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Apologies for the nerdiness on a first post.
In the midst of some last checks before releasing my first real font, I noticed an unpleasant beating frequency effect on repeated characters at smallish ppems when testing on Windows 7 with the "Natural" mode of DirectWrite ClearType rendering (tested via Firefox with the Anti-aliasing Tuner add-on). The characters phase in and out of different degrees of bluriness.
With a bit of investigation, it looks like this is due to fractional advance widths. I was hoping to be able to control for this by slightly adjusting the advance width phantom point via TrueType hinting. However, it looks like Windows is ignoring that in this rendering mode and using only the designed width instead.