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image-blob-reduce - downscale blobs with images inside

Wrapper for pica to work with blobs, with some sugar.

This is pica wrapper for convenient work with images from file input fields. While pica works with raw bitmaps, this package operates with "image files". Additional features are:

• [jpeg] Apply orientation to downscaled result.
• [jpeg] Keep metadata, but with patched orientation & removed original color profile.
• Easy to monkey-patch for your needs.

Demo

Install

npm install image-blob-reduce

Known issues

This package is not compaible with some minification options. If you use terser, disable evaluate or all compress options:

• { compress: { evaluate: false } }
• { compress: false }

This should not cause notable assets increase

Usage

const reduce = require('image-blob-reduce')();

//...

reduce
  .toBlob(image_blob, { max: 1000 })
  .then(blob => { ... });

If you load prebuild script in browser, use window.ImageBlobReduce

API

new ImageBlobReduce([options])

Create new reducer. Options:

• pica - instance of pica, if you wish different defaults or shareable webworkers pool.

Short call: require('image_blob_reduce')()

.toBlob(in_blob, options) => Promise(out_blob)

Downscale image to fit into max*max size. If blob contains jpeg, then orientation is applied and metadata from original image reused (with minimal change).

Options:

• max - max allowed image size.
• pica .resize() options - alpha, unsharpAmount, unsharpRadius, unsharpThreshold, cancelToken

.toCanvas(in_blob, options) => Promise(out_canvas)

The same as .toBlob(), but with canvas output.

.before(method_name, hook_fn)

Inject your custom handler before specified method. See .init() source code for example.

.after(method_name, hook_fn)

The same as .before(), but handler is injected after specified method.

.use(plugin_init, ...params) => this

Sugar to simplify assign of external plugins. Just calls plugin_init(this, ...params).

.utils

require('./lib/utils'), to simplify modifications.

Reexports

• ImageBlobReduce.pica => require('pica') - useful to customize pica options.

Customization

Since it's difficult to implement all possible options, this package is specially designed for easy customization. See source code first.

• You can inherit class & replace existing methods.
• You can add extra actions before/after existing method.
• You can override existing methods of instance.

For example, if you wish force output to be always jpeg with some quality:

const reducer = require('image-blob-reduce')();

reducer._create_blob = function (env) {
  return this.pica.toBlob(env.out_canvas, 'image/jpeg', 0.8)
    .then(function (blob) {
      env.out_blob = blob;
      return env;
    });
};

Or rewrite scaling logic, introducing min option instead:

const reducer = require('image-blob-reduce')();

reducer._calculate_size = function (env) {
  const scale_factor = env.opts.min / Math.min(env.image.width, env.image.height);

  if (scale_factor > 1) scale_factor = 1;

  env.transform_width = Math.max(Math.round(env.image.width * scale_factor), 1);
  env.transform_height = Math.max(Math.round(env.image.height * scale_factor), 1);
  
  return env;
};