For the past few days, I've been testing an iOS tool called Duet Display. Eric Cheng of DJI clued me in on the $15 app, and it's one of the more interesting and useful iPad utilities I've used so far. Simply, it allows you to use any iPad--whether it's an old 30-pin or current Lightning cable model--as a second screen for your Mac or PC. Yep, it's platform agnostic, and the desktop client is free. Using a 9.7-inch or 7.9-inch tablet as your secondary monitor may not sound like a great idea, and it's not something I would use on a regular basis. But since I keep both a laptop and my iPad in my backpack for most places I go, this is something that may have a lot of utility for frequent work travel.
The ability to use an iPad as a second display isn't new--iOS apps like Air Display have granted that ability for years. But those apps rely on a tethered or shared Wi-Fi connection, which limits the quality and responsiveness of the extended display image. The host computer is essentially sending compressed video over to the iPad, and that requires a lot of bandwidth. Duet Display uses a wired connection, so the only limiting factor is the host computer's ability to render and compress a desktop to send over the cable (Duet Display is admittedly a bit of a CPU and power hog, if you're running on laptop power). I was impressed by how good the desktop on my iPad Mini looked, and how responsive the cursor was as I moved windows between screens. It's not exactly zero lag, but darn close.
Setting up Duet was easy: you just buy the app on the iPad and install the host software on your PC or Mac. The host software runs in the background, and detects connected iPads as secondary displays when you launch the app. I love the "it just works" simplicity of the app, which isn't surprising since it was developed by ex-Apple display engineers.
Current Retina iPads have a resolution of 2048x1536. You unfortunately don't get all of those pixels as screen real estate with Duet Display. But that's ok--sending a full-res desktop at 60Hz over USB 2.0 would be taxing, to say the least. Instead, Duet gives you the option of three resolution options (Regular, Retina, and High Resolution) as well as two framerate choices (30fps and 60fps). At the basic Regular resolution, you get a second display that's effectively 1024x768, scaled to fit the iPad's screen. Retina setting doubles the pixel density at the same point dimensions (you don't get more real estate), and the high resolution setting acts as a 2048x1536 display. Text is almost unreadable on an iPad Mini at that size, so I kept to the Retina setting at 60fps. Duet also works in portrait orientation.
If I just wanted to browse the web or watch video on the iPad while multi-tasking, Duet Display and other screen extending apps aren't useful--it's better to just do those things natively in iOS. But the potential in Duet Display is efficiency in multi-tasking--the ability to treat the iPad screen as just another Mac OS desktop "space" and drag windows across. If you're editing video on a Macbook Pro, for example, the iPad screen would make a great place to hold all your video library folders. With photo editing, I can run Lightroom on one screen and Finder windows in the other. You get the benefits of the iPad's great IPS display for mobile multi-tasking.
Touch input also works through the app, though dragging your finger across a browser window selects text instead of scrolling the page. It's an interesting way to experiment with how Mac OS might work with a touchscreen. My recommendation is to install BetterTouchTool on the Mac, when then lets you create macro actions for touch gestures on the desktop and for window management (for example, resizing windows by dragging them to the sides of the screen). The iPad doesn't have pressure sensitivity on its touchscreen, but it would be cool if pressure abstraction with stylii tips worked with Photoshop.
While Duet Display works well for what it advertises, I really dig it for what it stands for: innovative ways to make more use of your tablets, new and old. Another example, the ability to use an iPad as a temporary external monitor for Mac Mini or Intel NUC box setups. The iPad has a wealth of inputs and outputs--a touchscreen display, camera, microphone, speaker, etc--all features that could use outside of content consumption on the tablet. I'm an advocate of any practical use for a tablet that keeps it out of storage or stuck in a messenger bag.
Duet Display costs $15, but it occasionally goes on sale for $10, which is a great price for its functionality.