react-native-flip/example/node_modules/pngjs/lib/filter-parse.js

<div align="center">

'use strict';

var interlaceUtils = require('./interlace');
var paethPredictor = require('./paeth-predictor');

function getByteWidth(width, bpp, depth) {
  var byteWidth = width * bpp;
  if (depth !== 8) {
    byteWidth = Math.ceil(byteWidth / (8 / depth));
  }
  return byteWidth;
}

var Filter = module.exports = function(bitmapInfo, dependencies) {

  var width = bitmapInfo.width;
  var height = bitmapInfo.height;
  var interlace = bitmapInfo.interlace;
  var bpp = bitmapInfo.bpp;
  var depth = bitmapInfo.depth;

  this.read = dependencies.read;
  this.write = dependencies.write;
  this.complete = dependencies.complete;

  this._imageIndex = 0;
  this._images = [];
  if (interlace) {
    var passes = interlaceUtils.getImagePasses(width, height);
    for (var i = 0; i < passes.length; i++) {
      this._images.push({
        byteWidth: getByteWidth(passes[i].width, bpp, depth),
        height: passes[i].height,
        lineIndex: 0
      });
    }
  }
  else {
    this._images.push({
      byteWidth: getByteWidth(width, bpp, depth),
      height: height,
      lineIndex: 0
    });
  }

  // when filtering the line we look at the pixel to the left
  // the spec also says it is done on a byte level regardless of the number of pixels
  // so if the depth is byte compatible (8 or 16) we subtract the bpp in order to compare back
  // a pixel rather than just a different byte part. However if we are sub byte, we ignore.
  if (depth === 8) {
    this._xComparison = bpp;
  }
  else if (depth === 16) {
    this._xComparison = bpp * 2;
  }
  else {
    this._xComparison = 1;
  }
};

Filter.prototype.start = function() {
  this.read(this._images[this._imageIndex].byteWidth + 1, this._reverseFilterLine.bind(this));
};

Filter.prototype._unFilterType1 = function(rawData, unfilteredLine, byteWidth) {

  var xComparison = this._xComparison;
  var xBiggerThan = xComparison - 1;

  for (var x = 0; x < byteWidth; x++) {
    var rawByte = rawData[1 + x];
    var f1Left = x > xBiggerThan ? unfilteredLine[x - xComparison] : 0;
    unfilteredLine[x] = rawByte + f1Left;
  }
};

Filter.prototype._unFilterType2 = function(rawData, unfilteredLine, byteWidth) {

  var lastLine = this._lastLine;

  for (var x = 0; x < byteWidth; x++) {
    var rawByte = rawData[1 + x];
    var f2Up = lastLine ? lastLine[x] : 0;
    unfilteredLine[x] = rawByte + f2Up;
  }
};

Filter.prototype._unFilterType3 = function(rawData, unfilteredLine, byteWidth) {

  var xComparison = this._xComparison;
  var xBiggerThan = xComparison - 1;
  var lastLine = this._lastLine;

  for (var x = 0; x < byteWidth; x++) {
    var rawByte = rawData[1 + x];
    var f3Up = lastLine ? lastLine[x] : 0;
    var f3Left = x > xBiggerThan ? unfilteredLine[x - xComparison] : 0;
    var f3Add = Math.floor((f3Left + f3Up) / 2);
    unfilteredLine[x] = rawByte + f3Add;
  }
};

Filter.prototype._unFilterType4 = function(rawData, unfilteredLine, byteWidth) {

  var xComparison = this._xComparison;
  var xBiggerThan = xComparison - 1;
  var lastLine = this._lastLine;

  for (var x = 0; x < byteWidth; x++) {
    var rawByte = rawData[1 + x];
    var f4Up = lastLine ? lastLine[x] : 0;
    var f4Left = x > xBiggerThan ? unfilteredLine[x - xComparison] : 0;
    var f4UpLeft = x > xBiggerThan && lastLine ? lastLine[x - xComparison] : 0;
    var f4Add = paethPredictor(f4Left, f4Up, f4UpLeft);
    unfilteredLine[x] = rawByte + f4Add;
  }
};

Filter.prototype._reverseFilterLine = function(rawData) {

  var filter = rawData[0];
  var unfilteredLine;
  var currentImage = this._images[this._imageIndex];
  var byteWidth = currentImage.byteWidth;

  if (filter === 0) {
    unfilteredLine = rawData.slice(1, byteWidth + 1);
  }
  else {

    unfilteredLine = new Buffer(byteWidth);

    switch (filter) {
      case 1:
        this._unFilterType1(rawData, unfilteredLine, byteWidth);
        break;
      case 2:
        this._unFilterType2(rawData, unfilteredLine, byteWidth);
        break;
      case 3:
        this._unFilterType3(rawData, unfilteredLine, byteWidth);
        break;
      case 4:
        this._unFilterType4(rawData, unfilteredLine, byteWidth);
        break;
      default:
        throw new Error('Unrecognised filter type - ' + filter);
    }
  }

  this.write(unfilteredLine);

  currentImage.lineIndex++;
  if (currentImage.lineIndex >= currentImage.height) {
    this._lastLine = null;
    this._imageIndex++;
    currentImage = this._images[this._imageIndex];
  }
  else {
    this._lastLine = unfilteredLine;
  }

  if (currentImage) {
    // read, using the byte width that may be from the new current image
    this.read(currentImage.byteWidth + 1, this._reverseFilterLine.bind(this));
  }
  else {
    this._lastLine = null;
    this.complete();
  }
};