BINARY DATA ADVENTURES IN BROWSER JAVASCRIPT

BINARY DATA ADVENTURES
IN BROWSER JAVASCRIPT
OR HILTCH / CTO @ STREAMRAIL

INTRO
• Background
• Evolution in the browser
• The Binary Arsenal in JS (type system,
inputs, outputs)
• Solving Cool, Practical Problems
00110110011100100101010100100
10101011101000001010100101010
01010010111010101001010101010
010101011111001010100010101110
0101010111101000000101010100
01011001010101010111100001010

TRADITIONALLY
• Outside of JS:  
Low level stuff: Compression (gzip/deflate), Bitfields (ACLs),
Networks (CRC32), Graphics (algorithms, positioning), ...
• Inside of JS:  
Solving weird issues

$http.post('/someUrl', {score:42}). 
success(function(data, status, headers, conﬁg) { 
// not today :-( 
}). 
error(function(data, status, headers, conﬁg) { 
// data=Object {‘error’: ‘Float64 is not int’} 
}

In JS, all numbers are of type Float64
But all bitwise ops are Int32!
Bitwise OR: number | 0
Bitwise NOT: ~~number
Shift: number >> 0
parseInt: parseInt(number, 10)
ﬂoor: Math.ﬂoor(number)
modulo: number - number % 1

BROWSERS, EVOLVED
• Type system: Blob, ArrayBuﬀer, TypedArray, DataView
• Inputs: 
Input methods: XHR 2, File API, Canvas, WebSockets, WebRTC, ...
• Outputs: 
MSE, Canvas, WebSockets, WebRTC, WebGL, ...

WHAT MAKES
TYPED ARRAYS FAST
• Passed to native interfaces completely AS IS (direct memory access)
• Native “endianness” - watch out!
• DataView adapter
Byte order
0x12345678 Hex inside a Uint32Array of 4
Little-endian: 78 56 34 12
12 34 56 78Big-endian:

STREAMING VIDEO
WITH JS
• No plugins (Netﬂix: silverlight, Facebook: ﬂash)
• MSE: Pure JS DASH/HLS (YouTube)
• DRM Support (EME)
• MSE + WebRTC: Bittorrent, in the browser! (WebTorrent)

‘USE ASM’
sys/libkern/strlen.c
 
size_t strlen(const char *str) {
const char *s;
for (s = str; *s; ++s);
return(s - str);
}
asm.js
 
function strlen(ptr) { 
ptr = ptr|0; 
var curr = 0; 
curr = ptr; 
while (MEM8[curr]|0 != 0) { 
curr = (curr + 1)|0; 
} 
return (curr - ptr)|0; 
}

a subset of JS for handling numbers faster
• OdinMonkey - AOT in FF (fallback to IonMonkey JIT)
• Support by all major browsers (to an extent)
• Typed Array used as “memory”
• All add/sub are 32 bit (number | 0)
• DI: function(stdlib, foreign, heap) 
‘USE ASM’

KRIPKEN TO THE RESCUE
C/C++ > LLVM > Emscripten > asm.js!
• Huge code bases (SQLite, BananaBread,J2ME VM, …) 
- close to impossible by hand
• Enjoy Clang/LLVM optimizations 
- decades of work done to optimize compiled code
• OpenGL > WebGL “for free”
- directly map a lot of OpenGL ES 2.0 command to WebGL
- not only for graphics rendering, also for GPU oﬄoading

• Use Docker to run Emscripten
- lots of software (emscripten, emscripten-fastcomp, emscripten-
fastcomp-clang, llvm clang)
- build your own or use one from DockerHub
KRIPKEN TO THE RESCUE

• Obtain bytes of video (and audio) by downloading them
• Decode the video (and audio) using native decoders in the browser/OS
• Animate the frames to create the video experience
 
VIDEO TAG EMULATION

• Obtain bytes of video (and audio) by downloading them
> XHR 2.0, response type “arraybuﬀer”
• Decode the video (and audio) using native decoders in the browser/OS
> Cross compile a native decoder to JS using Emscripten
• Animate the frames to create the video experience
> For each decoded pixel, create a 2D Texture, transform it to RGBA using
a fragment shader, and render it with WebGL using canvas
(“experimental-webgl”). 
VIDEO TAG EMULATION

YUV -> RGB
• Luma + (2 * Chroma) vs  
3 * (Chroma + Luma)
• Different needs, different inputs
• Color space conversion heavily
computational (floating point coefficients
matrix multiplication)

function yuv2canvas(buffer, width, height) {
var lumaSize = width * height;
var chromaSize = lumaSize >> 2;
webGLCanvas.YTexture
.fill(buffer.subarray(0, lumaSize));
webGLCanvas.UTexture
.fill(buffer.subarray(lumaSize, lumaSize + chromaSize));
webGLCanvas.VTexture
.fill(buffer.subarray(lumaSize + chromaSize,
lumaSize + 2 * chromaSize));
webGLCanvas.drawScene();
}
4:2:0
=
+

precision highp ﬂoat;
varying highp vec2 vTextureCoord;
uniform sampler2D YTexture;
uniform sampler2D UTexture;
uniform sampler2D VTexture;
const mat4 YUV2RGB = mat4 (
1.1643828125, 0 , 1.59602734375, -.87078515625,
1.1643828125, -0.39176171875, -0.81296875 , .52959375,
1.1643828125, 2.017234375 , 0 , -1.081390625,
0 , 0 , 0 , 1
);
void main(void) {
gl_FragColor = vec4(texture2D(YTexture, vTextureCoord).x,
texture2D(UTexture, vTextureCoord).x,
texture2D(VTexture, vTextureCoord).x,
1)
* YUV2RGB;
}
<SCRIPT TYPE="X-SHADER/X-FRAGMENT">

THANK YOU!
OR HILTCH / CTO @ STREAMRAIL

BINARY DATA ADVENTURES IN BROWSER JAVASCRIPT

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