# Add subtitles to videos

Robot: `/video/subtitle`

🤖/video/subtitle adds subtitles and closed captions to videos.

This <dfn>Robot</dfn> supports both SRT and VTT subtitle files.

Stage: ga

## Usage example

If you have file input fields in a form — one for a video and one for each subtitle language (named \`input\_video\`, \`input\_srt\_en\`, and \`input\_srt\_de\` via the HTML \`name\` attribute) — here’s how to add multiple subtitle streams with language tags to the video. Chain multiple \`/video/subtitle\` Steps with \`keep\_subtitles\` set to \`true\` so each new stream is added alongside existing ones:

```json
{
  "steps": {
    "add_english_subtitles": {
      "robot": "/video/subtitle",
      "use": {
        "steps": [
          {
            "name": ":original",
            "fields": "input_video",
            "as": "video"
          },
          {
            "name": ":original",
            "fields": "input_srt_en",
            "as": "subtitles"
          }
        ]
      },
      "language": "eng",
      "name": "English",
      "ffmpeg_stack": "v7"
    },
    "add_german_subtitles": {
      "robot": "/video/subtitle",
      "use": {
        "steps": [
          {
            "name": "add_english_subtitles",
            "as": "video"
          },
          {
            "name": ":original",
            "fields": "input_srt_de",
            "as": "subtitles"
          }
        ]
      },
      "language": "deu",
      "name": "Deutsch",
      "keep_subtitles": true,
      "ffmpeg_stack": "v7"
    }
  }
}
```

## Parameters

* `interpolate`: Controls whether Assembly Variables are interpolated for individual instruction fields.

  By default, most Robot instruction fields interpolate Assembly Variables. Set this to `false` to treat every instruction field as literal text, or set an individual field path to `false` to treat only that field as literal text. For Robot-specific fields that are literal by default, set this to `true` or set that field path to `true` to opt back into interpolation.

  Use field names such as `path`, or dotted paths such as `ffmpeg.vf` for nested objects.

* `output_meta`: Allows you to specify a set of metadata that is more expensive on CPU power to calculate, and thus is disabled by default to keep your Assemblies processing fast.

  For images, you can add `"has_transparency": true` in this object to extract if the image contains transparent parts and `"dominant_colors": true` to extract an array of hexadecimal color codes from the image.

  For images, you can also add `"blurhash": true` to extract a [BlurHash](https://blurha.sh) string — a compact representation of a placeholder for the image, useful for showing a blurred preview while the full image loads.

  For videos, you can add the `"colorspace": true` parameter to extract the colorspace of the output video.

  For videos, you can also add `"interlaced": true` to detect whether the video is interlaced. This combines the cheap ffprobe `field_order` flag with a bounded `idet` sampling pass over the first frames of the source, exposing `interlaced`, `field_order`, and a diagnostic `interlace_detection` object under `file.meta`. This is computationally expensive and billed accordingly.

  For audio, you can add `"mean_volume": true` to get a single value representing the mean average volume of the audio file.

  You can also set this to `false` to skip metadata extraction and speed up transcoding.

* `user_meta`: Adds custom metadata to each file emitted by this Robot without modifying the file’s contents.

  The values are merged with any existing `user_meta` carried by the input file. If both objects contain the same key, this Robot’s value takes precedence. Assembly Variables are supported, for example `{ "internal_file_id": "${file.id}" }`.

* `result`: Whether the results of this Step should be present in the Assembly Status JSON

* `queue`: Setting the queue to 'batch', manually downgrades the priority of jobs for this step to avoid consuming Priority job slots for jobs that don't need zero queue waiting times

* `force_accept`: Force a Robot to accept a file type it would have ignored.

  By default, Robots ignore files they are not familiar with.
  [🤖/video/encode](/docs/robots/video-encode.md), for
  example, will happily ignore input images.

  With the `force_accept` parameter set to `true`, you can force Robots to accept all files thrown at them.
  This will typically lead to errors and should only be used for debugging or combatting edge cases.

* `ignore_errors`: Ignore errors during specific phases of processing.

  Setting this to `["meta"]` will cause the Robot to ignore errors during metadata extraction.

  Setting this to `["execute"]` will cause the Robot to ignore errors during the main execution phase.

  Setting this to `true` is equivalent to `["meta", "execute"]` and will ignore errors in both phases.

* `use`: Specifies which Step(s) to use as input.

  * You can pick any names for Steps except `":original"` (reserved for user uploads handled by Transloadit)
  * You can provide several Steps as input with arrays:
    ```json
    {
      "use": [
        ":original",
        "encoded",
        "resized"
      ]
    }
    ```
  * You can also tag input Steps with `as` to pass semantic intent to robots:
    ```json
    {
      "use": [
        {
          "name": ":original",
          "as": "image"
        },
        {
          "name": ":original",
          "as": "mask"
        }
      ]
    }
    ```

  > [!Tip]
  > That's likely all you need to know about `use`, but you can view [Advanced use cases](/docs/topics/use-parameter.md).

* `ffmpeg`: A parameter object to be passed to FFmpeg. If a preset is used, the options specified are merged on top of the ones from the preset. For available options, see the [FFmpeg documentation](https://ffmpeg.org/ffmpeg-doc.html). Options specified here take precedence over the preset options.

* `ffmpeg_stack`: Selects the FFmpeg stack version to use for encoding. We currently recommend using "v7". The exact versions "v6.0.0", "v7.0.0", and "v8.0.0" are legacy values that remain accepted for backward compatibility. Deprecated "v5.x" values are also accepted.

* `width`: Width of the new video, in pixels.

  If the value is not specified and the `preset` parameter is available, the `preset`'s [supplied width](/docs/presets/video.md) will be implemented.

* `height`: Height of the new video, in pixels.

  If the value is not specified and the `preset` parameter is available, the `preset`'s [supplied height](/docs/presets/video.md) will be implemented.

* `preset`: Converts a video according to [pre-configured settings](/docs/presets/video.md).

  Starting with `ffmpeg_stack: "v7"`, you can use the value `'empty'` here if you specify your own FFmpeg parameters using the <dfn>Robot</dfn> or do not want Transloadit to set any encoding settings.

* `subtitles_type`: Determines if subtitles are added as a separate stream to the video (value `"external"`) that then can be switched on and off in your video player, or if they should be burned directly into the video (value `"burned"` or `"burn"`) so that they become part of the video stream.

* `border_style`: Specifies the style of the subtitle. Use the `border_color` parameter to specify the color of the border.

* `border_color`: The color for the subtitle border. The first two hex digits specify the alpha value of the color.

* `font`: The font family to use. Also includes boldness and style of the font.

  [Here](/docs/supported-formats/fonts.md) is a list of all supported fonts.

* `font_color`: The color of the subtitle text in `"rgb"`, `"rrggbb"`, or `"aarrggbb"` format. If 8 hex digits are provided, the first two specify the alpha value of the color (`"00"` is fully opaque, `"ff"` is fully transparent).

* `font_size`: Specifies the size of the text.

* `bold`: Specifies whether the subtitle text should be bold. Only applies to burned subtitles.

* `italic`: Specifies whether the subtitle text should be italic. Only applies to burned subtitles.

* `outline_width`: Specifies the width of the text outline in pixels. Only applies to burned subtitles with `"outline"` or `"box"` border styles. If not specified, the default width is used.

* `position`: Specifies the position of the subtitles.

* `language`: Specifies the language of the subtitle stream using a three-letter language code (e.g. `"eng"`, `"deu"`, `"spa"`). Only used if the subtitles are external. When adding multiple subtitle streams, set a different language for each stream so that video players can offer language selection.

* `name`: Specifies a human-readable name for the subtitle track (e.g. `"English"`, `"Deutsch"`, `"Español"`). Only used if the subtitles are external. This name is displayed in video player subtitle menus alongside the language.

* `keep_subtitles`: Specifies if existing subtitles in the input file should be kept or be replaced by the new subtitle. Only used if the subtitles are external. Set this to `true` when chaining multiple `/video/subtitle` Steps to add several subtitle streams (e.g. different languages) to the same video.