streampng

Reading, modifying and (experimentally) writing PNGs, chunk by chunk.

Install

$ npm install streampng

Tests

Requires node-tap.

$ npm test

API

StreamPng([streamOrBuffer])

@returns instance @throws TypeError @see Stream#pipe @see StreamPng#write

Inherits from Stream. Optionally takes a Stream or Buffer instance. If the argument is a Stream, it will pipe the stream to the new instance. If it's a buffer, the buffer will be passed to StreamPng#write.

Event: 'data'

function (data) { }

Emitted whenever StreamPng#write or StreamPng#end is called with data. Allows for transparent re-piping.

Event: 'end'

function () { }

Emitted when the end of the input stream is reached.

Event: 'error'

function (exception) { }

Emitted when there is an error parsing the PNG.

Event: 'signature'

function () { }

Emitted after the PNG signature has been verified.

Event: 'chunk'

function (chunk) { }

Emitted when a full chunk has been parsed.

Event: chunkType

function (chunk) { }

Emitted when a chunk of a certain type has been parsed. For example,

// do something with each tEXt chunk
png.on('tEXt', function(chunk) { ... })

Event: 'imagedata begin'

function () { }

Emitted when the first IDAT chunk is recieved.

StreamPng#write([buffer])

@returns instance @emits {'data', Buffer}

Writes data to the internal parser. Does nothing if no arguments are recieved. Emits a data event with the buffer that was passed in to allow for chaining pipes:

var outstream = fs.createWriteStream('node-logo.png');
var png = new StreamPng();
var opts = url.parse('http://nodejs.org/images/logo.png');

http.request(opts, function(res) {
  res.pipe(png).pipe(outstream);
}).end();

png.on('chunk', function(chunk) {
  console.dir(chunk);
});

In general, this shouldn't be called directly. Use Stream#pipe or pass a full buffer via the constructor.

StreamPng#end([buffer])

@returns instance @emits {'end'} @see StreamPng#write

Called when the input stream is done (or immediately if a Buffer was passed to the constructor).

This probably shouldn't ever have to be called directly.

StreamPng#inject(chunk, [opts, [conditionFn]])

@returns instance

Options

• test: Either 'all' or 'same'. Whether the condition should be checked against all chunks or just chunks of the same type.Defaults to 'same'.

Add a chunk to the PNG, optionally with a condition for inclusion.

Chunks will be added after directly after the IHDR chunk. conditionFn, if passed, will be called with every chunk of the same type. You can force the condition against every chunk by passing test: 'all' in the options.

Here is an example of conditionally adding a tEXt chunk with a License if one doesn't already exist.

// setup
var fs = require('fs'), StreamPng = require('./');

var instream = fs.createReadStream('example.png');
var outfile = fs.createWriteStream('example.license.png');
var png = StreamPng(instream);
var license = StreamPng.Chunk.tEXt({
  keyword: 'License',
  text: 'CC BY-SA 3.0'
});

// add a `License` text chunk if one doesn't already exist.
 png.inject(license, function(chunk) {
   if (chunk.get('keyword') === 'License')
      return false;
});

// write output to file
png.out().pipe(outfile);

Important: note the use of StreamPng#out before calling pipe – calling pipe() directly on the instance will pipe the input transparently, without modification.

StreamPng#out([callback])

@returns new Stream

Outputs the (potentially modified) PNG buffer. Can be used either callback style or stream style:

Callback example:

png.out(function (err, buf) {
  fs.writeFile('example.png', buf, function() {
    console.log('done');
   });
});

Stream example:

var infile = fs.createWriteStream('example.png');
png.out().pipe(infile).once('close', function() {
  console.log('done');
});

Common chunk methods

All chunk constructors take one argument, fields, which should be an object defining the fields necessary for the chunk.

Chunk#set(key, value)

@returns instance

Set a chunk attribute. Use this rather than directly accessing attributes to ensure that cached buffers are correctly invalidated.

Chunk#get(key)

@returns value

Gets a chunk attribute.

Chunk#inflate([callback])

@returns instance

Callback: function (err, buffer) { }

For chunks that have compressed fields (iTXt, zTXt, iCCP), inflates the compressed data, sets the appropriate attribute on the chunk (text for textual data, profile for ICC profile data) and executes callback either either the zlib error or the inflated buffer.

Chunk#length()

@returns Integer

Length of the data portion of the chunk. For the length of the entire chunk including length, type and CRC segments, add 12 to the result.

Chunk#getComputedCrc()

@returns Buffer

Calculate the CRC for the chunk.

Specific chunk attributes

All incoming chunks will have type and crc fields.

Chunk.IHDR, Chunk.ImageHeader

• width: Width of the image in pixels
• height: Height of the image in pixels
• colourType: The type of image. Must be one of the following values:
  • IHDR.GREYSCALE
  • IHDR.TRUECOLOUR
  • IHDR.INDEXED_COLOUR
  • IHDR.GREYSCALE_ALPHA
  • IHDR.TRUECOLOUR_ALPHA
• bitDepth: The following values are valid depending on colour type:
  • greyscale: [1, 2, 4, 8 ,16]
  • truecolour: [8, 16]
  • indexed: [1, 2, 4, 8]
  • greyscale with alpha: [8, 16]
  • truecolour with alpha: [8, 16]
• interlaceMethod: Whether the image is interlaced. The png specification recognizes the following values:
  • IHDR.NONE
  • IHDR.ADAM7: Adam7 style interlacing.

Chunk.tIME, Chunk.LastModified

• year: Four digit year (e.g, 2046)
• month: Month, starting at 1 for January. (valid: 1-12)
• day: Day of the month, starting at 1. (valid: 1-31)
• hour: In 24 hour format (valid: 0-23)
• minute: Minutes (valid: 0-59)
• second: Seconds (valid: 0-60, to allow for leap second)

Chunk.zTXt, Chunk.CompressedText

• keyword: According to the specification, should use Latin-1 character set and be less than 79 characters (not enforced by this implementation).
• compressionMethod: How the image is compressed. The only method currently defined in the png specification is 0 for zlib deflate/inflate.
• text: Uncompressed textual data.
• compressedText: Buffer of compressed text.

Chunk.tEXt, Chunk.TextualData

• keyword: According to the specification, should use Latin-1 character set and be less than 79 characters (not enforced by this implementation).
• text: The textual data.

Chunk.iTXt, Chunk.InternationalText

• keyword: According to the specification, should use Latin-1 character set and be less than 79 characters (not enforced by this implementation).
• translatedKeyword: Can be UTF-8. Optional.
• languageTag: Language of the text and translatedKeyword fields.
• compressed: true if the text field should be compressed before outputting, false otherwise.
• text: The textual data.
• compressedText: Buffer of compressed text.

Chunk.pHYs, Chunk.PhysicalDimensions

• pixelsPerUnit: How many pixels per unit are there in each dimensions. Can be an array [1, 10] or an object {x: 14, y: 27}
• unitSpecifier: Valid values are:
  • 0 for unknown
  • 1 for meters

Chunk.sPLT, Chunk.SuggestedPalette

• paletteName: The palette name is case-sensitive, and subject to the same restrictions as the keyword parameter for the tEXt chunk. Palette names shall contain only printable Latin-1 characters and spaces (only character codes 32-126 and 161-255 decimal are allowed).
• sampleDepth: The sPLT sample depth shall be 8 or 16.
• palette: An array of objects with the following fields:
  • red: Red component of sample. Max value of 255 or 65535.
  • green: Green component of sample. Max value of 255 or 65535.
  • blue: Blue component of sample. Max value of 255 or 65535.
  • alpha: Alpha component of sample. Max value of 255 or 65535.
  • frequency: Frequency of sample appearance. Range 0 to 65535

Chunk.iCCP, Chunk.ICCProfile

• profileName: The profile name is case-sensitive, and subject to the same restrictions as the keyword parameter for the tEXt chunk. Palette names shall contain only printable Latin-1 characters and spaces (only character codes 32-126 and 161-255 decimal are allowed).
• profile: Buffer of the profile data.
• compressedProfile: Compressed Buffer of the profile data.

Chunk.sRGB, Chunk.StandardRGBColourSpace

• renderingIntent: Should be one of the following constants:
  • sRGB.PERCEPTUAL, for images preferring good adaptation to the output device gamut at the expense of colorimetric accuracy, such as photographs.
  • sRGB.RELATIVE_COLORIMETRIC, for images requiring colour appearance matching (relative to the output device white point), such as logos.
  • sRGB.SATURATION, for images preferring preservation of saturation at the expense of hue and lightness, such as charts and graphs.
  • sRGB.ABSOLUTE_COLORIMETRIC, for images requiring preservation of absolute colorimetry, such as previews of images destined for a different output device (proofs).

Chunk.sBIT, Chunk.SignificantBits

All fields have a range of 0 to image sample depth, inclusive.

When type is greyscale:

• greyscale

When type is truecolour or indexed colour:

• red
• green
• blue

When type is greyscale with alpha

• greyscale
• alpha

When type is truecolour with alpha

• red
• green
• blue
• alpha

Chunk.gAMA, Chunk.Gamma

• gamma: Float representing the image gamma.

Chunk.cHRM, Chunk.Chromaticities

Optional. If present, can appear exactly once.

All fields can be arrays in the form of [x, y] or objects with { x: value, y : value}

• whitePoint
• red
• green
• blue

Chunk.PLTE, Chunk.Palette

Can appear exactly once. Required for images of type 3, optional for types 2 and 6. Must not appear for types 0 and 4.

-colours: Array of objects. Max length of 256, all values must be between 0-255

• red
• green
• blue

Chunk.tRNS, Chunk.Transparency

Specifies either alpha values that are associated with palette entries (for indexed-colour images) or a single transparent colour (for greyscale and truecolour images).

Type 0, value between 0 and (2bitdepth-1)

• greyscale: 0-65535

Type 2, all values between 0 and (2bitdepth-1)

• red
• green
• blue

Type 3, all values between 0 and 255

• palette: Array ordered by palette index.

Chunk.hIST, Chunk.Histogram

Gives the approximate usage frequency of each colour in the palette. A histogram chunk can appear only when a PLTE chunk appears.

• frequencies

Chunk.bKGD, Chunk.BackgroundColour

Specifies a default background colour to present the image against. Optional; if present, must appear no more than once.

greyscale types:

• greyscale: Between 0 and (2bitdepth-1)

truecolour types, all values between 0 and (2bitdepth-1)

• red
• green
• blue

index colour:

• paletteIndex: Index of the palette entry to be used as the background.

Chunk.IDAT, Chunk.ImageData

Contains the actual image data which is the output stream of the compression algorithm

• data: compressed image data.

Chunk.IEND, Chunk.ImageTrailer

Marks the end of the PNG datastream. The chunk's data field is empty.

Chunk.oFFs, Chunk.Offset

Gives the position on a printed page at which the image should be output when printed alone. It can also be used to define the image's location with respect to a larger screen or other application-specific coordinate system.

• position: Position on the page, accessible by [x, y] or {x: value, y: value}. Value is a signed 32-bit integer.

• unitSpecifier: One of the following constants:

  • oFFs.PIXELS: unit is pixels
  • oFFs.MICRONS: unit is microns

Chunk.pCAL, Chunk.PixelCalibration

When a PNG file is being used to store physical data other than color values, such as a two-dimensional temperature field, the pCAL chunk can be used to record the relationship (mapping) between stored pixel samples, original samples, and actual physical values. The pCAL data might be used to construct a reference color bar beside the image, or to extract the original physical data values from the file. It is not expected to affect the way the pixels are displayed. Another method should be used if the encoder wants the decoder to modify the sample values for display purposes.

• name: The calibration name is case-sensitive, and subject to the same restrictions as the keyword parameter for the tEXt chunk. Palette names shall contain only printable Latin-1 characters and spaces (only character codes 32-126 and 161-255 decimal are allowed).
• originalZero: 32-bit signed integer.
• originalMaximum: 32-bit signed integer.
• equationType: Defines the pixel mapping equation: One of the following constants:
  • pCAL.LINEAR
  • pCAL.BASE_E_EXPONENTAL
  • pCAL.ARIBTRARY_BASE_EXPONENTIAL
  • pCAL.HYPERBOLIC
• parameters: Array of n length where n=2 for linear, n=3 for either of the exponential types and n=4 for hyperbolic equations. Contains objects with:
  • unit: Unit name
  • parameter: Value of the parameter

Chunk.sCAL, Chunk.Scale

While the pHYs chunk is used to record the physical size of the image itself as it was scanned or as it should be printed, certain images (such as maps, photomicrographs, astronomical surveys, floor plans, and others) may benefit from knowing the actual physical dimensions of the image's subject for remote measurement and other purposes. The sCAL chunk serves this need.

• unitSpecifier: The following constants are defined:
  • sCAL.METERS: Units are meters
  • sCAL.RADIANS: Units are radians
• width: Floating point representing the unit width per pixel.
• height: Floating point representing the unit height per pixel.

Chunk.gIFg, Chunk.GifControl

The gIFg chunk is provided for backward compatibility with the GIF89a Graphic Control Extension.

• disposalMethod: The following constants are defined
  • gIFg.NONE
  • gIFg.BACKGROUND
  • gIFg.PREVIOUS
• userInput: Whether or not user input is required to activate animate. Defaults to false.
• delay: How long between frames in hundredths of seconds.

Chunk.gIFx, Chunk.GifApplication

The gIFx chunk is provided for backward compatibility with the GIF89a Application Extension. The Application Extension contains application-specific information.

• appIdentifier: Up to eight printable ASCII characters used to identify the application
• authCode: Buffer, application specific. Up to three bytes.
• appData: Buffer, application specific.

Chunk.sTER, Chunk.Stereogram

When present, the sTER chunk indicates that the datastream contains a stereo pair of subimages within a single PNG image.

• mode: One of the following constants:

  • sTER.CROSS_FUSE: right-eye image appears at the left and the left-eye image appears at the right, suitable for cross-eyed free viewing.

  • sTER.DIVERGING_FUSE: left-eye image appears at the left and the right-eye image appears at the right, suitable for divergent (wall-eyed) free viewing.