Mark Jardine and Paul Haddad of Tapbots are the breakout designers and engineers behind Weightbot (iTunes link), one of the most original user experiences released on the iPhone App Store to date. Continuing the iPhone blog's behind-the-scenes look at iPhone application development, Mark and Paul were kind enough to take time and discuss their ideas on interface and interactivity, and how what more we might expect from Apple's next generation mobile platform.

TiPb: How was approaching the iPhone interface for this App different than how you would have approached an interface for another platform?

Mark: It was really different coming from a web design background. 320x460 isn't a lot of space to work with and then you have to factor in the huge difference in input devices. A person's finger is a lot less accurate than a mouse cursor. At the same time, I wasn't designing a website so I was freed from a lot of rules and conventions I've been following over the past 8 years. So my initial approach was pretty simple. If Weightbot was an actual physical device, how could I make it usable and fun at the same time?

Paul: The thing I found challenging about dealing with the iPhone interface is that users want a silky smooth and yet the iPhone and iPod touch are both very slow compared to any modern Mac. I spent a lot of time juggling things around in order to get a consistent 60 fps for all the various animations within Weightbot. Coming from our current Multi Gigahertz, Multi Gigabyte and Multi Core world where for the most part you don't have to worry about performance to a platform like the iPhone where every cycle and byte counts is a big change.

TiPb: Weightbot's interface is obviously both creative and innovative. Is the robot theme something that came to expressly for that app, or does it exemplify a personal style of interaction that you can see pushing further in future with different types of Apps.

Mark: The whole robot theme sort of just fell into place. That happens a lot with the work I do. I can't always sit down with a piece of paper and try to think up a whole concept from scratch. I prefer to just dive into the work and then see what ideas pop up from it.

Paul came to me one day telling me he wanted to test the iPhone market and was going to start off simple with a weight tracking app. He asked if I was interested in helping out and didn't want to spend more than a week or two on it. I think the original plan was to use all of the standard UI elements provided in the SDK, but I thought I'd let my imagination run a little and then see what Paul would think. My initial sketch was simple. It was just the shape of the iphone with the lcd as seen on the app today. One thing popped into mind at that point. It was Eve from Wall-e. Everything just fell into place from there. Weightbot was inspired by Eve, and the Tapbots idea was based on the whole concept of different utility robots in Wall-e. That was such a great movie. When I was in high school, my dream was to become a 3d modeler for Pixar. Obviously, my career went off on a tangent (along with this interview).

So now that you know the story of where Tapbots and Weightbot came from, obviously you can expect there to be more robot applications in the future.

TiPb: Given the stylization, was it difficult to balance form and function, and make sure the interface didn't take over or drown out the underlying App?

Mark: Over the years, my web design philosophy has matured. I used to be all about the flash, bang, whiz type of websites, but one day I just realized that people just want to use the site or get the information they need. That's not to say flashy sites don't have their place. I've just changed my philosophy on it. So my take on all UI design is function first, and then wrap a solid, usable design around it. The same approach was taken with Weightbot. I pictured a user balancing on his/her scale, and easily being able to enter weight data into the app with one hand and without needing too much precision or concentration.

The Wii fit is probably the best example of simplicity because it weighs you and stores your weight data automatically. It's also somewhat fun to do. The big flaw with it is you have to turn on your TV and Wii just to do it and that can get tedious if you do it everyday. I know because I have one and hardly ever turn it on anymore. So Weightbot needed to be very simple, fun enough to be motivational, and not feel like a chore to use. I'd like to think we did a pretty decent job on it.

TiPb: Did developing an interface for the iPhone change the way you may approach future interfaces on other platforms? Did you learn anything special from developing for the iPhone?

Mark: New experiences always help shape the way you think about other things. I've definitely learned a lot from the experience and I'm sure it will influence my work on the web. Maybe not from a visual standpoint, but definitely from a user experience point of view.

TiPb: You use touch and accelerometer based interactions in your Apps. Are developers fully tapping the potential of iPhone interactivity yet? What more, if anything, could Apple surface to help developers create even better user interactions?

Mark: Some developers are doing really cool stuff with it. The multi-touch screen, accelerometer, and even the mic are all input devices for the iPhone. Developers should use them to their app's advantage. Our main purpose for using them was to simplify the interface. Imagine having a "graph" button in Weightbot to switch to the graph. The button would have to be small so it doesn't make the interface busy, but then the user has to look for the button. Using the accelerometer was a no brainer. It keeps our interface clean and it's very intuitive.

Google Earth makes really good use of the accelerometer and multi-touch controls. I think games are also pushing how users can interact with the iPhone. I think the possibilities are almost limitless so we will be seeing a lot of great solutions in the future.

Paul: I don't think most developers fully tapping the iPhone at all. It'll probably be another year before we see a large number of apps that really push the iPhone. Unfortunately in the current iPhone SDK there's very little defined support for detecting things like two finger swipes or shaking the iPhone. You pretty much have to depend on accessing the accelerometer and Event code at fairly low levels. Once Apple provides some higher level support for these features you are going to see more and more developers using them in innovative ways.

TiPb: What worked out especially well for you with Weightbot? Any favorite features or elements that you're especially happy with?

Mark: I'm happy most with the pure simplicity of it. Part of me wants to say this is the final version. But we get a lot of feature requests and Weightbot is nothing without people using it. So we will do our best to make our users happy without sacrificing on the initial vision of Weightbot.

Paul: I really like the Date Slider screen, maybe its because I spent an obscene amount of time tuning it, but I really enjoy the effect of flicking the Slider and hearing it tick as it switches dates.

Mark, Paul, we sincerely appreciate your taking the time to share with us and our readers!

Tapbots is Mark and Paul's website, and their premiere application, Weightbot, is available via the iTunes App Store.