Adding and reading png iTXt data with streams
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png-itxt

Windows Mac/Linux
Windows Build status Build Status

js-standard-style

Tool for adding and reading textual data in PNG images using streams. All three textual chunks (iTXt, zTXt and tEXt) can be both read and written by the tool. Chunks can be filtered by chunk type and keyword as required. Compressing and decompressing of data, where appropriate, is handled transparently to the user so you only ever see the uncompressed values.

Three different methods are provided for using the tool:

  • command line
  • client side browser (thanks to browserify)
  • node library (via require)

Install with

npm install png-itxt

Using in Node

To use the tool in your node programs you must first require the module. If installed with npm then you could write the following.

var pngitxt = require('png-itxt')

Constants

The module exports constants for the types of the textual chunks. These can be accessed as follows.

  pngitxt.iTXt  // type for iTXt chunks
  pngitxt.zTXt  // type for zTXt chunks
  pngitxt.tEXt  // type for tEXt chunks

Data Format

Chunk data should be provided and will be returned as an object as shown below. Note that this is the full declaration for a iTXt chunk as will be produced by the program. However not all fields are required when passing information to the program. Additionally some of these fields are not relevant to zTXt and tEXt chunks.

{
  type: 'iTXt',
  keyword: 'keyword',
  value: 'value',
  language: '',
  translated: '',
  compressed: false,
  compression_type: 0
}

See the table below for details of which fields are relevent to each chunk and what their default values are.

Field Name Chunks Default
type ALL iTXt
keyword ALL None - must be specified for set
value ALL None - must be specified for set
lanuage iTXt empty string
translated iTXt empty string
compressed iTXt, zTXt false apart from zTXt chunks
compression_type iTXt, zTXt 0 (only valid value)

set - Writing iTXt data

The set function can be used to write new textual chunks to a image or replace existing ones. The following example shows how a new iTXt chunk can be added with the keyword pizza and the value delicious. While only these two options are specified here any of the other fields identified earlier can be specified. If compressed is set to true then the value will be compressed before it is written to the image.

fs.createReadStream('input.png')
  .pipe(pngitxt.set( { keyword: 'pizza', value: 'delicious' } ))
  .pipe(fs.createWriteStream('output.png'))

The set function will overwrite all the chunks of the same type and the keyword. In the previous example is there were already iTXt chunks with the keyword pizza (the spec allows for more than one chunk with the same keyword) then the values stored in those chunks would be lost.

It is also possible to have different types of textual chunk with the same keyword. An option is provided to allow you to replace all textual chunks with the same keyword regardless of their type. To do this specify the value true as the second parameter to the function.

fs.createReadStream('input.png')
  .pipe(pngitxt.set( { keyword: 'pizza', value: 'delicious' }, true ))
  .pipe(fs.createWriteStream('output.png'))

In this case if there were already any iTXt, zTXT or tEXt chunks with the keyword pizza then they would both be replaced by a single iTXt chunk with the new value.

Using set to Remove Chunks

In the special case where you pass null as the value to be stored then all chunks that would normally have been replaced will simply be removed. For example the following example would result in all the textual chunks with the keyword pizza being removed from the image.

fs.createReadStream('input.png')
  .pipe(pngitxt.set( { keyword: 'pizza', value: null }, true ))
  .pipe(fs.createWriteStream('output.png'))

Exceptions

An exception will be thrown if you pass in an unknown chunk type.

// This will cause an exception to be thrown.
fs.createReadStream('input.png')
  .pipe(pngitxt.set( { type: 'causeanexception',
        keyword: 'pizza', value: 'delicious' } ))
  .pipe(fs.createWriteStream('output.png'))

get - Reading iTXt data

Retrieval of information is achieved using the get function and callbacks. The callback will be invoked each time a matching chunk is found. It cannot be assumed that callback will only be called once for a particular keyword. You are guaranteed that your callback will be called at least once even if no matching chunks are found - in that case a null is provided instead of data. By default the get function will return any textual chunks that match the criteria provided. If you just want one type of chunk (e.g iTXt) then see the section of filters.

Callback Signature

The callback to the get function is given two parameters. The first is used to indicate whether or not an error was encountered. The second returns the data found in the format outlined above. In the case of an error the data provided may not be null. For example if the error was caused when trying to inflate a compressed value then all the other information collected about the chunk will be returned with the error.

function pngtxtCallback (err, data) {
  if (!err && data) {
    console.log(data.keyword, ":", data.value)
  }
}

Finding a specific keyword

To find the text associated with a specific keyword you must call get and provide both the keyword and a callback. Passing null as the keyword will cause all textual chunks to be passed to the callback. If the keyword is not found the callback will be called with null for both parameters. Note as mentioned previously this example will return any textual chunk that has the specified keyword. If you just want a specific type of chunk then see the section on filters below.

fs.createReadStream('input.png')
  .pipe(pngitxt.get('pizza', function (err, data) {
    if (!err && data) {
      console.log(data.keyword, ":", data.value)
    }
  }))

Using Regular Expressions

Instead of providing a keyword you can provide a regular expression and all chunks whose keywords match the regular expression will be passed to the callback function. If no chunks are found then the callback will be called once with null for both parameters.

fs.createReadStream('input.png')
  .pipe(pngitxt.get(/Pi[z]{2}a/i, function (err, data) {
    if (!err && data) {
      console.log(data.keyword, ":", data.value)
    }
  }))

Finding all textual chunks

To find all chunks simply provide a callback that takes two parameters. If no textual chunks are found the callback will be called once with null for both parameters. Otherwise it will be called for each block.

fs.createReadStream('input.png')
  .pipe(pngitxt.get(function (err, data) {
    if (!err && data) {
      console.log(data.keyword, ":", data.value)
    }
  }))

Filtering by Chunk Type

To filter the chunks by type you can provide an array of acceptable chunk types. However for you convience three wrapper functions are provided if you are you are just looking for chunks of a particular type. These functions are shown below and, apart from the filters array, they have the exact same parameters as the normal get function.

function callback (err, data) {
  // do something with result.
}


fs.createReadStream('input.png')
  // Read all the iTXt blocks with keyword cat
  .pipe(pngitxt.getitxt('cat', callback))
  // Read all the iTXt blocks regardless of keyword
  .pipe(pngitxt.getitxt(callback))
  // Read all the zTXt blocks with keyword cat
  .pipe(pngitxt.getztxt('cat', callback))
  // Read all the zTXt blocks regardless of keyword
  .pipe(pngitxt.getztxt(callback))
  // Read all the tEXt blocks with keyword cat
  .pipe(pngitxt.gettext('cat', callback))
  // Read all the tEXt blocks regardless of keyword
  .pipe(pngitxt.gettext(callback))

To specify a specific combination of chunk types use an array as shown in the following example where the data is filtered so that you only get tEXt and iTXt chunks with the keyword pizza.

// This will find all the tEXt chunks with the keyword pizza
fs.createReadStream('input.png')
  .pipe(pngitxt.get('pizza', [ pngitxt.tEXt, pngitxt.iTXt ], function (err, data) {
  if (!err && data) {
      console.log(data.keyword, ":", data.value)
    }
  }))

Please note that even if you are only looking for one type of chunk the value must be passed as an array or an exception will be thrown. You can specify whatever combination of chunk types that you want but be aware that specifying all 3, as shown below, is the equivalent of having no filter.

// This is the equivalent of having no filter
fs.createReadStream('input.png')
  .pipe(pngitxt.get('pizza', [ pngitxt.tEXt, pngitxt.zTXt, pngitxt.iTXt ],
    function (err, data) {
    if (!err && data) {
      console.log(data.keyword, ":", data.value)
    }
  }))

If no keyword is specified then the filter array can be provided as the first parameter to the function.

// This will find all the iTXt and zTXt chunks.
fs.createReadStream('input.png')
  .pipe(pngitxt.get([pngitxt.iTXt, pngitxt.zTXt], function (err, data) {
    if (!err && data) {
      console.log(data.keyword, ":", data.value)
    }
  }))

Exceptions

In all but two cases errors are indicated by passing them to the callback function. However if no callback function is provided or the filter arguement is incorrect (i.e. it is not an array or doesn't contain any valid chunk types) then exceptions will be thrown to indicate this.

// This will cause an exception as there
// is no callback function provided and the
// filter is not correct.
fs.createReadStream('input.png')
  .pipe(pngitxt.get("pizza", 'blah'))

Command Line Tool

The command line tool, called png-itxt is provided in the bin folder and will be automatically linked from the node_modules/.bin folder if installed with npm. The two commands it supports are as follows.

  • png-itxt get
  • png-itxt set

png-itxt get

This command allows you to seach an image for textual chunks of any type and will output the result in JSON format. For example the following command will search the input.png file for a iTXT or zTXt chunk that has the keyword pizza and then output the results to standard output.

png-itxt get -k pizza -f iTXt,zTXt input.png

A complete list of the options for the get command are shown below.

  Usage: png-itxt-get [options] <file.png>

  Tool for getting textual information from PNG images

  Options:

    -h, --help                 output usage information
    -V, --version              output the version number
    -k, --keyword <keyword>    keyword to serach for
    -r, --regexp <expression>  regular expression to search by
    -i, --ignorecase           makes the search case insensitive
    -s, --stdin                reads the image data from standard input
    -f, --filter <chunktypes>  comma seperated list of chunk types to return
    -o, --output <file>        write results to a file

png-itxt set

This command allows you to add iTXt chunk to an image. For example the following command will add a chunk with keyword pizza and the value delicious to an image from the file input.png and then save it to output.png. The command can also be used to read data from stdin and output the result to stdout so that you can pipe the information between commands. Note that this tool will only add iTXt chunks.

png-itxt set -k pizza -d delicious -o output.png input.png

A full list of all the avilable options available for use with the set command are shown below.

  Usage: png-itxt-set [options] <fileIn.png>

  Tool for setting textual information into a PNG images. If no output method is specified output will be put into {fileIn}_out.png

  Options:

    -h, --help               output usage information
    -V, --version            output the version number
    -k, --keyword <keyword>  keyword to set the value for
    -d, --data <chunkdata>   data to store with the keyword
    -f, --file <datafile>    text file to read the value from
    -v, --valuein            read data to store from standard input
    -c, --compress           compress the value stored in the chunk
    -s, --stdin              read png data from standard input
    -p, --pipe               redirect output to stdout for processing
    -o, --output <file>      file to output PNG data to