John Gruber mentioned it during the 6th episode of the new Talk Show in a conversation with Guy English: subpixel rendering does only work in one orientation of an LCD display. If you flip a display from the “native” orientation by 90 degrees and keep the subpixel rendering activated the resulting text rendering quality will be reduced compared to the “native” orientation.
The reason is that the algorithm for subpixel rendering takes the order and orientation of the RGB subpixels into account. It only works this way and if you flip the orientation the effect will no longer work.
Apparently, this issue is singlehandedly responsible for not having an implementation of subpixel rendering on iOS.
This Wikipedia page basically confirms the conclusion but on the other hand also breaks the information that Microsoft’s implementation of subpixel rendering (called ClearType) is positively supported in both landscape and portrait in, guess what, Windows CE 5.0 (emphasis mine):
ClearType on Windows CE-based devices implements only a vertical striping algorithm. When the screen is rotated, it will still display ClearType using the vertical striping algorithm. ClearType is available in all orientations. Legibility is not compromised by using vertical striping on horizontal striped devices. For example, ClearType works well in the tablet PC devices.
Another page on MSDN (timestamped 6/7/2012) says (emphasis mine):
However, ClearType is dependent on the orientation and ordering of the LCD stripes. Currently, ClearType is implemented only for LCDs with vertical stripes that are ordered RGB. In particular, this affects tablet PCs, where the display can be oriented in any direction, and those screens that can be turned from landscape to portrait.
Unfortunately, the article that claims subpixel rendering works in both orientations in Windows CE does not carry a time-stamp but seeing the Windows CE 5.0 is not exactly bleeding-edge technology it is safe to assume that either Microsoft managed to square the circle in Windows CE and later forgot about how to do it1 or else the claim is simply wrong.
Call me biased, but as subpixel rendering is not available in iOS I tend to agree to the latter theory.
In related news, Gruber mentioned that Microsoft’s ClearType, according to his understanding, has been being implemented on a different code base than Apple’s version. As far as I understood the story from listening to episode 2.0.5 of the podcast This Developer’s Life, Apple has at one point in time simply licensed the technology of subpixel rendering from Microsoft.
Whether or not this means that the implementation of subpixel rendering in OSX is unrelated to ClearType in Windows does not change the fact that Microsoft invented it in the first place. But still, I found the story amusing.
Even more so because, in a typical Microsoft move, the company has buried the ability to switch on subpixel rendering somewhere deep in the preferences dialogs of Windows xp so that only geeks would ever discover it.
Thanks to this, the introduction of the technology on a broad scale could indeed more or less rightfully be credited to Apple that made it the default setting in OS X.
Interestingly, first descriptions of the surface tablet also consistently mention the term ClearType in the tablet’s feature descriptions. This was also discussed in said episode of the Talk Show and the conclusion is that this time Microsoft simply redefined the term ClearType without actually bothering to implement subpixel rendering for both landscape and portrait orientation, supposedly with the desperate intent to counter the iPad’s retina display.
I’ll be staying tuned to see how this stunt works out.