pngjs/lib/filter-parse.js

PNG encoder/decoder in pure JS, supporting any bit size & interlace, async & sync with full test suite.

"use strict";

let interlaceUtils = require("./interlace");
let paethPredictor = require("./paeth-predictor");

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

let Filter = (module.exports = function (bitmapInfo, dependencies) {
  let width = bitmapInfo.width;
  let height = bitmapInfo.height;
  let interlace = bitmapInfo.interlace;
  let bpp = bitmapInfo.bpp;
  let depth = bitmapInfo.depth;

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

  this._imageIndex = 0;
  this._images = [];
  if (interlace) {
    let passes = interlaceUtils.getImagePasses(width, height);
    for (let 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
) {
  let xComparison = this._xComparison;
  let xBiggerThan = xComparison - 1;

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

Filter.prototype._unFilterType2 = function (
  rawData,
  unfilteredLine,
  byteWidth
) {
  let lastLine = this._lastLine;

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

Filter.prototype._unFilterType3 = function (
  rawData,
  unfilteredLine,
  byteWidth
) {
  let xComparison = this._xComparison;
  let xBiggerThan = xComparison - 1;
  let lastLine = this._lastLine;

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

Filter.prototype._unFilterType4 = function (
  rawData,
  unfilteredLine,
  byteWidth
) {
  let xComparison = this._xComparison;
  let xBiggerThan = xComparison - 1;
  let lastLine = this._lastLine;

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

Filter.prototype._reverseFilterLine = function (rawData) {
  let filter = rawData[0];
  let unfilteredLine;
  let currentImage = this._images[this._imageIndex];
  let byteWidth = currentImage.byteWidth;

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