The iPhone 4 features a vastly superior display resolution (614400 pixels) over previous iPhone models, containing quadruple the 153600-pixel display of the iPhone 3GS. The screen is the same physical size, so those extra dots are used for additional detail — twice the detail horizontally, and twice vertically. For developers only using Apple’s user interface elements, most of the work is already done for you.
While apps that take advantage of Apple’s native user interface elements require a lot less work when designing for the Retina display, we’re here to talk about highly custom, graphic-driven apps that need a fair amount of work to take full advantage of the Retina display
Just-in-time Resolution Independence
Another approach to handling widely different resolutions and pixel densities is to draw everything using code or vector-based images (like PDFs) at runtime. Without trying to stereotype anyone, it’s usually the approach engineering-types like. It’s clean, simple and elegant. It lets you design or code once, and display at any resolution, even at fractional scales.
Ahead-of-time Resolution Independence
The best quality results — and the method Apple chose for the iPhone 3GS to iPhone 4 transition — comes from pre-rendered images, built for specific devices, at specific resolutions: bespoke designs for each required size, if you will. It’s more work, but pre-rendering images ensures everything always looks as good as possible.
Fortunately, Apple chose to exactly double the resolution for the iPhone 4, and for using ahead-of-time resolution independence. As complex as the process is now, things would have been far worse if they had chosen a fractional scale for the display.