gif-encoder

gif-encoder

Streaming GIF encoder

This is built as part of the gifsockets project. It is forked from gif.js to allow for a streaming API and performance optimization.

Getting Started

Install the module with: npm install gif-encoder

// Create a 10 x 10 gif
var GifEncoder = require('gif-encoder');
var gif = new GifEncoder(10, 10);

// using an rgba array of pixels [r, g, b, a, ... continues on for every pixel]
// This can be collected from a <canvas> via context.getImageData(0, 0, width, height).data
var pixels = [0, 0, 0, 255/*, ...*/];

// Collect output
var file = require('fs').createWriteStream('img.gif');
gif.pipe(file);

// Write out the image into memory
gif.writeHeader();
gif.addFrame(pixels);
// gif.addFrame(pixels); // Write subsequent rgba arrays for more frames
gif.finish();

Documentation

gif-encoder exports GifEncoder, a constructor function which extends readable-stream@~1.1.9. This means you can use any streams1/streams2 functionality. I will re-iterate what this means below.

// streams1
var gif = new GifEncoder(10, 10);
gif.on('data', console.log);
gif.on('end', process.exit);

// streams2
var gif = new GifEncoder(10, 10);
gif.on('readable', function () {
  console.log(gif.read());
});

new GifEncoder(width, height, [options])

Constructor for a new GifEncoder

• width Number - Width, in pixels, of the GIF to output
• height Number - Height, in pixels, of the GIF to output
• options Object - Optional container for any options
  • highWaterMark Number - Number, in bytes, to store in internal buffer. Defaults to 64kB.

NEVER CALL .removeAllListeners(). NO DATA EVENTS WILL BE ABLE TO EMIT.

We implement the GIF89a specification which can be found at

http://www.w3.org/Graphics/GIF/spec-gif89a.txt

Events

Event: data

function (buffer) {}

Emits a Buffer containing either header bytes, frame bytes, or footer bytes.

Event: end

function () {}

Signifies end of the encoding has been reached. This will be emitted once .finish() is called.

Event: error

function (error) {}

Emits an Error when internal buffer is exceeded. This occurs when you do not read (either via .on('data') or .read()) and we cannot flush prepared data.

If you have a very large GIF, you can update options.highWaterMark via the Constructor.

Event: readable

function () {}

Emits when the stream is ready to be .read() from.

Event: writeHeader#start/stop

function () {}

Emits when at the start and end of .writeHeader().

Event: frame#start/stop

function () {}

Emits when at the start and end of .addFrame()

Event: finish#start/stop

function () {}

Emits when at the start and end of .finish()

Settings

gif.setDelay(ms)

Set milliseconds to wait between frames

• ms Number - Amount of milliseconds to delay between frames

setFrameRate(framesPerSecond)

Set delay based on amount of frames per second. Cannot be used with gif.setDelay.

• framesPerSecond Number - Amount of frames per second

setDispose(disposalCode)

Set the disposal code

• disposalCode Number - Alters behavior of how to render between frames
  • If no transparent color has been set, defaults to 0.
  • Otherwise, defaults to 2.

Values :    0 -   No disposal specified. The decoder is
                  not required to take any action.
            1 -   Do not dispose. The graphic is to be left
                  in place.
            2 -   Restore to background color. The area used by the
                  graphic must be restored to the background color.
            3 -   Restore to previous. The decoder is required to
                  restore the area overwritten by the graphic with
                  what was there prior to rendering the graphic.
         4-7 -    To be defined.

Taken from http://www.w3.org/Graphics/GIF/spec-gif89a.txt

setRepeat(n)

Sets amount of times to repeat GIF

• n Number
  • If n is -1, play once.
  • If n is 0, loop indefinitely.
  • If n is a positive number, loop n times.

setTransparent(color)

Define the color which represents transparency in the GIF.

• color Hexadecimal Number - Color to represent transparent background
  • Example: 0x00FF00

setQuality(quality)

Set the quality (computational/performance trade-off).

• quality Positive number
  • 1 is best colors, worst performance.
  • 20 is suggested maximum but there is no limit.
  • 10 is the default, provided an even trade-off.

Input/output

read([size])

Read out size bytes or until the end of the buffer. This is implemented by readable-stream.

• size Number - Optional number of bytes to read out

writeHeader()

Write out header bytes. We are following GIF89a specification.

addFrame(imageData)

Write out a new frame to the GIF.

• imageData Array - Array of pixels for the new frame. It should follow the sequence of r, g, b, a and be 4 * height * width in length.

finish()

Write out footer bytes.

Low-level

For performance in gifsockets, we needed to open up some lower level methods for fancy tricks.

Don't use these unless you know what you are doing.

flushData()

We have a secondary internal buffer that collects each byte from writeByte. This is to prevent create a new Buffer and data event for every byte of data.

This method empties the internal buffer and pushes it out to the stream buffer for reading.

pixels

Internal store for imageData passed in by addFrame.

analyzeImage(imageData)

First part of addFrame; runs setImagePixels(removeAlphaChannel(imageData)) and runs analyzePixels().

• imageData Array - Same as that in addFrame

removeAlphaChannel(imageData)

Reduces imageData into a Uint8Array of length 3 * width * height containing sequences of r, g, b; removing the alpha channel.

• imageData Array - Same as that in addFrame; array containing r, g, b, a sequences.

setImagePixels(pixels)

Save pixels as this.pixels for image analysis.

• pixels Array - Same as imageData from addFrame
  • GifEncoder will mutate the original data.

writeImageInfo()

Second part of addFrame; behavior varies on if it is the first frame or following frame.

In either case, it writes out a bunch of bytes about the image (e.g. palette, color tables).

outputImage()

Third part of addFrame; encodes the analyzed/indexed pixels for the GIF format.

Donating

Support this project and others by twolfson via gittip.

Contributing

In lieu of a formal styleguide, take care to maintain the existing coding style. Add unit tests for any new or changed functionality. Lint via grunt and test via npm test.

UNLICENSE

As of Nov 11 2013, Todd Wolfson has released all code differences since initial fork from gif.js to the public domain.

These differences have been released under the UNLICENSE.

At the gif.js time of forking, gif.js was using the MIT license.