dwv/decoders/dwv/rle.js

DICOM Web Viewer.

// namespaces
// (do not use dwv since it is the exported module name)
var dwvdecoder = dwvdecoder || {};

/**
 * RLE (Run-length encoding) decoder class.
 * @constructor
 */
dwvdecoder.RleDecoder = function () {};

/**
 * Decode a RLE buffer.
 * @param {Array} buffer The buffer to decode.
 * @param {Number} bitsAllocated The bits allocated per element in the buffer.
 * @param {Boolean} isSigned Is the data signed.
 * @param {Number} sliceSize The size of a slice
    (number of rows per number of columns).
 * @param {Number} samplesPerPixel The number of samples per pixel (3 for RGB).
 * @param {Number} planarConfiguration The planar configuration.
 * @returns The decoded buffer.
 * @see http://dicom.nema.org/dicom/2013/output/chtml/part05/sect_G.3.html
 */
dwvdecoder.RleDecoder.prototype.decode = function (buffer,
  bitsAllocated, isSigned, sliceSize, samplesPerPixel, planarConfiguration) {

  // bytes per element
  var bpe = bitsAllocated / 8;

  // input
  var inputDataView = new DataView(buffer.buffer, buffer.byteOffset);
  var inputArray = new Int8Array(buffer.buffer, buffer.byteOffset);
  // output
  var outputBuffer = new ArrayBuffer(sliceSize * samplesPerPixel * bpe);
  var outputArray = new Int8Array(outputBuffer);

  // first value of the RLE header is the number of segments
  var numberOfSegments = inputDataView.getInt32(0, true);

  // index increment in output array
  var outputIndexIncrement = 1;
  var incrementFactor = 1;
  if (samplesPerPixel !== 1 && planarConfiguration === 0) {
    incrementFactor *= samplesPerPixel;
  }
  if (bpe !== 1) {
    incrementFactor *= bpe;
  }
  outputIndexIncrement *= incrementFactor;

  // loop on segments
  var outputIndex = 0;
  var inputIndex = 0;
  var remainder = 0;
  var maxOutputIndex = 0;
  var groupOutputIndex = 0;
  for (var segment = 0; segment < numberOfSegments; ++segment) {
    // handle special cases:
    // - more than one sample per pixel: one segment per channel
    // - 16bits: sort high and low bytes
    if (incrementFactor !== 1) {
      remainder = segment % incrementFactor;
      if (remainder === 0) {
        groupOutputIndex = maxOutputIndex;
      }
      outputIndex = groupOutputIndex + remainder;
      // 16bits data
      if (bpe === 2) {
        outputIndex += (remainder % bpe ? -1 : 1);
      }
    }

    // RLE header: list of segment sizes
    var segmentStartIndex = inputDataView.getInt32((segment + 1) * 4, true);
    var nextSegmentStartIndex = inputDataView.getInt32((segment + 2) * 4, true);
    if (segment === numberOfSegments - 1 || nextSegmentStartIndex === 0) {
      nextSegmentStartIndex = buffer.length;
    }
    // decode segment
    inputIndex = segmentStartIndex;
    var count = 0;
    while (inputIndex < nextSegmentStartIndex) {
      // get the count value
      count = inputArray[inputIndex];
      ++inputIndex;
      // store according to count
      if (count >= 0 && count <= 127) {
        // output the next count+1 bytes literally
        for (var i = 0; i < count + 1; ++i) {
          // store
          outputArray[outputIndex] = inputArray[inputIndex];
          // increment indexes
          ++inputIndex;
          outputIndex += outputIndexIncrement;
        }
      } else if (count <= -1 && count >= -127) {
        // output the next byte -count+1 times
        var value = inputArray[inputIndex];
        ++inputIndex;
        for (var j = 0; j < -count + 1; ++j) {
          // store
          outputArray[outputIndex] = value;
          // increment index
          outputIndex += outputIndexIncrement;
        }
      }
    }

    if (outputIndex > maxOutputIndex) {
      maxOutputIndex = outputIndex;
    }
  }

  var decodedBuffer = null;
  if (bitsAllocated === 8) {
    if (isSigned) {
      decodedBuffer = new Int8Array(outputBuffer);
    } else {
      decodedBuffer = new Uint8Array(outputBuffer);
    }
  } else if (bitsAllocated === 16) {
    if (isSigned) {
      decodedBuffer = new Int16Array(outputBuffer);
    } else {
      decodedBuffer = new Uint16Array(outputBuffer);
    }
  }

  return decodedBuffer;
};