Node.js server for composing map tiles using modestmaps.js

modestmaps-static.js

var MM = require('modestmaps'),
    Canvas = require('canvas'),
    Image = Canvas.Image;
    get = require('get'),
    express = require('express');

function renderStaticMap(provider, dimensions, zoom, location, callback) {

  var canvas = new Canvas(dimensions.x, dimensions.y),
      ctx = canvas.getContext('2d');

  var centerCoordinate = provider.locationCoordinate(location).zoomTo(zoom);

  function pointCoordinate(point) {
    // new point coordinate reflecting distance from map center, in tile widths
    var coord = centerCoordinate.copy();
    coord.column += (point.x - dimensions.x/2) / provider.tileWidth;
    coord.row += (point.y - dimensions.y/2) / provider.tileHeight;
    return coord;
  };

  function coordinatePoint(coord) {
    // Return an x, y point on the map image for a given coordinate.
    if (coord.zoom != zoom) {
      coord = coord.zoomTo(zoom);
    }
    var point = new MM.Point(dimensions.x/2, dimensions.y/2);
    point.x += provider.tileWidth * (coord.column - centerCoordinate.column);
    point.y += provider.tileHeight * (coord.row - centerCoordinate.row);
    return point;
  }

  var startCoord = pointCoordinate(new MM.Point(0,0)).container(),
      endCoord = pointCoordinate(dimensions).container(); 

  var numRequests = 0,
      completeRequests = 0;

  function checkDone() {
    if (completeRequests == numRequests) {
      callback(null, canvas);
    }
  }

  function getTile(url, p) {
    new get(url).asBuffer(function(error,data) { 
      if (error) {
        callback(url + ' error: ' + error);
      }
      else {
        var img = new Image();
        img.src = data;
        ctx.drawImage(img, p.x, p.y, provider.tileWidth, provider.tileHeight);
        completeRequests++;
        checkDone();
      }
    });
  }

  for (var column = startCoord.column; column <= endCoord.column; column++) {
    for (var row = startCoord.row; row <= endCoord.row; row++) {
      var c = new MM.Coordinate(row, column, zoom),
          url = provider.getTileUrl(c),
          p = coordinatePoint(c);
      getTile(url, p);
      numRequests++;
    }
  }
  
}

// just one for now...
var providers = {
  osm: new MM.TemplatedMapProvider("http://tile.openstreetmap.org/{Z}/{X}/{Y}.png")
}

var app = express.createServer();

app.get('/map', function(req,res) {
  var provider = providers[req.param("provider") || "osm"], // default osm
      width = req.param("width") || 800,
      height = req.param("height") || 600,
      dimensions = new MM.Point(width, height),
      zoom = parseInt(req.param("zoom") || 1),
      lat = parseFloat(req.param("lat") || 0.0),
      lon = parseFloat(req.param("lon") || 0.0),
      location = new MM.Location(lat, lon);
  renderStaticMap(provider, dimensions, zoom, location, function(err,canvas) {
    if (err) {    
      res.send(new Error(err));
    } else {
      res.header('Content-Type', 'image/png');
      res.send(canvas.toBuffer());
    }
  });
});

app.listen(3000);