Concatenate videos

🤖/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.

Itʼs possible to concatenate a virtually infinite number of video files using 🤖/video/concat.

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:

{
  "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 an input Step with as to pass semantic intent to robots:
- 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: "v6.0.0")
Selects the FFmpeg stack version to use for encoding. These versions reflect real FFmpeg versions. We currently recommend to use "v6.0.0". Deprecated "v5.x" values are accepted for backward compatibility.
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 |
Converts a video according to pre-configured settings.

If you specify your own FFmpeg parameters using the Robot's and/or do not not want Transloadit to set any encoding setting, starting ffmpeg_stack: "v6", you can use the value 'empty' here.
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.
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.

Enhancing FFmpeg for superior encoding performance July 30, 2015
Introducing four new Robots to enhance media processing September 24, 2015
New pricing model for future Transloadit customers February 7, 2018
No-code real-time video uploading with Bubble & Transloadit August 2, 2019

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 an input Step with as to pass semantic intent to robots:

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: "v6.0.0")

Selects the FFmpeg stack version to use for encoding. These versions reflect real FFmpeg versions. We currently recommend to use "v6.0.0". Deprecated "v5.x" values are accepted for backward compatibility.

`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`

Converts a video according to pre-configured settings.

If you specify your own FFmpeg parameters using the Robot's and/or do not not want Transloadit to set any encoding setting, starting ffmpeg_stack: "v6", you can use the value 'empty' here.

`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.

`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.

Concatenate videos

Note

Usage example

Parameters

interpolate

output_meta

result

queue

force_accept

ignore_errors

use

Tip

ffmpeg

ffmpeg_stack

width

height

preset

video_fade_seconds

audio_fade_seconds

chapter_markers

transition

transition_duration

Related blog posts

Note

Usage example

Parameters

interpolate

output_meta

result

queue

force_accept

ignore_errors

use

Tip

ffmpeg

ffmpeg_stack

width

height

preset

video_fade_seconds

audio_fade_seconds

chapter_markers

transition

transition_duration

Related blog posts

`interpolate`

`output_meta`

`result`

`queue`

`force_accept`

`ignore_errors`

`use`

`ffmpeg`

`ffmpeg_stack`

`width`

`height`

`preset`

`video_fade_seconds`

`audio_fade_seconds`

`chapter_markers`

`transition`

`transition_duration`

`interpolate`

`output_meta`

`result`

`queue`

`force_accept`

`ignore_errors`

`use`

`ffmpeg`

`ffmpeg_stack`

`width`

`height`

`preset`

`video_fade_seconds`

`audio_fade_seconds`

`chapter_markers`

`transition`

`transition_duration`