awoodruff/README.md

## README.md

      
    Raw
  

              README.md
            
          
    This is an attempt at a slightly D3-ish way of drawing a dot density map on a canvas element. In short, each polygon is drawn (invisibly) with a unique color, then random points are thrown at polygon bounding boxes and the pixel color is used for a point-in-polygon test. This example draws one dot for every 2 people in central Boston census blocks, for a total of approximately 132,000 dots.

  
## blocks.json

      
Display the source blob

    
Display the rendered blob

    
    Raw
  

              blocks.json
            
          
      Sorry, something went wrong. Reload?
      Sorry, we cannot display this file.
      Sorry, this file is invalid so it cannot be displayed.
      
          Viewer requires iframe.
      
    
## index.html
<html>
<head>
	<title>Dot density map</title>
	<script src="http://d3js.org/d3.v3.min.js" charset="utf-8"></script>
	<script src="http://d3js.org/topojson.v1.min.js"></script>
</head>
<body>

<script>

var width = 1160,
	height = 700;

// invisible map of polygons
var polyCanvas = d3.select("body")
	.append("canvas")
	.attr("width",width)
	.attr("height",height)
	.style("display","none");

// using this div to crop the map; it has messy edges
var container = d3.select("body")
	.append("div")
	.style({
		"position": "relative",
		"width": (width-200) + "px",
		"height": (height-200) + "px",
		"overflow": "hidden"
	});

// canvas for dot map
var dotCanvas = container
	.append("canvas")
	.attr("width",width)
	.attr("height",height)
	.style({
		"position": "absolute",
		"top": "-100px",
		"left": "-100px"
	});

var projection = d3.geo.albers()
	.rotate([71.083,0])
	.center([0,42.3581])
	.parallels([40,44])
	.scale(880000)
	.translate([width / 2, height / 2]);

var path = d3.geo.path().projection(projection);

var polyContext = polyCanvas.node().getContext("2d"),
	dotContext = dotCanvas.node().getContext("2d");

var features;

d3.json( "blocks.json", function(error, blocks){
	features = topojson.feature(blocks, blocks.objects.massblocks_central).features;

	// draw the polygons with a unique color for each
	var i=features.length;
	while(i--){
		var r = parseInt(i / 256),
			g = i % 256;
		drawPolygon( features[i], polyContext, "rgb(" + r + "," + g + ",0)" );
	};

	// pixel data for the whole polygon map. we'll use color for point-in-polygon tests.
	var imageData = polyContext.getImageData(0,0,width,height);

	// now draw dots
	i=features.length;
	while(i--){

		var pop = features[i].properties.POP10 / 2;	// one dot = 2 people
		if ( !pop ) continue;

		var bounds = path.bounds(features[i]),
			x0 = bounds[0][0],
			y0 = bounds[0][1],
			w = bounds[1][0] - x0,
			h = bounds[1][1] - y0,
			hits = 0,
			count = 0,
			limit = pop*10,	// limit tests just in case of infinite loops
			x,
			y,
			r = parseInt(i / 256),
			g = i % 256;

		// test random points within feature bounding box
		while( hits < pop-1 && count < limit ){	// we're done when we either have enough dots or have tried too many times
			x = parseInt(x0 + Math.random()*w);
			y = parseInt(y0 + Math.random()*h);

			// use pixel color to determine if point is within polygon. draw the dot if so.
			if ( testPixelColor(imageData,x,y,width,r,g) ){
				drawPixel(x,y,0,153,204,255);	// #09c, vintage @indiemaps
				hits++;
			}
			count ++;
		}
	}
});

function testPixelColor(imageData,x,y,w,r,g){
	var index = (x + y * w) * 4;
	return imageData.data[index + 0] == r && imageData.data[index + 1] == g;
}

function drawPolygon( feature, context, fill ){
	var coordinates = feature.geometry.coordinates;
	context.fillStyle = fill || "#000";
	context.beginPath();
	coordinates.forEach( function(ring){
		ring.forEach( function(coord, i){
			var projected = projection( coord );
			if (i == 0) {
                context.moveTo(projected[0], projected[1]);
            } else {
                context.lineTo(projected[0], projected[1]);
            }
		});
	});
	context.closePath();
	context.fill();
}

// there are faster (or prettier) ways to draw lots of dots, but this works
function drawPixel (x, y, r, g, b, a) {
	dotContext.fillStyle = "rgba("+r+","+g+","+b+","+(a/255)+")";
	dotContext.fillRect( x, y, 1, 1 );
}

</script>

</body>
</html>
	<html>
	<head>
	<title>Dot density map</title>
	<script src="http://d3js.org/d3.v3.min.js" charset="utf-8"></script>
	<script src="http://d3js.org/topojson.v1.min.js"></script>
	</head>
	<body>

	<script>

	var width = 1160,
	height = 700;

	// invisible map of polygons
	var polyCanvas = d3.select("body")
	.append("canvas")
	.attr("width",width)
	.attr("height",height)
	.style("display","none");

	// using this div to crop the map; it has messy edges
	var container = d3.select("body")
	.append("div")
	.style({
	"position": "relative",
	"width": (width-200) + "px",
	"height": (height-200) + "px",
	"overflow": "hidden"
	});

	// canvas for dot map
	var dotCanvas = container
	.append("canvas")
	.attr("width",width)
	.attr("height",height)
	.style({
	"position": "absolute",
	"top": "-100px",
	"left": "-100px"
	});

	var projection = d3.geo.albers()
	.rotate([71.083,0])
	.center([0,42.3581])
	.parallels([40,44])
	.scale(880000)
	.translate([width / 2, height / 2]);

	var path = d3.geo.path().projection(projection);

	var polyContext = polyCanvas.node().getContext("2d"),
	dotContext = dotCanvas.node().getContext("2d");

	var features;

	d3.json( "blocks.json", function(error, blocks){
	features = topojson.feature(blocks, blocks.objects.massblocks_central).features;

	// draw the polygons with a unique color for each
	var i=features.length;
	while(i--){
	var r = parseInt(i / 256),
	g = i % 256;
	drawPolygon( features[i], polyContext, "rgb(" + r + "," + g + ",0)" );
	};

	// pixel data for the whole polygon map. we'll use color for point-in-polygon tests.
	var imageData = polyContext.getImageData(0,0,width,height);

	// now draw dots
	i=features.length;
	while(i--){

	var pop = features[i].properties.POP10 / 2; // one dot = 2 people
	if ( !pop ) continue;

	var bounds = path.bounds(features[i]),
	x0 = bounds[0][0],
	y0 = bounds[0][1],
	w = bounds[1][0] - x0,
	h = bounds[1][1] - y0,
	hits = 0,
	count = 0,
	limit = pop*10, // limit tests just in case of infinite loops
	x,
	y,
	r = parseInt(i / 256),
	g = i % 256;

	// test random points within feature bounding box
	while( hits < pop-1 && count < limit ){ // we're done when we either have enough dots or have tried too many times
	x = parseInt(x0 + Math.random()*w);
	y = parseInt(y0 + Math.random()*h);

	// use pixel color to determine if point is within polygon. draw the dot if so.
	if ( testPixelColor(imageData,x,y,width,r,g) ){
	drawPixel(x,y,0,153,204,255); // #09c, vintage @indiemaps
	hits++;
	}
	count ++;
	}
	}
	});

	function testPixelColor(imageData,x,y,w,r,g){
	var index = (x + y * w) * 4;
	return imageData.data[index + 0] == r && imageData.data[index + 1] == g;
	}

	function drawPolygon( feature, context, fill ){
	var coordinates = feature.geometry.coordinates;
	context.fillStyle = fill \|\| "#000";
	context.beginPath();
	coordinates.forEach( function(ring){
	ring.forEach( function(coord, i){
	var projected = projection( coord );
	if (i == 0) {
	context.moveTo(projected[0], projected[1]);
	} else {
	context.lineTo(projected[0], projected[1]);
	}
	});
	});
	context.closePath();
	context.fill();
	}

	// there are faster (or prettier) ways to draw lots of dots, but this works
	function drawPixel (x, y, r, g, b, a) {
	dotContext.fillStyle = "rgba("+r+","+g+","+b+","+(a/255)+")";
	dotContext.fillRect( x, y, 1, 1 );
	}

	</script>

	</body>
	</html>