@voxelmed/charlsjs

charls-js

JS/WebAssembly build of CharLS

Try It Out!

Try it in your browser here

Install

Install this in your JavaScript project using npm:

# NOTE - this is not published yet so won't work yet...
#npm install --save-dev charls-js 

Usage

Before using this library, you must wait for it to be initialized:

const charls = require('charlsjs')
charls.onRuntimeInitialized = async _ => {
    // Now you can use it
}

To decode a JPEG-LS image, create a decoder instance, copy the JPEG-LS bitstream into its memory space, decode it, copy the decoded pixels out of its memory space and finally, delete the decoder instance.

function decode(jpeglsEncodedBitStream) {
  // Create a decoder instance
  const decoder = new charls.JpegLSDecoder();
  
  // get pointer to the source/encoded bit stream buffer in WASM memory
  // that can hold the encoded bitstream
  const encodedBufferInWASM = decoder.getEncodedBuffer(jpeglsEncodedBitStream.length);
  
  // copy the encoded bitstream into WASM memory buffer
  encodedBufferInWASM.set(jpeglsEncodedBitStream);
  
  // decode it
  decoder.decode();
  
  // get information about the decoded image
  const frameInfo = decoder.getFrameInfo();
  const interleaveMode = decoder.getInterleaveMode();
  const nearLossless = decoder.getNearLossless();
  
  // get the decoded pixels
  const decodedPixelsInWASM = decoder.getDecodedBuffer();
  
  // TODO: do something with the decoded pixels here (e.g. display them)
  // The pixel arrangement for color images varies depending upon the
  // interleaveMode parameter, see documentation in JpegLSDecode::getInterleaveMode()
  
  // delete the instance.  Note that this frees up memory including the
  // encodedBufferInWASM and decodedPixelsInWASM invalidating them. 
  // Do not use either after calling delete!
  decoder.delete();
}

const jpeglsEncodedBitStream = // read from file, load from URL, etc
decode(jpeglsEncodedBitStream)

See examples for browsers and nodejs. Also read the API documentation for JpegLSDecoder.hpp and JpegLSEncoder.hpp

Building

See information about building here

Design

Read about the design considerations that went into this library here

Performance

Read about the encode/decode performance of this library with NodeJS 14, Google Chrome and FireFox vs Native here