {{out_1}}"
  }'

curl --request GET \
  --url https://api.rendi.dev/v1/commands/<command_id> \
  --header 'X-API-KEY: <api-key>'

How the FFmpeg command works

-i {{in_video}} — input video
-ss 00:00 -to 01:00 — only burn subtitles into the first minute (drop this to process the full video)
-vf subtitles={{in_srt}} — subtitles video filter reads the SRT file
-c:v libx264 -crf 20 — re-encode video (required, since the filter modifies frames)
-c:a aac -b:a 192k — re-encode audio
{{out_1}} — output file

For soft subtitles (a separate subtitle track that can be toggled off), use -c:s mov_text with an MP4 container and skip the -vf filter.

Response

{
  "output_files": {
    "out_1": {
      "file_id": "cb5b9186-...",
      "size_mbytes": 13.95,
      "duration": 60.0,
      "file_type": "video",
      "file_format": "mp4",
      "storage_url": "https://storage.rendi.dev/temp_files/.../subtitled.mp4",
      "width": 854,
      "height": 480,
      "codec": "h264"
    }
  },
  "status": "SUCCESS",
  "command_type": "FFMPEG_COMMAND"
}