robinhouston · August 4, 2013 17:49
diff --git a/README.md b/README.md
diff --git a/index.html b/index.html
 <!DOCTYPE html>
 <meta charset="utf-8">
 <title>Spiral flow</title>
 <script src="http://bl.ocks.org/robinhouston/raw/6096562/rAF.js" charset="utf-8"></script>
 <script src="streamer.js" charset="utf-8"></script>
 <style>
    canvas { background-color: #4C4C4C; }
 </style>

 <canvas width=960 height=500></canvas>

 <script>
    var NUM_STREAMS = 8000,
        MAX_AGE = 50,
        FADE_RATE = 0.05,
        BORDER = 100;
  
    var canvas = document.getElementsByTagName("canvas")[0],
        cx = canvas.getContext("2d"),
        streamer = Streamer({
            canvas: canvas,
            num_streams: NUM_STREAMS,
            max_age: MAX_AGE,
            fade_rate: FADE_RATE,
            border: BORDER,
            velocity: to_px(rotate(Math.PI/4, double_spiral(spiral(1/2, Math.PI/6))))
        });
    
    cx.strokeStyle = "rgba(255,255,255,0.5)";
    cx.fillStyle = "rgba(255,255,255,0.05)";
    cx.fillRect(0, 0, canvas.width, canvas.height);
    streamer.start();
    
    // * Vector field transformers
    
    // Rotate the vector field by θ
    function rotate(θ, f) {
        var cos_θ = Math.cos(θ), sin_θ = Math.sin(θ);
        return function(x, y, t) {
            return f(x*cos_θ - y*sin_θ, x*sin_θ + y*cos_θ, t);
        };
    }
    
    // Transform the vector field f under the mobius transformation z ↦ (z-1)/(z+1)
    function double_spiral(f) {
        return function(x, y, t) {
            // Let (x0, y0) be the inverse image of (x, y) ...
            var r0 = (x-1)*(x-1) + y*y,
                x0 = (1 - x*x - y*y) / r0,
                y0 = 2*y / r0,
            
            // Find the velocity v0 at (x0, y0)
                v0 = f(x0, y0, t),
                vx = v0[0], vy = v0[1];
            
            // and hit v0 with the Jacobian at (x0, y0)
            var denom = Math.pow((x0+1)*(x0+1) + y0*y0, 2) / 4,
                a = 2 * (y0*y0 - (x0+1)*(x0+1)),
                b = 4*(x0+1)*y0;
            return [
                ( a * vx - b * vy ) / denom,
                ( b * vx + a * vy ) / denom
            ];
        };
    }
    
    // Transform a vector field from origin-centred coordinates to pixel coords
    function to_px(f) {
        return function(x_px, y_px, t) {
            var divisor = Math.min(canvas.width, canvas.height)/4,
                x = (x_px - canvas.width/2) / divisor,
                y = (y_px - canvas.height/2) / divisor;
            
            return f(x, y, t);
        };
    }
    
    
    // * Primitive vector fields
    
    function spiral(r, theta) {
        var log_r = Math.log(r);
        return function (x, y, t) {
            return [
                x*log_r - y*theta,
                x*theta + y*log_r
            ];
        };
    }
    
    
    
 </script>
diff --git a/streamer.js b/streamer.js
 function Streamer(options) {
    if (!options.hasOwnProperty("canvas")) throw "The 'canvas' option must be specified";
    if (!options.hasOwnProperty("velocity")) throw "The 'velocity' option must be specified";
    
    var canvas = options.canvas,
        cx = canvas.getContext("2d"),
        w = canvas.width, h = canvas.height,
    
        num_streams = options.num_streams || 1000,
        ms_per_repeat = options.ms_per_repeat || 1000,
        max_age = options.max_age || 10,
        fade_rate = options.fade_rate || 0.02,
        border = options.border || 100,
        velocity = options.velocity;

    function fadeCanvas(alpha) {
        cx.save();
        cx.globalAlpha = alpha;
        cx.globalCompositeOperation = "copy";
        cx.drawImage(canvas, 0, 0);
        cx.restore();
    }

    var streams = initStreams();

    function initStreams() {
        var streams = [];
    
        for (var i=0; i<num_streams; i++)
            streams.push([
                Math.round(Math.random() * (w + 2*border)) - border,
                Math.round(Math.random() * (h + 2*border)) - border,
                Math.round(Math.random() * max_age) + 1
            ]);
    
        return streams;
    }

    function frame(t) {
        fadeCanvas(1 - fade_rate);
        
        cx.save();
        cx.setTransform(1, 0, 0, 1, 0, 0);
        
        for (var i=0; i<streams.length; i++) {
            var stream = streams[i];
            if (stream[2] == 0) {
                stream[0] = Math.round(Math.random() * (w + 2*border)) - border;
                stream[1] = Math.round(Math.random() * (h + 2*border)) - border;
                stream[2] = MAX_AGE;
            }
            
            var v = velocity(stream[0], stream[1], t);
            if (v[0] <= w && v[1] <= h) {
                cx.beginPath();
                cx.moveTo(stream[0], stream[1]);
                cx.lineTo(stream[0] + v[0], stream[1] + v[1]);
                cx.stroke();
            
                stream[0] += v[0];
                stream[1] += v[1];
            }
            stream[2]--;
        }
        
        cx.restore();
    }

    var first_timestamp, animation_frame_id;
    function loop(timestamp) {
        if (!first_timestamp) first_timestamp = timestamp;
        frame(((timestamp - first_timestamp) % ms_per_repeat) / ms_per_repeat);
        animation_frame_id = requestAnimationFrame(loop);
    }

    return {
        "start": function() { animation_frame_id = requestAnimationFrame(loop); },
        "stop": function() { cancelAnimationFrame(animation_frame_id); }
    };
 }
	<!DOCTYPE html>
	<meta charset="utf-8">
	<title>Spiral flow</title>
	<script src="http://bl.ocks.org/robinhouston/raw/6096562/rAF.js" charset="utf-8"></script>
	<script src="streamer.js" charset="utf-8"></script>
	<style>
	canvas { background-color: #4C4C4C; }
	</style>

	<canvas width=960 height=500></canvas>

	<script>
	var NUM_STREAMS = 8000,
	MAX_AGE = 50,
	FADE_RATE = 0.05,
	BORDER = 100;

	var canvas = document.getElementsByTagName("canvas")[0],
	cx = canvas.getContext("2d"),
	streamer = Streamer({
	canvas: canvas,
	num_streams: NUM_STREAMS,
	max_age: MAX_AGE,
	fade_rate: FADE_RATE,
	border: BORDER,
	velocity: to_px(rotate(Math.PI/4, double_spiral(spiral(1/2, Math.PI/6))))
	});

	cx.strokeStyle = "rgba(255,255,255,0.5)";
	cx.fillStyle = "rgba(255,255,255,0.05)";
	cx.fillRect(0, 0, canvas.width, canvas.height);
	streamer.start();

	// * Vector field transformers

	// Rotate the vector field by θ
	function rotate(θ, f) {
	var cos_θ = Math.cos(θ), sin_θ = Math.sin(θ);
	return function(x, y, t) {
	return f(xcos_θ - ysin_θ, xsin_θ + ycos_θ, t);
	};
	}

	// Transform the vector field f under the mobius transformation z ↦ (z-1)/(z+1)
	function double_spiral(f) {
	return function(x, y, t) {
	// Let (x0, y0) be the inverse image of (x, y) ...
	var r0 = (x-1)(x-1) + yy,
	x0 = (1 - xx - yy) / r0,
	y0 = 2*y / r0,

	// Find the velocity v0 at (x0, y0)
	v0 = f(x0, y0, t),
	vx = v0[0], vy = v0[1];

	// and hit v0 with the Jacobian at (x0, y0)
	var denom = Math.pow((x0+1)(x0+1) + y0y0, 2) / 4,
	a = 2 * (y0y0 - (x0+1)(x0+1)),
	b = 4(x0+1)y0;
	return [
	( a * vx - b * vy ) / denom,
	( b * vx + a * vy ) / denom
	];
	};
	}

	// Transform a vector field from origin-centred coordinates to pixel coords
	function to_px(f) {
	return function(x_px, y_px, t) {
	var divisor = Math.min(canvas.width, canvas.height)/4,
	x = (x_px - canvas.width/2) / divisor,
	y = (y_px - canvas.height/2) / divisor;

	return f(x, y, t);
	};
	}


	// * Primitive vector fields

	function spiral(r, theta) {
	var log_r = Math.log(r);
	return function (x, y, t) {
	return [
	xlog_r - ytheta,
	xtheta + ylog_r
	];
	};
	}



	</script>
	function Streamer(options) {
	if (!options.hasOwnProperty("canvas")) throw "The 'canvas' option must be specified";
	if (!options.hasOwnProperty("velocity")) throw "The 'velocity' option must be specified";

	var canvas = options.canvas,
	cx = canvas.getContext("2d"),
	w = canvas.width, h = canvas.height,

	num_streams = options.num_streams \|\| 1000,
	ms_per_repeat = options.ms_per_repeat \|\| 1000,
	max_age = options.max_age \|\| 10,
	fade_rate = options.fade_rate \|\| 0.02,
	border = options.border \|\| 100,
	velocity = options.velocity;

	function fadeCanvas(alpha) {
	cx.save();
	cx.globalAlpha = alpha;
	cx.globalCompositeOperation = "copy";
	cx.drawImage(canvas, 0, 0);
	cx.restore();
	}

	var streams = initStreams();

	function initStreams() {
	var streams = [];

	for (var i=0; i<num_streams; i++)
	streams.push([
	Math.round(Math.random() * (w + 2*border)) - border,
	Math.round(Math.random() * (h + 2*border)) - border,
	Math.round(Math.random() * max_age) + 1
	]);

	return streams;
	}

	function frame(t) {
	fadeCanvas(1 - fade_rate);

	cx.save();
	cx.setTransform(1, 0, 0, 1, 0, 0);

	for (var i=0; i<streams.length; i++) {
	var stream = streams[i];
	if (stream[2] == 0) {
	stream[0] = Math.round(Math.random() * (w + 2*border)) - border;
	stream[1] = Math.round(Math.random() * (h + 2*border)) - border;
	stream[2] = MAX_AGE;
	}

	var v = velocity(stream[0], stream[1], t);
	if (v[0] <= w && v[1] <= h) {
	cx.beginPath();
	cx.moveTo(stream[0], stream[1]);
	cx.lineTo(stream[0] + v[0], stream[1] + v[1]);
	cx.stroke();

	stream[0] += v[0];
	stream[1] += v[1];
	}
	stream[2]--;
	}

	cx.restore();
	}

	var first_timestamp, animation_frame_id;
	function loop(timestamp) {
	if (!first_timestamp) first_timestamp = timestamp;
	frame(((timestamp - first_timestamp) % ms_per_repeat) / ms_per_repeat);
	animation_frame_id = requestAnimationFrame(loop);
	}

	return {
	"start": function() { animation_frame_id = requestAnimationFrame(loop); },
	"stop": function() { cancelAnimationFrame(animation_frame_id); }
	};
	}