# Concatenate videos

Robot: `/video/concat`

🤖/video/concat concatenates several videos together.

> [!Note]
> Input videos may have differing dimensions and streams - the Robot can handle this fine. It will pre-transcode the input videos if necessary before concatenation at no additional cost.

It’s possible to concatenate a virtually infinite number of video files using [🤖/video/concat](/docs/robots/video-concat.md).

Stage: ga

## Usage example

If you have a form with 3 file input fields and want to concatenate the uploaded videos in a specific order, instruct Transloadit using the \`name\` attribute of each input field. Use this attribute as the value for the \`fields\` key in the JSON, and set \`as\` to \`video\_\[\[index]]\`. Transloadit will concatenate the files based on the ascending index order:

```json
{
  "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`: 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.

* `video_fade_seconds`: 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](/docs/robots/video-encode.md) Step and use our `ffmpeg` parameter as shown in this [demo](/demos/video-encoding/concatenate-fade-effect.md).

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

* `audio_fade_seconds`: 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.md) Step and use our `ffmpeg` parameter as shown in this [demo](/demos/audio-encoding/ffmpeg-fade-in-and-out.md).

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

* `chapter_markers`: 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`: Controls how bundled inputs are ordered when no explicit numbered alias for the input type is used. Numbered aliases end with a numeric suffix such as `_1`.

  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`: 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`: 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.