lerc

What is lerc?

The 'lerc' npm package is used for encoding and decoding LERC (Limited Error Raster Compression) files. LERC is a format developed by Esri for compressing raster data with controlled loss, making it suitable for geospatial applications where precision is important.

What are lerc's main functionalities?

Decode LERC data

This feature allows you to decode LERC data from a file. The decoded data can then be used for further processing or analysis.

const lerc = require('lerc');
const fs = require('fs');

// Read LERC file
const lercData = fs.readFileSync('path/to/lerc/file.lerc');

// Decode LERC data
const decodedData = lerc.decode(lercData);
console.log(decodedData);

Encode data to LERC format

This feature allows you to encode raster data into the LERC format. The encoded data can then be saved to a file for storage or transmission.

const lerc = require('lerc');
const fs = require('fs');

// Example data to encode
const data = {
  width: 256,
  height: 256,
  pixels: new Float32Array(256 * 256)
};

// Encode data to LERC format
const encodedData = lerc.encode(data);

// Write encoded data to file
fs.writeFileSync('path/to/output/file.lerc', encodedData);

Other packages similar to lerc

geotiff

The 'geotiff' package is used for reading and writing GeoTIFF files, which are a common format for geospatial raster data. Unlike LERC, GeoTIFF does not focus on compression but rather on storing georeferenced raster data. It is more versatile in terms of metadata and geospatial information but may result in larger file sizes.

pngjs

The 'pngjs' package is used for encoding and decoding PNG images. While PNG is a lossless compression format, it is not specifically designed for geospatial data. It is more general-purpose and widely supported but may not offer the same level of control over compression errors as LERC.

jpeg-js

The 'jpeg-js' package is used for encoding and decoding JPEG images. JPEG is a lossy compression format that is widely used for photographic images. It is not suitable for applications requiring precise control over compression errors, making it less ideal for geospatial data compared to LERC.

README

Lerc JS

Rapid decoding of Lerc compressed raster data for any standard pixel type, not just rgb or byte.

Breaking changes

• Web Assembly support is now required.
• Lerc.load() must be invoked and the returned promise must be resolved prior to Lerc.decode. This only needs to be done once per worker (or the main thread). There's no extra cost when invoked multiple times as the internal wasm loading promise is cached.

Get started

npm install lerc

// es module
import * as Lerc from 'lerc';

// commonJS
const Lerc = require('lerc');

// use umd via a script tag
<script type="text/javascript" src="https://unpkg.com/lerc@latest/LercDecode.min.js"></script>

Sample usage

await Lerc.load();

const arrayBuffer = await fetch('http://elevation3d.arcgis.com/arcgis/rest/services/WorldElevation3D/Terrain3D/ImageServer/tile/0/0/0')
  .then(response => response.arrayBuffer());
const pixelBlock = Lerc.decode(arrayBuffer);
const { height, width, pixels, mask } = pixelBlock;
for (let i = 0; i < height; i++) {
  for (let j = 0; j < width; j++) {
    if (!mask || mask[i * width + j]) {
      // do something with valid pixel (i,j)
    }
  }
}

// use options
const pixelBlock = Lerc.decode(arrayBuffer, {
  inputOffset: 10, // start from the 10th byte (default is 0)
  returnInterleaved: true // only applicable to n-depth lerc blobs (default is false)
});

API Reference

Lerc

A module for decoding LERC blobs.

load([options]) ⇒ Promise ⏏

Load the dependencies (web assembly). Check whether dependencies has been loaded using Lerc.isLoaded(). The loading promise is cached so it can be invoked multiple times if needed.

Kind: Exported function

Param	Type	Description
[options.locateFile]	(wasmFileName?: string, scriptDir?: string) => string	The function to locate lerc-wasm.wasm. Used when the web assembly file is moved to a different location.

decode(input, [options]) ⇒ Object ⏏

A function for decoding both LERC1 and LERC2 byte streams capable of handling multiband pixel blocks for various pixel types.

Kind: Exported function

Param	Type	Description
input	ArrayBuffer	The LERC input byte stream
[options]	object	The decoding options below are optional.
[options.inputOffset]	number	The number of bytes to skip in the input byte stream. A valid Lerc file is expected at that position.
[options.noDataValue]	number	It is recommended to use the returned mask instead of setting this value.
(Deprecated) [options.returnPixelInterleavedDims]	boolean	will be removed in next release, use returnInterleaved instead.
[options.returnInterleaved]	boolean	(ndepth LERC2 only) If true, returned depth values are pixel-interleaved.

Result Object Properties

Name	Type	Description
width	number	Width of decoded image.
height	number	Height of decoded image.
pixels	array	[band1, band2, …] Each band is a typed array of width * height * depthCount.
pixelType	string	The type of pixels represented in the output.
mask	mask	Typed array with a size of width*height, or null if all pixels are valid.
statistics	array	[statistics_band1, statistics_band2, …] Each element is a statistics object representing min and max values
(Deprecated) dimCount	number	Will be removed in next release, use depthCount instead.
depthCount	number	Depth count
[bandMasks]	array	[band1_mask, band2_mask, …] Each band is a Uint8Array of width * height * depthCount.

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Licensing

Copyright © 2017-2023 Esri

Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at

http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.

A local copy of the license and additional notices are located with the source distribution at:

http://github.com/Esri/lerc/