[HarfBuzz] Should MarkToBase attachment zero the mark advance?

Denis Jacquerye moyogo at gmail.com
Mon Aug 23 13:17:09 PDT 2010

On Mon, Aug 23, 2010 at 7:46 PM, Jonathan Kew <jonathan at jfkew.plus.com> wrote:
> Hi Behdad,
> I notice that the code in MarkArray::apply() includes the following:
>    hb_internal_glyph_position_t &o = c->buffer->pos[c->buffer->i];
>    o.x_advance = 0;
>    o.y_advance = 0;
>    o.x_offset  = base_x - mark_x;
>    o.y_offset  = base_y - mark_y;
>    o.back      = c->buffer->i - glyph_pos;
> i.e. in addition to setting x_offset and y_offset so as to position the mark glyph, it also explicitly overrides any existing x_advance and y_advance values for the glyph, settings them to zero. In many cases, this is harmless (though redundant), as mark glyphs are typically designed with zero advance anyway.
> However, I'm seeing problems in several monospaced fonts as a result of this; an example is DejaVuSansMono.ttf. Here, the mark glyphs have the same (non-zero) advance as the rest of the glyphs -- logical, I suppose, for a fixed-width font. There is a GPOS 'mark' feature that positions diacritics such as the U+03xx range. The trouble is that this feature actually executes TWO lookups for these glyphs: first, it does a MarkToBase Attachment (type 4), to place the diacritic over the base glyph, AND THEN it does a Single Adjustment that modifies the advance of the diacritic glyph by the negative of its original advance. This is clearly intended to make it become zero-width; but because harfbuzz has already zeroed the advance, it now ends up with a NEGATIVE advance, and the result is that the next glyph completely overprints the accented character.
> So unless you know of specific reasons why it is necessary to zero the x_advance and y_advance values here (are there examples of fonts where the rendering is incorrect without this?), I'd suggest removing those two lines, as in the attached patch.
> With that change, I'm getting the expected rendering with DejaVuSansMono. (The same issue occurs with the Consolas font on Windows7, for example.)
> JK


DejaVu SansMono uses this trick because renderers are'nt handling
zero-width characters in proper monowidth fonts. I guess Consolas
started doing it for the same reasons.

In a perfect world, this should be done in the renderer, and not by
using GPOS on marks.

Denis Moyogo Jacquerye --- http://home.sus.mcgill.ca/~moyogo
African Network for Localisation http://www.africanlocalisation.net/
Nkótá ya Kongó míbalé --- http://info-langues-congo.1sd.org/
DejaVu fonts --- http://www.dejavu-fonts.org/

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