Concatenate videos

🤖/video/concat concatenates several videos together.

Usage example

{
  "steps": {
    "concatenated": {
      "robot": "/video/concat",
      "use": {
        "steps": [
          {
            "name": ":original",
            "fields": "first_video_file",
            "as": "video_1"
          },
          {
            "name": ":original",
            "fields": "second_video_file",
            "as": "video_2"
          },
          {
            "name": ":original",
            "fields": "third_video_file",
            "as": "video_3"
          }
        ]
      }
    }
  }
}

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.

• video_fade_seconds

  string | number(default: 1)

  When used this adds a video fade in and out effect between each section of your concatenated video. The float value is used so if you want a video delay effect of 500 milliseconds between each video section you would select 0.5, however, integer values can also be represented.

  This parameter does not add a video fade effect at the beginning or end of your video. If you want to do so, create an additional 🤖/video/encode Step and use our ffmpeg parameter as shown in this demo.

  Please note this parameter is independent of adding audio fades between sections.

• audio_fade_seconds

  string | number(default: 1)

  When used this adds an audio fade in and out effect between each section of your concatenated video. The float value is used so if you want an audio delay effect of 500 milliseconds between each video section you would select 0.5, however, integer values can also be represented.

  This parameter does not add an audio fade effect at the beginning or end of your video. If you want to do so, create an additional [🤖/video/encode](/docs/robots/video-encode/] Step and use our ffmpeg parameter as shown in this demo.

  Please note this parameter is independent of adding video fades between sections.

• chapter_markers

  boolean(default: false)

  When set to true, the concatenated video will contain chapter markers at the positions where the input videos are joined. Each chapter will be titled with the basename of the corresponding input video file.

  This is useful for navigation in video players that support chapter-based seeking.

• sort_by

  auto | basename | import_order(default: "basename")

  Controls how bundled inputs are ordered when no explicit numbered alias such as video_1 or document_1 is used.

  The default "basename" keeps the legacy natural basename sorting behavior.

  Set this to "import_order" to preserve the order of array-based import steps when all input files carry complete import order metadata. "auto" has the same import-order preference with natural basename sorting as fallback.

• transition

  none | crossfade | fade_to_black(default: "none")

  Specifies the transition effect to apply between concatenated video clips.

  • "none" (default): No transition effect. Videos are joined end-to-end.
  • "crossfade": Applies a crossfade transition where the end of one clip gradually blends into the beginning of the next clip. Both video and audio are crossfaded.
  • "fade_to_black": Applies a fade-to-black transition where each clip fades out to black before the next clip fades in from black.

  When using "crossfade" or "fade_to_black", the transition_duration parameter controls how long the transition lasts. Note that crossfade transitions will reduce the total output duration since clips overlap during the transition.

• transition_duration

  string | number(default: 1)

  The duration of the transition effect in seconds. Only applies when transition is set to "crossfade" or "fade_to_black".

  For example, a value of 1.0 creates a 1-second transition between clips. The value can be a float for sub-second precision (e.g., 0.5 for 500 milliseconds) and must be greater than 0 whenever transitions are enabled.

  For crossfade transitions, this is the overlap duration where both clips are visible. For fade_to_black transitions, this is the total time for the fade out and fade in (half for each). The applied transition is capped at half of the shorter clip in each transition pair.

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