[Webkit-unassigned] [Bug 102374] [EFL, Cairo?, Freetype?] Subpixel Layout results are vertically off-by-minus-one

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=102374





--- Comment #3 from Dominik Röttsches (drott) <dominik.rottsches at intel.com>  2012-11-20 04:56:01 PST ---
Turns out, cairo is only using the Freetype clamping path if cairo_font_options_set_hint_metrics is set to CAIRO_HINT_METRICS_DEFAULT or CAIRO_HINT_METRICS_ON, see http://cgit.freedesktop.org/cairo/tree/src/cairo-ft-font.c?id=1.12.4#n1888

WebCore's Cairo backend does not modify the DEFAULT setting, which results in always receiving integer-clamped values SimpleFontData::platformInit().

As mentioned, the clamping in FreeType can lead to incorrect, negative line gap values which are exposed when enabling subpixel layout. 

Since WebCore maintains the font metrics in floats, there's no reason we should clamp early, allowing such bugs. I suggest to disable cairo's metrics hinting and receive correct, proportionally scaled values in SimpleFontDataFreetype.cpp.

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