Scans and indexes the specified PNG files, and and yields a reader-writer object with the following properties:

• length - Total number of bytes that can be hidden in the PNG files.
• getByte()
• setByte()
• write()
• read()
• save()

See below for documentation of the read-write functions.

Arguments

• pngFileNames - An array of file paths. Each file path must be for an existing valid PNG file.
• callback - function(err, stash). If the PNGs file were successfully indexed, err will be undefined. stash will be reader-writer object mentioned in description above.

Example

var pngStashMux = require('png-stash-mux');

pngStashMux(["test1.png", "test2.png"], function(err, stash) {
  console.log("Available bytes: " + stash.length);
});

Reads a single byte from the "invisible" bits of the PNG files.

May trigger a save() of previously changed data. See "Notes" below.

Arguments

• index - Position among all bytes composed by the PNGs' "invisible" bits. Must be less than the pngStashMux instance's length property.
• callback - function(err, byte). err will be undefined if read was successful. Otherwise it will represent an error. byte is the read value.

Example

var pngStashMux = require('png-stash-mux');

pngStashMux(["frame0.png", "frame1.png", "frame2.png"], function(err, stash) {
  stash.getByte(1000, function(err, b1) {
  stash.getByte(2000, function(err, b2) {
  stash.getByte(3000, function(err, b3) {
    if (b1 == 0xaa && b2 == 0x44 && b3 == 0xff) {
      console.log("Watermark detected!");
    } else {
      console.log("No watermark detected.");
    }
  })})});
});

Writes a single byte to the "invisible" bits of the PNGs, but does not immediately save it to disk. To save, call save().

May trigger a save() of previously changed data. See "Notes" below.

Arguments

• index - Position among all bytes composed by the PNGs' "invisible" bits. Must be less than the pngStash instance's length property.
• value - Byte to store. Must be an integer in the 8-bit range (0 < value < 256).
• callback - function(err). err will be undefined if save was successful. Otherwise it will represent an error.

Example

var pngStashMux = require('png-stash-mux');

pngStashMux(["frame0.png", "frame1.png", "frame2.png"], function(err, stash) {
  stash.setByte(1000, 0xaa);
  stash.setByte(2000, 0x44);
  stash.setByte(3000, 0xff);
  stash.save(function(err) {
    console.log(err || "Watermark inserted.");
  });
});

Reads a sequence of bytes from the PNGs' "invisible" bits, and returns them as a Buffer.

May trigger a save() of previously changed data. See "Notes" below.

Arguments

• offset - default=0. At which byte position to start reading from.
• length - default=stash.length. How many bytes to read.
• callback - function(err, data). err will be undefined if save was successful. Otherwise it will represent an error. data is all requested bytes in a Buffer.

Example

var pngStashMux = require('png-stash-mux');

pngStashMux(["bunny.png", "kitten.png"], function(err, stash) {
  var messageLength = stash.getByte(0) * 65536
                    + stash.getByte(1) * 256
                    + stash.getByte(2);
  var message = stash.read(messageLength, 3);
  console.log("Found message: " + message);
});

Writes a sequence of bytes to the PNGs' "invisible" bits, but does not save to disk. To save to disk, call save().

May trigger a save() of previously changed data. See "Notes" below.

Arguments

• data - Bytes to store in the PNGs. Must be string or Buffer. If string, will be UTF-8 encoded.
• offset - default=0. At which byte position to start writing to.
• length - default=all. How many bytes from data to write. Be aware that the .length of a string may not correspond to the number of bytes in its resulting UTF-8 representation.
• callback - function(err). err will be undefined if save was successful. Otherwise it will represent an error.

Example

var pngStashMux = require('png-stash-mux');

pngStash(["bunny.png", "kitten.png"], function(err, stash) {
  var message = new Buffer("Hello there!");
  stash.setByte(0, message.length >> 16 & 0xff);
  stash.setByte(1, message.length >>  8 & 0xff);
  stash.setByte(2, message.length >>  0 & 0xff);
  stash.write(message, 3);
});

Stores the PNG data back into the files. You want to call this function after writing data to the stash instance.

May be triggered sproadically as a result of other operations. See "Notes" below for details.

Arguments

• callback - function(err). err will be undefined if save was successful. Otherwise it will represent an error.

Example

// See examples for stash.write() and stash.setByte().