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fal-ai/workflow-utilities/pick-image-by-index

Choose the Nth image from an image URL list for workflows.
Inference
Commercial use
Schema
LLMs
PlaygroundAPI

Table of contents

About

Select a single image from an image_urls array by 1-based index.

Returns the URL at the requested position without fetching or re-uploading the image. The output exposes both an image object and a single-element images array so downstream nodes can reference image.url or images.0.url interchangeably.

This unblocks workflows that need to parameterize array selection beyond hardcoded position 0 references such as $node-X.images.0.url.

Out-of-range fallback: When index exceeds the length of image_urls, the response falls back to the image specified by fallback_index (default first-image) instead of failing. This lets workflow authors safely request "the nth image" without knowing the array length up front.

Validation:

  • image_urls empty: 422 "image_urls cannot be empty" (no fallback; there is nothing to pick)
  • index < 1: 422 "Index must be 1 or greater. Received: {value}"

1. Calling the API#

Install the client#

The client provides a convenient way to interact with the model API.

npm install --save @fal-ai/client

Setup your API Key#

Set FAL_KEY as an environment variable in your runtime.

export FAL_KEY="YOUR_API_KEY"

Submit a request#

The client API handles the API submit protocol. It will handle the request status updates and return the result when the request is completed.

import { fal } from "@fal-ai/client";

const result = await fal.subscribe("fal-ai/workflow-utilities/pick-image-by-index", {
  input: {
    image_urls: ["https://storage.googleapis.com/falserverless/example_inputs/ltxv-image-input.jpg", "https://storage.googleapis.com/falserverless/example_inputs/wan_s2v_cat.png"],
    index: 1
  },
  logs: true,
  onQueueUpdate: (update) => {
    if (update.status === "IN_PROGRESS") {
      update.logs.map((log) => log.message).forEach(console.log);
    }
  },
});
console.log(result.data);
console.log(result.requestId);

2. Authentication#

The API uses an API Key for authentication. It is recommended you set the FAL_KEY environment variable in your runtime when possible.

API Key#

In case your app is running in an environment where you cannot set environment variables, you can set the API Key manually as a client configuration.
import { fal } from "@fal-ai/client";

fal.config({
  credentials: "YOUR_FAL_KEY"
});

3. Queue#

Submit a request#

The client API provides a convenient way to submit requests to the model.

import { fal } from "@fal-ai/client";

const { request_id } = await fal.queue.submit("fal-ai/workflow-utilities/pick-image-by-index", {
  input: {
    image_urls: ["https://storage.googleapis.com/falserverless/example_inputs/ltxv-image-input.jpg", "https://storage.googleapis.com/falserverless/example_inputs/wan_s2v_cat.png"],
    index: 1
  },
  webhookUrl: "https://optional.webhook.url/for/results",
});

Fetch request status#

You can fetch the status of a request to check if it is completed or still in progress.

import { fal } from "@fal-ai/client";

const status = await fal.queue.status("fal-ai/workflow-utilities/pick-image-by-index", {
  requestId: "764cabcf-b745-4b3e-ae38-1200304cf45b",
  logs: true,
});

Get the result#

Once the request is completed, you can fetch the result. See the Output Schema for the expected result format.

import { fal } from "@fal-ai/client";

const result = await fal.queue.result("fal-ai/workflow-utilities/pick-image-by-index", {
  requestId: "764cabcf-b745-4b3e-ae38-1200304cf45b"
});
console.log(result.data);
console.log(result.requestId);

4. Files#

Some attributes in the API accept file URLs as input. Whenever that's the case you can pass your own URL or a Base64 data URI.

Data URI (base64)#

You can pass a Base64 data URI as a file input. The API will handle the file decoding for you. Keep in mind that for large files, this alternative although convenient can impact the request performance.

Hosted files (URL)#

You can also pass your own URLs as long as they are publicly accessible. Be aware that some hosts might block cross-site requests, rate-limit, or consider the request as a bot.

Uploading files#

We provide a convenient file storage that allows you to upload files and use them in your requests. You can upload files using the client API and use the returned URL in your requests.

import { fal } from "@fal-ai/client";

const file = new File(["Hello, World!"], "hello.txt", { type: "text/plain" });
const url = await fal.storage.upload(file);

Read more about file handling in our file upload guide.

5. Schema#

Input#

image_urls list<string>* required

Array of image URLs to select from

index integer* required

1-based position of the image to return. Value 1 returns the first image, 2 returns the second, and so on.

fallback_index FallbackIndexEnum

Behavior when index exceeds the length of image_urls. When first-image, returns the first image in the array. When last-image, returns the last image in the array. This input has no effect when index is within range, when index is below 1, or when image_urls is empty (the latter two cases still return validation errors). Default is first-image. Default value: "first-image"

Possible enum values: first-image, last-image

{
  "image_urls": [
    "https://storage.googleapis.com/falserverless/example_inputs/ltxv-image-input.jpg",
    "https://storage.googleapis.com/falserverless/example_inputs/wan_s2v_cat.png"
  ],
  "index": 1,
  "fallback_index": "first-image"
}

Output#

image Image* required

The image at the requested position

images list<Image>* required

Single-element array containing the selected image. Included so downstream nodes that expect an images reference (matching merge and other utilities) work without changes.

{
  "image": {
    "url": "https://storage.googleapis.com/falserverless/example_inputs/ltxv-image-input.jpg"
  },
  "images": [
    {
      "url": "https://storage.googleapis.com/falserverless/example_inputs/ltxv-image-input.jpg"
    }
  ]
}

Other types#

AudioFile#

url string* required

URL of the audio file

content_type string* required

Content type of the audio file

file_name string* required

Name of the audio file

file_size integer* required

Size of the audio file in bytes

EQBand#

frequency float* required

Center frequency in Hz (20-20000)

width float

Bandwidth in Hz Default value: 100

gain float* required

Gain in dB (-30 to 30, negative = cut, positive = boost)

Image#

url string* required

The URL where the file can be downloaded from.

content_type string

The mime type of the file.

file_name string

The name of the file. It will be auto-generated if not provided.

file_size integer

The size of the file in bytes.

width integer

The width of the image in pixels.

height integer

The height of the image in pixels.

SubtitleSegment#

start float* required

Start time in seconds (e.g., 0.0 for beginning, 5.5 for 5.5 seconds)

end float* required

End time in seconds (must be greater than start time)

text string* required

Subtitle text to display (supports multiple lines with )

SceneChangeInfo#

frame_number integer* required

Frame number of the scene change

pts integer* required

Presentation timestamp in stream time base units

timestamp float* required

Timestamp of the scene change in seconds

score float* required

Scene change score (0-1). Higher values indicate a bigger visual change between frames.

File#

url string* required

The URL where the file can be downloaded from.

content_type string

The mime type of the file.

file_name string

The name of the file. It will be auto-generated if not provided.

file_size integer

The size of the file in bytes.