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iiif-processor

Build Status Maintainability Test Coverage

This module provides a full-featured image processor supporting the IIIF Image API versions 2.1 and 3.0. It covers only the image processing pipeline itself, leaving input and output to the caller.

Install with NPM

npm install iiif-processor --save

Usage

const IIIF = require('iiif-processor');

const processor = new IIIF.Processor(url, streamResolver, opts);
  • url (string, required) - the URL of the IIIF resource to process
  • streamResolver (function, required) – a callback function that returns a readable image stream for a given request (see below)
  • opts:
    • dimensionFunction (function) – a callback function that returns the image dimensions for a given request (see below)
    • max (object) – optional maximum size constraints of an image that can be returned
      • width (integer) - the maximum pixel width of the returned image
      • height (integer) - the maximum pixel height of the returned image
      • area (integer) - the maximum total number of pixels in the returned image
    • includeMetadata (boolean) – if true, all metadata from the source image will be copied to the result
    • density (integer) – the pixel density to be included in the result image in pixels per inch
      • This has no effect whatsoever on the size of the image that gets returned; it's simply for convenience when using the resulting image in software that calculates a default print size based on the height, width, and density
    • pathPrefix (string) – the template used to extract the IIIF version and API parameters from the URL path (default: /iiif/{{version}}/) (see below)
    • version (number) – the major version (2 or 3) of the IIIF Image API to use (default: inferred from /iiif/{version}/)

Examples

Full Self-Contained Application

See the TinyIIIF example.

Stream Resolver

The calling function must supply the processor with a Stream Resolver callback function, which takes information about the request (id and baseUrl) and returns an open Readable Stream from which the source image can be read.

Pairtree File Source

function streamResolver({ id, baseUrl }) {
  let imagePath = '/path/to/image/root/' + id.match(/.{1,2}/g).join('/') + '/image.tif';
  return fs.createReadStream(imagePath);
}

The Stream Resolver can also take an async callback as its second parameter, in which case it should return the value of applying the callback to the stream. This allows the function to do its own cleanup.

Amazon S3 Bucket Source

const AWS = require('aws-sdk');

async function streamResolver({ id, baseUrl }, callback) {
  let s3 = new AWS.S3();
  let key = id + '.tif';
  let request = s3.getObject({ Bucket: 'my-tiff-bucket', Key: key });
  let stream = request.createReadStream();
  try {
    return await callback(stream);
  } finally {
    stream.end().destroy();
    request.abort();
  }
}

Dimension Function

The calling function can also supply the processor with an optional Dimension callback that takes information about the request (id and baseUrl) and returns the dimensions of the source image. This allows for caching dimensions and avoiding an expensive image request.

The function should return either:

  • a {width: w, height: h} object indicating the dimensions of the source image
  • an array of {width: w, height: h} objects indicating the dimensions of all of the pages available within the source image, if it is a multi-resolution image (e.g., a pyramidal TIFF), e.g.: 
    [
  { width: 14499, height: 12069 },
  { width: 7249, height: 6034 },
  { width: 3624, height: 3017 },
  { width: 1812, height: 1508 },
  { width: 906, height: 754 },
  { width: 453, height: 377 },
  { width: 226, height: 188 }
]

Providing the dimensions of all available pages allows the processor to choose the most efficient starting image for the size requested.

async function dimensionFunction({ id, baseUrl }) {
  let dimensions = lookDimensionsUpInDatabase(id);
  return { width: dimensions.width, height: dimensions.height };
}

Path Prefix

The pathPrefix constructor option provides a tremendous amount of flexibility in how IIIF URLs are structured. The prefix includes one placeholder {{version}}, indicating the major version of the IIIF Image API to use when interpreting the rest of the path.

  • The pathPrefix must start and end with /.
  • The pathPrefix must include the {{version}} placeholder unless the version constructor option is specified. If both are present, the constructor option will take precedence.
  • To allow for maximum flexibility, the pathPrefix is interpreted as a JavaScript regular expression. For example, /.+?/iiif/{{version}}/ would allow your path to have arbitrary path elements before /iiif/. Be careful when including greedy quantifiers (e.g., + as opposed to +?), as they may produce unexpected results. / characters are treated as literal path separators, not regular expression delimiters as they would be in JavaScript code.

Processing

Promise

const IIIF = require('iiif-processor');

let url = 'http://iiif.example.com/iiif/2/abcdefgh/full/400,/0/default.jpg'
let processor = new IIIF.Processor(url, streamResolver, { dimensionFunction });
processor.execute()
  .then(result => handleResult(result))
  .catch(err => handleError(err));

Async/Await

const IIIF = require('iiif-processor');

let url = 'http://iiif.example.com/iiif/2/abcdefgh/full/400,/0/default.jpg'
let processor = new IIIF.Processor(url, streamResolver, { dimensionFunction });
try {
  let result = await processor.execute();
  return result;
} catch (err) {
  handleError(err);
}

id / baseUrl

The stream resolver and dimensions function functions both accept an object with id and baseUrl specified.

For instance, for the request:

https://example.org/iiif/assets/42562145-0998-4b67-bab0-6028328f8319.png/10,20,30,40/pct:50/45/default.png

The id parameter is 42562145-0998-4b67-bab0-6028328f8319.png and the baseUrl is https://example.org/iiif/assets.

Contributing

Contributions are welcome in the form of bug reports, suggestions, pull requests, and/or documentation.

If you're working on a PR for this project, create a feature branch off of main.

This project uses the debug library for selective debugging output. To view all IIIF-related debug messages, set the environment variable DEBUG=iiif-processor:*. To view just the main or transformer contexts, set DEBUG=iiif-processor:main or DEBUG=iiif-processor:transform.

This repository follows the Samvera Community Code of Conduct and language recommendations. Please do not create a branch called master for this repository or as part of your pull request; the branch will either need to be removed or renamed before it can be considered for inclusion in the code base and history of this repository.

License

node-iiif is available under the Apache 2.0 license.