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I’ve come across a bug in iOS causing rendering errors with Postscript flavoured fonts. Some characters are rendered lighter than others in landscape mode (subpixels), but when you zoom in or switch to portrait mode (greyscale) they appear correctly again. Tim Brown (edit: Tim Ahrens) over at Typekit looked into the issue with the font in question (I’ve only tested Mobile Safari on iPad, but Tim also sees these problems on the iPod):
I think you have discovered a rather weird bug in the iOS type rendering engine. I can reproduce this and have tried to figure out what exactly it is that causes the problem. It must be in the actual outline drawings of the letters in question, not the hinting, features or any other font metadata.
It seems that the rasterizer is not 100% numerically stable and runs into problems when certain constellations of nodes and BCPs occur. It is difficult to pin down exactly what these constellations are, which is not too surprising considering it is a bug.
I had a closer look at the v, which rendered strangely on my iPod. After a bit of playing around, it turned out that tweaking one curve is enough to make it render correctly (see the arrow on the attached image). Looking at this example, and how the other glyphs are drawn, I suspect that it could be related to "retracted" BCPs, although in some cases, they don't seem to cause any problems. I personally also work with retracted BCPs a lot and I am not aware of any rule or convention that forbids this. That said, the only fix I can think of right now would be to "extract" the BCPs.
Have anyone else experience this? Can someone shed any light?