glif.js

glif.js is a fork of omggif that speeds up gif decoding using WebGL.

install

For those using bower

$ bower install glif.js

usage

raw decoding

var gr = new GifReader(byteArray);
var glif = new GLIF(canvas);

gr.decodeAndGLIF(frame_num, glif);

• byteArray is a Uint8Array containing the data to be decoded.
• canvas is a HTMLCanvasElement where the gif will be displayed.
• frame_num is the frame to display

animating

Use viewer.js to help animate the gif.

var view = viewer(canvas, data, callback);
var timeoutID = view(false);

• canvas is a HTMLCanvasElement where the gif will be displayed.
• data is one of ArrayBuffer, Uint8Array, or GifReader (from omggif).
• callback is a function that is called after each frame is drawn. One parameter is passed - the timeoutID of the setTimeout() call. clearTimeout(timeoutId) will stop the animation.

viewer returns a function (view) to draw the next frame of the gif. The function takes one argument, once. If once is true, then only one frame is drawn. The function returns the timeoutID of the first frame.

how

GIF decoding is slow because LZW decompression must be performed serially. But the next step, mapping palette indices to RGB colors, is easily parallelizable by the GPU.

More specifically, it turns this slow for loop:

for (var i = 0, il = index_stream.length; i < il; ++i) {
	var index = index_stream[i];

	if (xleft === 0) {  // Beginning of new scan line
		op += scanstride;
		xleft = framewidth;
	}

	if (index === trans) {
		op += 4;
	} else {
		var r = buf[palette_offset + index * 3];
		var g = buf[palette_offset + index * 3 + 1];
		var b = buf[palette_offset + index * 3 + 2];
		pixels[op++] = r;
		pixels[op++] = g;
		pixels[op++] = b;
		pixels[op++] = 255;
	}
	--xleft;
}

into this fragment shader:

varying highp vec2 vTextureCoord;

uniform sampler2D uIndexStream;
uniform sampler2D uPalette;

uniform bool uTransparency;
uniform mediump float uTransparent;

void main(void) {
	mediump float uIndex = texture2D(uIndexStream, vTextureCoord).r;

	if (uTransparency && uIndex == uTransparent) {
		gl_FragColor = vec4(0.0, 0.0, 0.0, 0.0);
	} else {
		gl_FragColor = texture2D(uPalette, vec2(uIndex, 0.5));
	}
}

License

(c) Dean McNamee <dean@gmail.com>, 2013.

Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to
deal in the Software without restriction, including without limitation the
rights to use, copy, modify, merge, publish, distribute, sublicense, and/or
sell copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in
all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS
IN THE SOFTWARE.