Doing this is straightforward with our Windows desktop application. But we have always been interested in finding a cross-platform browser-based way of doing the same thing. Our hope was to increase user adoption two ways: by supporting the increasingly-popular Mac platform, and by eliminating the requirement that first time uploaders download and install software.

Some of the things we considered: ActiveX, Firefox Plugin, Java applet, Flash, Adobe AIR, and Silverlight. We wanted a solution that was cross-browser and cross-platform, worked in the browser rather than a separate install, had a reasonable first-time user experience, and didn’t require the user to click through any scary security warnings.

After eliminating those that didn’t meet our criteria, we were down to Flash and Silverlight. To do what we wanted, we needed to be able to have the user select multiple files, read the bytes locally, compute a SHA1 hash, load the bytes into an image and perform manipulations like rotation and scaling, re-encode the resulting images to JPEG, and upload them to our server. Unfortunately, Flash can’t read the bytes locally, and Silverlight can’t do image manipulations or re-encode to JPEG.

At this point we thought we were out of luck. For a while we chatted about this problem with others to see if they had insights, and we’d always jokingly conclude that you’d really need to build a hybrid Flash/Silverlight application to do what we wanted. There would always be jolly consensus that such an idea was too silly to pursue. We even came up with a silly name for the “new” RIA platform: FlashLight.

But as we thought about it more, we found it less and less silly. If we could think of Flash and Silverlight as Javascript libraries instead of monolithic app platforms, there was really no reason that we couldn’t use the functionality from both to achieve our goals. It probably wouldn’t be a happy developer-tool supported experience, but — hey — we like writing code.

So next we tried to figure out if we could use Javascript as the core and call into Flash and Silverlight as needed. It turns out Silverlight provides really great Javascript connectivity. Essentially, you just decorate your types with attributes to make them accessible from Javascript. Flash’s Javascript interaction looked painful at first, but then I found this great add-on from the Flex SDK called FABridge that made it easy.

The plan was to use Silverlight to read the JPEG files, then pass those bytes over to Flash for image processing. But in the middle is Javascript, and Javascript doesn’t deal with binary data very well. Keeping it simple, we decided to base64 encode the bytes and pass them through Javascript as strings. Both Silverlight and Flash have libraries to deal with base64-encoded data.

Once we had the pieces in place, we were able to crank out our web-based uploader in about a week by writing a little bit of C#, and moderate amounts of Javascript and ActionScript. You can see the results on Photosleeve. (You’ll have to sign up for an account and click on Add Photos in the upper right.)

Thinking of Flash and Silverlight as complementary Javascript libraries instead of competing app platforms allowed us to build an app that leveraged Silverlight’s client-side file access and Flash’s client-side image manipulation.

Have other exciting apps been built using this technique? Are there others waiting to be built?