font hinting produces nonsensical results for tilde-a

[Maciej Katafiasz]

This is a problem with hinting, so I'm assigning it tentatively to FT, because I don't really know where the problem lies. As described below the problem is present on an installed Ubuntu 10.04, as well as completely pristine 11.04 run from a USB stick.

Description:
Font hinting fails completely for U+00E3 LATIN SMALL LETTER A WITH TILDE (ã). For instance in the "Ubuntu" font at 18pt size it's rendered indistinguishable from a macron, whereas at 19pt it's very clearly a tilde. The problem is not limited to a single font, or a single size, or only for small enough sizes. Another font exhibiting the same problem is Trebuchet MS -- the tilde looks perfect at 32pt, but at 33pt it's almost a straight line with barely visible trace of waves. Same for lower sizes: below 14pt it's rendered as a macron, 15 and 16 have it look right, then macron up until 23pt, then 24pt is great again but 25pt is a macron, etc.

The same problem is not present for U+0303 COMBINING TILDE. A combined with it (ã) is rendered properly and visibly at all point sizes down to 8pt. At 7pt it does look like a straight line, but it's hard to judge because the font is so tiny.

The attached .gif is a small animation clearly showing the issue.

[Maciej Katafiasz (mathrick)]

To add to the above, and especially given the behaviour with Trebuchet MS, where it's nice at a nice, round size of 16, and twice that 32, but fails at one point larger, I'd wager it's overzealous pixel grid snapping distorting the shape at sizes which have the tilde's waves come ever so slightly outside the grid. But I dunno how to influence that, so I can't really track down the root cause myself.

[Maciej Katafiasz (mathrick)]

So the problem is only limited to LCD subpixel aliasing, when I select the antialiasing type of "best contrast", "best shapes" or "monochrome", tilde remains a tilde. All these antialising however cause huge variations in the font widths and kerning, as well as causing perceived emboldening at the 12→13pt switch. Subpixel antialiasing produces fonts that are visually consistent across point sizes, but clearly does something too eagerly to the tilde.