How can I use an iPad with a Retina display as an external monitor?
I have three 23" Samsung LCD monitors, and I've had all three hooked up to my MacBook Pro, for a total of four displays. Problem is, the MacBook Pro's Retina display has totally spoiled me. The Samsungs now look terrible in comparison, so much so that I stick to the single MacBook Pro display as much as possible, until I have to go to multiple monitors for whatever task I'm working on (I'm a Java software engineer).
I got to thinking if it's possible to use an iPad with Retina display as an external monitor. Even better, if I could just buy iPad display replacement parts and use those as external monitors, that'd be rad, but I don't know (a) if it's possible, and (b) what kind of connector they would require.
How can I directly connect two iPads or two standalone iPad Retina displays to my MacBook Pro?
Update: There are now several competitors, such as Luna Display, as well as native support in iPadOS 13 + macOS Catalina, called Sidecar.
The app Duet Display lets iOS devices connect to a Mac that is running at least OS 10.9. The Mac needs duet software which is a free download and connects via Lightning port to the Mac in question to send the video.
The Mac app is a free download and the iOS app is available for purchase on the iOS App Store.
There are a few Apps on the App Store that will allow your iPad to be used as an external display using your WiFi connection.
Here are some examples:
Air Display
SplashTop Remote Display
It would definitely not be possible to connect an iPad to the MacBook without an app designed to do that on the iPad and a daemon on your MacBook, and it would have to use some tricky protocol passing data through the Lightning connector as audio (or some other way that the app could access). There is no way to connect to an iOS device by Thunderbolt that I know of.
Connecting just the iPad display to the computer would be as close to impossible as to make it unfeasible. The displays in iOS devices use the DSI protocol, so would need custom-made circuit boars for the connectors, and custom low-level drivers to interface with the GPU, and the OS.
The only truly high-DPI (>120) monitors that I've seen are for medical applications (e.g. this one), and they only give quotes when you contact them.
So for the moment, Wifi display apps such as Air Display are the best solution for Retina displays. Or, the Apple Thunderbolt Display has one of the highest DPIs of consumer-grade monitors, at 109 DPI, but this is still a far cry from the 227 DPI of my 13-inch Retina MacBook Pro.