beta
# Generate a preview thumbnail
š¤/file/preview generates a thumbnail for any uploaded file to preview its content, similar to the thumbnails in desktop file managers.
This Robot's purpose is to generate a meaningful preview image for any file, in such a way that the resulting thumbnail highlights the file's content. The goal is not to losslessly present the original media in a smaller way. Instead, it is to maximize the chance of a person recognizing the media at a glance, while being visually pleasing and consistent with other previews. The generation process depends on the file type. For example, the Robot can extract artwork from media files, frames from videos, generate a waveform for audio files, and preview the content of documents and images. The details of all available strategies are provided in the next section.
If no file-specific thumbnail can be generated because the file type is not supported, a generic icon containing the file extension will be generated.
The default parameters ensure that the Robot always generates a preview image with the predefined dimensions and formats, to allow an easy integration into your application's UI. In addition, the generated preview images are optimized by default to reduce their file size while keeping their quality.
## Usage example
Generate a preview thumbnail for any uploaded file:
```
{
"steps": {
"previewed": {
"robot": "/file/preview",
"use": ":original",
"height": 400,
"width": 300,
"format": "png"
}
}
}
```
## Parameters
*
### `interpolate`
`boolean | Record`\
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 | boolean | Array`\
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 [BlurHashā ](https://blurha.sh) string ā 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`
`Record`(default: `{}`)\
Adds custom metadata to each file emitted by this Robot without modifying the fileās contents.\
The values are merged with any existing `user_meta` carried by the input file. If both objects contain the same key, this Robotās value takes precedence. Assembly Variables are supported, for example `{ "internal_file_id": "${file.id}" }`.
*
### `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.[š¤/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`
`boolean | Array`(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 | Array