Displaying and navigating large custom maps offlin

My question is how to efficiently display large custom maps in an offline Phonegap application, allowing them to be panned and zoomed smoothly while still supporting older mobile devices?

I’m developing a mobile application that involves using geolocation to navigate walking routes in remote areas where it’s likely the user won’t have a signal and therefore an internet connection. It’s important that the app works well with Android 2.2+ (so SVG is not an option) as well as iOS4+.

I’ve drawn custom vector maps using Adobe Illustrator at resolutions appropriate to each route, the average being about 2000x2000 pixels and the largest of which so far results in an image 4000x2400 pixels.

I’ve chosen to go with Phonegap/JQM rather than native simply because I come from a web programming background and it seemed the fastest way to get a user interface up and running without needing to delve into native code too much, although I’ve written a couple of Phonegap plugins using native code for the purposes of power and screen management.

The application needs to allow the user to pan around the map (by dragging) and zoom in/out (by pinching) between about 25% to 200% of the original image size.

Most of the testing I’ve done has been on an HTC Desire running Android 2.3.3 and an HTC Wildfire running Android 2.2 since these are likely to be some of the lowest spec devices the app is going to have to run on.

I’ve tried out various approaches to display the map (detailed below), but so far each has proved unfit for purpose either because the memory usage of the app is too great, the storage space required makes the app too large to download or the CPU usage is too intensive causing lag when panning/zooming.

Any suggestions much appreciated. Thanks in advance.

Approaches I’ve tried:

1. Display map as raster PNG using tag

This was the first approach I tried. Exporting the 4000x2400 pixel image from Illustrator as a 128 colour PNG-8 resulted in a 746Kb file. I panned the image by absolutely positioning it relative to the viewport and zoomed the image by scaling the width/height attributes of the tag. The problem with this approach was that even at a 1:1 zoom level, the Android application used 60Mb of RAM for the image and zooming in to 200% caused this to increase 120Mb, causing the app to crash on the HTC Wildfire.

2. Display portions of raster PNG using HTML5 canvas

To avoid the problem of zooming-in causing a proportional increase in memory usage, I tried loading the image via JS then copying the portion of the image to be displayed to a canvas the size of the viewport, something like:

var canvas = $(‘canvas#mycanvas’);
canvas.width = $(window).width;
canvas.height = $(window).height;
...
var img = new Image();
img.src = “map.png”;
...
var context = canvas[0].getContext("2d");
context.drawImage(img, x, y, w, h, 0, 0, canvas.width, canvas.height);

where x,y is the top-left corner within the source image defined by panning and w,h is the area size within the source image determined by zoom level

The problem here was that large map images were somehow losing quality while in memory (I can only assume there’s some upper memory limit which is resulting in dithering), causing the maps to look distorted in the app: see here for an example screenshot

3. Display map as vector using HTML5 canvas

A bit of Googling led me to discover ai2canvas, an Illustrator plugin that enables you to export artwork as vectors displayed in an HTML5 canvas. The result of the export is an html file containing a chunk of JS which represents all the paths in illustrator as bezier curves. Exporting my 4000x2400 map resulted in a 550Kb html file containing the vector paths. In my test app, I rendered the entire map to an in-memory canvas (not attached to the DOM) of 4000x2400 pixels, then copied the relevant portions of it to a viewport-sized canvas using context.drawImage() with the in-memory canvas as the source. On the HTC Wildfire, although the initial render of all the bezier curves to the in-memory canvas took around 2000ms, copying between canvases was fast enough to allow smooth panning and zooming. The problem was when I looked at the memory usage of the app, it was using 120Mb for the in-memory canvas once all the vectors had rendered.

I tried a second approach using the vector map; instead of rendering all the vectors to a large in-memory canvas, I made the app calculate which vector paths were visible within the viewport at the current pan position/zoom level during each drag/pinch event and only draw the visible vectors to the viewport-sized canvas. While this reduced the required memory usage to 10Mb, the CPU cycles required to perform these calculations on every drag/pinch cycle made the app lag so much on the old android phones it was unusable.

4. Display map using offline tiling

Using map tiler, I created PNG tiles for my maps at zoom levels from 25% to 100%. In my test app, I was then able to lazy load the tiles on demand reducing memory usage and producing a smooth pan/zoom experience even on the HTC Wildfire. I thought I’d found the solution until I looked at the size of the APK produced: for my 4000x2400 map, map tiler produced 4Mb of tile images. Most of my maps are around 2000x2000 pixels, resulting in about 2Mb of tiles. The code of my proper application plus the Phonegap overhead is another 2Mb.

My intention is to release a series of apps available on the Android/Apple markets, each with a set of around 10 maps, but with tiling each map weighs in at between 1-4Mb so the resulting app becomes a very large download.

In case this is of interest to anyone else, I solved this by using map tiling in the end, using a tool called pnqnq to create 8-bit PNGs constrained to 256 colours. The resulting set of tiles for my 4000x2000 map was about 800K in size as opposed to 4Mb for PNG-24, which was an acceptable size for assets in my Android and iOS applications.