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SCVNGR is a platform for quickly and easily building location-based mobile games. Each game is all about doing challenges at places. Go here and take a photo, go there and solve this riddle. You happen to be at this coffee shop? Awesome! Try this challenge and earn a couple points! SCVNGR powers games for all sorts of institutions ranging from Princeton to Harvard to the Smithsonian Institutes to SIGGRAPH and even the U.S Navy.

If you're attending Google I/O this year, you'll get to try out our mobile game at the conference! (Don't forget to bring your Android phone, if you've got one!) I'm not going to give it all away here, but I do want to talk about one of the especially cool features that we're rolling out using some neat Google APIs.

One of the biggest challenges that our game-builders face is how to build location-based challenges that truly verify the user has actually made it to the right location. There are some non-technical solutions to this problem, such as creating riddles that require the user to be there to solve them (i.e. what is the third word on the fourth line of the plaque on the back wall) or taking photos which are then verified manually by the community or the game developer themselves.

We've also looked at a number of more technical solutions. The most obvious being to take the geo-tagged coordinates of each of the locations with a game and then use GPS to ensure that the player is within a certain radius of the location. Unfortunately, GPS verification has issues when the locations are indoors (as many are) and can vary greatly in accuracy across difference devices.

A new option, one that we're launching in a couple of weeks, uses QR codes planted at locations within the game-board. Players must scan these QR codes to verify that they've made it to the right spot. We're using the Google Charts API as an easy way to programmatically generate QR codes. Of course, generating and planting the QR codes is only half of the equation. You've also got to be able to decode them from the phones during the game. We experimented with a couple of options as to how to best achieve this.

Our first thought was to simply have the players snap pictures of the QR code which we'd then post back to our server, decode and respond with whether or not that was in fact the right QR code (or a QR code at all). The benefit of this solution was only having to utilize one QR code processing library for both our iPhone and Android applications. But we ran into a couple issues right up front:

1. The time cost incurred by having to transmit a reasonably high-resolution image to our servers.
   
2. Most players aren't very good at taking pictures of QR codes. They move the lens of the camera very close and snap the picture. (It's actually best to take the picture from 12-24 inches away to enable the camera to focus sharply on the QR code.) This led to a high-number of failed submissions before we were actually able to recognize a valid QR code.

So we turned to the ZXing project (pronounced Zebra Crossing) which is an open source barcode processing library written in Java and highly suited to the Android environment. Running the ZXing code right on the device rid us of the time-delay introduced by transmitting images to the server. But we still had the issue of the high-fail rate of the user snapping unsuitable images. Rather than trying to implement any form of "auto-scanning", we've chosen to simply grab images from the camera every 1/8 of a second or so, scan them and stop the process once a QR code is recognized.

As for the iPhone, ZXing has an objective-c port for the iPhone, but in order to grab images in real-time from the camera, you'll have to use a private API call for iPhone OS 3.1. Luckily, Apple has officially authorized its public usage until it's made public.

We're hard at work integrating QR codes into the great game that we're building for Google I/O and we hope you'll get a chance to play. The I/O team will let you know when the game is ready! The SCVNGR team will also be there in person, so please come by and say hello! We'd love to get your thoughts on the game or just chat about some of the Google APIs that we've used within SCVNGR.