Integrating Openlayers and HTML5 Canvas (Revisited)

 cartography, html5, openlayers html5 canvas  Comments Off on Integrating Openlayers and HTML5 Canvas (Revisited)
Jan 282014

The WordPress stats tell me there is still a lot of interest in our previous post on integrating OpenLayers and HTML5 Canvas from way back in 2010.
Time has passed, technology has moved on and I’ve started buying shoes in bulk like Mr Magorium. So below, I provide an update on how I integrate OL and HTML5 Canvas 3 years on.

Previously my approach was to replace each individual tile image with a corresponding canvas element, the same size as the tile (typically 256*256). Also we used JQuery to capture tile rendering events. The updated approach is to capture tile images as they are rendered by OpenLayers using OL built in event listeners and then draw these onto a single HTML5 canvas element.
Apart from being more efficient and producing cleaner, more robust code, this approach has the advantage that you can use HTML5 to draw shapes, lines and manipulate pixels on a single canvas tile, crossing tile boundaries. This is particularly useful for drawing lines and shapes using paths (e.g. lineTo() , moveTo() functions).

To demonstrate this I’ve set up a simple demo that shows the HTML5 Canvas adjacent to a simple OpenLayers map, where the canvas version (on the right hand side) is manipulated to show a grayscale and inverted version of the original map image (grayscale is triggered by loadend and the invert function by moveend) The source code is available on EDINAs gitHub page. ( and on JS Fiddle page.
The solution hinges on using the OpenLayers.Layer loadend event to capture the tiles when OpenLayers has finished loading all the tiles for a layer, and also the OpenLayers.Map moveend event, which OpenLayers triggers when it has dealt with the user panning the map. The former is shown in the code snippet below:
// register loadend event for the layer so that once OL has loaded all tiles we can redraw them on the canvas. Triggered by zooming and page refresh."loadend", layer, function()

// create a canvas if not already created


var mapCanvas = document.getElementById("mapcvs" ) ; // get the canvas element
var mapContainer = document.getElementById("OpenLayers.Map_2_OpenLayers_Container") ; // WARNING: Brittle to changes in OL

if(mapCanvas !== null)
var ctx = mapCanvas.getContext("2d") ;
var layers = document.getElementsByClassName("olLayerDiv") ; // WARNING: Brittle to changes in OL

// loop through layers starting with base layer
for(var i = 0 ; i < layers.length ; i++)

var layertiles = layers[i].getElementsByClassName("olTileImage") ; // WARNING: Brittle to changes on OL

// loop through the tiles loaded for this layer
for(var j = 0 ; j < layertiles.length ; j++ )

var tileImg = layertiles[j] ;
// get position of tile relative to map container
var offsetLeft = tileImg.offsetLeft;
var offsetTop = tileImg.offsetTop ;
// get postion of map container
var left = Number(,"p"))) ; // extract value from style e.g. left: 30px
var top = Number(,"p"))) ;
// draw the tile on the canvas in same relative postion it appears in OL map
ctx.drawImage(tileImg, offsetLeft + left, offsetTop + top) ;


greyscale(mapCanvas, 0, 0, mapCanvas.width, mapCanvas.height) ;
// uncomment below to toggle OL map on /off can only be done after layer has loaded
// = "none" ;


Note that some of the code here comes with a health warning. The DOM functions used to navigate the OpenLayers hierarchy is susceptible to changes in the Open Layers API so you need to use a local copy of OpenLayers (as is case in GitHub sample) rather than point to the OpenLayers URL (as is case in the JS Fiddle version).  Also note that all layers are drawn to the Canvas, not just the one that Open Layers triggered the loadend event for. This is necessary to ensure that the order of layers is maintained. Another issue to be aware of when using Canvas drawing methods on maps  is the likelihood of a CrossOrigin tainting error. This is due to map images being loaded from a different domain to that of the HTML5 code. The error will not get triggered simply by drawing the tiles to canvas using the drawImage() function, but does fail when you attempt pixel manipulation using functions such as putImageData() . OpenLayers handles this using the Cross-Origin-Resource-Sharing protocol which by default is set to ‘anonymous’ as below. So long as the map server you are pointing to is configured to handle CORS requests from anonymous sources you will be fine.

layer.tileOptions = {crossOriginKeyword: ‘anonymous’} ;

Would be interested to hear if others are doing similar or have other solutions to doing Canvasy things with OpenLayers.

OpenLayers Canvas Capture

Mbtiles and Openlayers

 html5, mapbox, mbtiles, mobile, openlayers, phonegap  Comments Off on Mbtiles and Openlayers
Jun 072013

Mbtiles and Openlayers

I was testing the feasibility of adding an overlay to openlayers map that is displayed on a mobile/tablet device .

The overlay is going to be in mbtiles format the made popular by MapBox.

The mbtiles db will be accessed locally on the device this useful when bandwidth is poor or non 3g tablets .

The mbtiles format is described here.

Its is basically a sqlite database that holds a collection of  x,y,z indexed tiles.

Webkit based browsers including mobile versions support this although its not actually part of the Html5 spec.

The main issue of using mbtiles locally is actually getting the database into the right location.

Another is the speed at which the device can render the images. The overhead in extracting blob images to the resulting  base64 encoded images.

There are a couple of ways this can be done however.

Getting Mbtiles on Device/Browser

With Phonegap

You can use the  FileTransfer object in phonegap to copy the database locally from a server. It will be downloaded to the Documents folder on the iphone by default.

example code to download an mbtiles db.

var fail = function (error) {

var doOnce = window.localStorage.getItem("doOnce");

   window.requestFileSystem(LocalFileSystem.PERSISTENT, 0, function(fileSystem) {
       fileSystem.root.getFile('testDB2.db', {create: true, exclusive: false}, function(fileEntry) {
           var localPath = fileEntry.fullPath;
           if (device.platform === "Android" && localPath.indexOf("file://") === 0) {
               localPath = localPath.substring(7);
           console.log("LOCAL PATH  "+ localPath);
           var ft = new FileTransfer();
           localPath, function(entry) {
               console.log("successful download");
           }, fail);
       }, fail);
     }, fail);

Use the phonegap web sql plugin  and open the database like.


The benefit of using a phonegap sqllite plugin – allows flexibility where you download the mbtile db to and removes the device dependant limits on database size.

Also if a browser drops native web sql support then it doesn’t matter.


Rather than download a remote database you could copy over a local database at startup.

The simple way to add a prepopulated SQLite DB in PhoneGap from this blog

If you want to keep it an entirely non-native web app based solution or desktop browser (webkit based – Chrome Safari you might be able to use a tool like.

There are more suggestion on stackoverflow here but I not tried them.

By using the syncing by creating an empty local mbtiles database and then populating it by inserts via data from the server is going to adversely affect performance. I have not tried this so I dont know how well it would work.

OpenLayers integration

First thing is to subclass an Openlayers TMS class.

* Map with local storage caching.
* @params options:
*     serviceVersion - TMS service version
*     layerName      - TMS layer name
*     type           - layer type
*     isBaseLayer    - is this the base layer?
*     name         - map name
*     url            - TMS URL
*     opacity        - overlay transparency
var MapWithLocalStorage = OpenLayers.Class(OpenLayers.Layer.TMS, {
   initialize: function(options) {

       this.serviceVersion = options.serviceVersion;
       this.layername = options.layerName;
       this.type = options.type;

       this.async = true;

       this.isBaseLayer = options.isBaseLayer;

           this.opacity = options.opacity;

       OpenLayers.Layer.TMS.prototype.initialize.apply(this, [,
   getURLasync: function(bounds, callback, scope) {
       var urlData = this.getUrlWithXYZ(bounds);
       webdb.getCachedTilePath( callback, scope, urlData.x, urlData.y , urlData.z, urlData.url);
   getUrlWithXYZ: function(bounds){
          bounds = this.adjustBounds(bounds);
       var res =;
       var x = Math.round((bounds.left - this.tileOrigin.lon) / (res * this.tileSize.w));
       var y = Math.round((bounds.bottom - / (res * this.tileSize.h));
       var z = this.serverResolutions != null ?
           OpenLayers.Util.indexOf(this.serverResolutions, res) :
  + this.zoomOffset;

       //inverty for openstreetmap rather than google style TMS
       var ymax = 1 << z;
       var y = ymax - y -1;
       var path = this.serviceVersion + "/" + this.layername + "/" + z + "/" + x + "/" + y + "." + this.type;

       var url = this.url;
       if (OpenLayers.Util.isArray(url)) {
           url = this.selectUrl(path, url);
       return { url: url + path, x:x, y:y, z:z};

   getURL: function(bounds) {
       return OpenLayers.Layer.XYZ.prototype.getURL.apply(this, [bounds]);


this.async = true;

as it will have to receive images from the local sqlite database asynchronously  as  web sql has an asynchronous callback style API.

       var ymax = 1 << z;

       var y = ymax – y -1;

All this does is invert the y axis tile to handle openstreetmap not required for google style TMS.

The is a good site that describes the various types of TMS around.

The Database Setup

"use strict";
var webdb = {};

function getWebDatabase(){
   if(typeof(openDatabase) !== 'undefined'){
       webdb = undefined;
   return webdb;
} = function() {
 var dbSize = 50 * 1024 * 1024; // 50MB
 webdb.db = openDatabase("'testDB2", "1.0", "Cached Tiles", dbSize);

webdb.onError = function(tx, e) {
 console.warn("There has been an error: " + e.message);

webdb.onSuccess = function(tx, r) {
 console.log("Successful Database tx " );

webdb.createTablesIfRequired = function() {
   console.log("Creating DataBase Tables");
 var db = webdb.db;
 db.transaction(function(tx) {
   tx.executeSql("CREATE TABLE IF NOT EXISTS " +
                 "tiles(zoom_level INTEGER, tile_column INTEGER, tile_row INTEGER, tile_data TEXT, mapName TEXT)", [], webdb.onSuccess,

                 " tile_index on tiles(zoom_level, tile_column, tile_row, mapName)", [], webdb.onSuccess,

function hexToBase64(str) {
   var hexString = str.replace(/([da-fA-F]{2}) ?/g, "0x$1 ");
   var hexArray = hexString.split(" ");
   var len = hexArray.length;
   var binary ='';
   for (var i = 0; i < len; i++) {
       binary += String.fromCharCode( hexArray[ i ] )
   //getting a stack error on large images
   //var binary = String.fromCharCode.apply(null, hexArray);
   return window.btoa(binary);

webdb.getCachedTilePath = function(callback, scope, x, y, z, url ){
   var db = webdb.db;
   var resultsCallback = function(tx, rs) {
       console.log('resultsCallback *********************' );
       console.log('rs.rows.length ' + rs.rows.length);

       if(callback) {
           if( rs.rows.length > 0 ) {
               var rowOutput  = rs.rows.item(0);
               var tile_data = rowOutput['tile_data'];
               //strip off the hex prefix
               tile_data = tile_data.substring(2);

           } else {
     , url);
   db.transaction(function(tx) {
       tx.executeSql("SELECT quote(tile_data) as tile_data FROM tiles where zoom_level=? AND tile_column=? AND tile_row=?", [z,x,y], resultsCallback,


When you have larger blobs in the database you can’t use the overloaded array version of String.fromCharCode as I was getting stack memory issue on the device. (iphone).

So you have to loop through and build it manually.

You have to use the quote function on the tile_data blob to turn it into a hex  string.

“SELECT quote(tile_data) as tile_data

Then trim the hex prefix X’ of the hex string before base64ing.

Testing if you just want to test the javascript /html5 with mbtiles you can copy your mbtiles database to the correct folder .

/Users/murrayking/Library/Application Support/iPhone Simulator/6.1/Applications/667F70EF-D002-425D-86C9-5027C965C518/Library/WebKit/LocalStorage/file__0/0000000000000001.db on a mac

or Chrome on mac as well.

Users/murrayking/Library/Application Support/Google/Chrome/Default/databases/http_localhost_8080/13


This approach is a bit convoluted.

Esp  the conversion of the blob to base64 and performance is a bit poor on older devices. But on newer devices its acceptable.  And as devices become more powerful it will become less issue as with all html5 javascript type things.

Not tried it yet on Android but should work. Worked in the Chrome browser on the linux box.

It does allow you to use rich openlayers framework cross platform without having to invest in native versions.

Also you can debug and test using a desktop browser which is fast before doing proper testing on the actual device.

Example Screenshot working on iphone3g using Phonegap and Mbtiles.

Development version based on our Fieldtrip GB app available on android and iphone.

Overlay is historic map in mbtiles format from the National Library of Scotland.


Debugging on Chrome non-native

working on chrome

Mar 252013

First of all – apologies for this blog going quiet for so long. Due to resource issues its been hard to keep up with documenting our activities. All the same we have been quietly busy continuing work on geo mobile activity and I’m please to announce that we have now releases our Fieldtrip GB app in the Google Play Store  


We expect the iOS version to go through the Apple App Store  in a few weeks.

Over the next few weeks I’ll be posting to blog with details of how we implemented this app and why we choose certain technologies and solutions.

Hopefully this will prove a useful resource to the community out there trying to do similar things.

A brief summary. The app uses PhoneGap and OpenLayers so is largely using HTML5 web technologies but wrapped up in a native framework. The unique mapping uses OS Open data including Strategi , Vector Map District  and Land-Form PANORAMA mashed together with path and cycleway data from OpenStreetMap and Natural England.


Fourth International Augmented Reality Standards Meeting

 AR standards, arml, augmented reality, augmented reality browsers, html5, karml, kml, Layar, mobile tech  Comments Off on Fourth International Augmented Reality Standards Meeting
Oct 282011

I’m just back from the Fourth International AR Standards Meeting that took place in Basel, Switzerland and trying hard to collect my thoughts after two days of intense and stimulating discussion. Apart from anything else, it was a great opportunity to finally meet some people I’ve known from email and discussion boards  on “the left hand side of the reality-virtuality continuum“.

Christine  Perry, the driving spirit, inspiration and editor at large of  AR Standards Group has done a fantastic job bringing so many stakeholders together representing Standards Organisations such as the OGC, Khronos, Web3d Consortium, W3C, OMA and WHATWG  Browser and SDK vendors such as Wikitude, Layar, Opera, ARGON and Qualcomm AR and hardware manufacturers ( Canon, SonyEricsson, NVIDIA) as well as several solution providers such as MOB Labs and mCrumbs – oh and a light sprinkling of academics ( Georgia Tech, Fraunhofer iDG ).

I knew I’d be impressed and slightly awe struck by these highly accomplished people, but what did  surprise me was the lack of  any serious turf fighting. Instead, there was a real sense of pioneering spirit in the room.  Of course everyone had their own story to tell (which just happened to be a story that fitted nicely into their organizational interests), but it really was more about people trying to make some sense of a confusing landscape of technologies and thinking in good faith about what we can do to make it easier.  In particular, it seemed clear that the Standards Organizations felt they could separate the problem space fairly cleanly between their specialist area of interest (geospatial, 3d, hardware/firmware, AR content, web etc). The only area where these groups had significant overlap was on sensor APIs, and some actions were taken to link in with the various Working Groups working on sensors to reduce redundancies.

In seemed to me that there was some agreement about how things will look for AR Content Providers and developers (eventually). Most people appeared to favour the idea of  declarative content mark-up language working in combination with a  scripting language (Javascript) similar to the geolocation API model. Some were keen on the idea of this all being embedded into a standard web browsers Document Object Model. Indeed, Rob Manson, from MobLabs has already achieved a prototype AR experience using various existing (pseduo) standards for web sensor and processing APIs. The two existing markup content proposals ARML and KARML are both based on the OGC’s KML, but even here the idea would be to eventually integrate a KML content and styling model into a generic html model, perhaps following the html/css paradigm.

This shared ambition to  converge AR standards with generic web browser standards is  a recognition that the convergence of hardware, sensors, 3d, computer vision and geo location is a bigger phenomenon than AR browsers or augmented reality. AR is just the first manifestation of this convergence and “anywhere, anytime” access to the virtual world as discussed by Rob Manson on his blog.

To a certain extent, the work we have been discussing here on geo mobile blog, using HTML5 to create web based mapping applications, is a precursor to a much broader sensor enabled web that uses devices such as camera, GPS, compass etc. not just to enable 2d mapping content but all kinds of application that can exploit the sudden happen-chance of  millions of people carrying around dozens of sensors, cameras and powerful compute/graphic processors in their pockets.

Coming back from this meeting, I’m feeling pretty upbeat about the prospects for AR and emerging sensor augmented web. Let’s hope we are able to keep the momentum going for the next meeting in Austin.