Add subtitles to videos

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

Usage example

{
  "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

  boolean | Record<string, boolean>

  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

  Record<string, boolean> | boolean | Array<string>

  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 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.

• result

  boolean(default: false)

  Whether the results of this Step should be present in the Assembly Status JSON

• queue

  batch

  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

  boolean(default: false)

  Force a Robot to accept a file type it would have ignored.

  By default, Robots ignore files they are not familiar with. 🤖/video/encode, 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

  boolean | Array<meta | execute>(default: [])

  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

  string | Array<string> | Array<object> | object

  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:

    {
      "use": [
        ":original",
        "encoded",
        "resized"
      ]
    }
    

  • You can also tag input Steps with as to pass semantic intent to robots:

    {
      "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.

• ffmpeg

  object

  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. Options specified here take precedence over the preset options.

• ffmpeg_stack

  v6 | v7 | v8 | string(default: "v7")

  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

  string | number | null

  Width of the new video, in pixels.

  If the value is not specified and the preset parameter is available, the preset's supplied width will be implemented.

• height

  string | number | null

  Height of the new video, in pixels.

  If the value is not specified and the preset parameter is available, the preset's supplied height will be implemented.

• preset

  android | android-high | android-low | android_high | android_low | dash-1080p-video | dash-1080p_video | 195 more ↓

  Converts a video according to pre-configured settings.

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

• subtitles_type

  burned | external | burn(default: "external")

  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

  box | outline | shadow(default: "outline")

  Specifies the style of the subtitle. Use the border_color parameter to specify the color of the border.

• border_color

  string(default: "40000000")

  The color for the subtitle border. The first two hex digits specify the alpha value of the color.

• font

  string(default: "Arial")

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

  Here is a list of all supported fonts.

• font_color

  string | string(default: "FFFFFF")

  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

  string | number(default: 16)

  Specifies the size of the text.

• bold

  boolean(default: false)

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

• italic

  boolean(default: false)

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

• outline_width

  string | number | null

  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

  bottom | bottom-left | bottom-right | center | left | right | top | 2 more ↓(default: "bottom")

  Specifies the position of the subtitles.

• language

  string | null

  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

  string | null

  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

  boolean(default: false)

  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.

Demos

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