Sub-pixel rendering of fonts on the iPad

Question

Sub-pixel font rendering like ClearType dramatically improves font display resolution and improves screen readability. How would I program sub-pixel rendering of a font (in general), and how can this be achieved on the iPad (C, C++, or Objective-C on an iOS device)? Fonts are quite blurry at certain sizes on the iPad, and I know that the iPad's display would work well with this technique...

So, how would I develop a font rendering engine for the iPad (e.g. How do I even access sub-pixels? Do I use OpenGL? Is there an existing open-source font rendering engine written in C, C++, or Objective-C for Mac OS X?)?

Answer 1

Each pixel on the iPad is a rectangle of red, green, and blue components, so one might think that sub-pixel font rendering would be a good choice for the device.

But consider that this device can be easily changed from portrait to landscape modes, and applications are expected to respond to that change. This would imply that your sub-pixel font mechanism would also have to respond to that change, and you would need two separate sub-pixel descriptions for each font.

Now throw in the fact that developers expect to be able to write universal applications that run on the pad and the phones in a single purchase/download. But look at the different pixel configurations on the various generations of the phones in the image below. Each of those, recall, would need to describe fonts differently in portrait mode and landscape mode. Now you have an explosion of font descriptions.

Now recall that we're speaking of portable devices where the most precious resource is the battery, and sub-pixel font rendering is more computationally intensive.

I'm guessing that this is not too different from the thought process that led Apple to eschew sub-pixel font rendering in favor of hoping that display technology increases pixel density to the point where it is no longer necessary (the retina display on the iPhone 4 being the first step in that direction.)

I would wager that in some future edition of the iPad, we'll have a display with similar density, and it won't matter as much. Any effort that you invest trying to invent a sub-pixel font rendering mechanism for your iPad application will immediately become obviated at that point, so I would recommend not going down that path.