[HarfBuzz] default features for vertical text runs

[Behdad Esfahbod]

Thanks John for getting to the bottom of this.  I'm pushing this out now.

On 13-08-09 12:59 AM, John Daggett wrote:
> 
> The code in hb-ot-shape.cc defines default features, distinguishing
> between defaults for horizontal and vertical text runs:
> 
>   static hb_tag_t common_features[] = {
>     HB_TAG('c','c','m','p'),
>     HB_TAG('l','i','g','a'),
>     HB_TAG('l','o','c','l'),
>     HB_TAG('m','a','r','k'),
>     HB_TAG('m','k','m','k'),
>     HB_TAG('r','l','i','g'),
>   };
>   
>   
>   static hb_tag_t horizontal_features[] = {
>     HB_TAG('c','a','l','t'),
>     HB_TAG('c','l','i','g'),
>     HB_TAG('c','u','r','s'),
>     HB_TAG('k','e','r','n'),
>     HB_TAG('r','c','l','t'),
>   };
>   
>   /* Note:
>    * Technically speaking, vrt2 and vert are mutually exclusive.
>    * According to the spec, valt and vpal are also mutually exclusive.
>    * But we apply them all for now.
>    */
>   static hb_tag_t vertical_features[] = {
>     HB_TAG('v','a','l','t'),
>     HB_TAG('v','e','r','t'),
>     HB_TAG('v','k','r','n'),
>     HB_TAG('v','p','a','l'),
>     HB_TAG('v','r','t','2'),
>   };
>   
> OpenType actually has two mutually exclusive vertical text layout
> models, one based on 'vert', the other based on 'vrt2'.  There's never
> a situation where both should be enabled.
> 
> Within vertical text runs, by default Chinese characters and kana are
> placed upright while subspans of Latin text are laid out horizontally
> and rotated into place within the vertical run.  One way to achieve
> this is to separate the run into vertical and horizontal runs and
> shape the subspans with the 'vert' feature on to enable vertical
> alternates (e.g. the comma mark in Japanese [、] changes to a version
> with the comma in the upper-right corner of the em-box).
> 
> The 'vrt2' model embeds rotated Latin glyphs within the font which
> obviates the need for the layout engine to slice up the text run.  A
> layout engine simply enables 'vrt2' and glyphs are automatically
> placed correctly.  Vertical alternates are also enabled via the 'vrt2'
> feature, so the set of substitutions applied are effectively a
> superset of the 'vert' substitutions.  In some controlled environments
> where the fonts and contents are known, the 'vrt2' model can simplify
> application logic.  However, it's not commonly used, the more general
> model is the one in which the layout engine handles orientation
> decisions.  Formalizing the orientation of different codepoints in
> Unicode has recently been a discussion within Unicode, leading to the
> recent publication of TR50.
> 
> I also checked with some of the Adobe engineers who originally
> registered the vertical text features in OpenType.  Their opinion was
> that 'vkrn', 'vpal' and 'valt' should all be off by default, despite
> what's written in the feature descriptions for these features.
> 
> So I think the resulting vertical defaults should be reduced to just
> 'vert', since that's the most common model used for vertical layout:
> 
>   static hb_tag_t vertical_features[] = {
>     HB_TAG('v','e','r','t'),
>   };
> 
> Cheers,
> 
> John Daggett
> Mozilla Japan
> 
> diff --git a/src/hb-ot-shape.cc b/src/hb-ot-shape.cc
> index 6a0c786..5b35abd 100644
> --- a/src/hb-ot-shape.cc
> +++ b/src/hb-ot-shape.cc
> @@ -59,16 +59,11 @@ static hb_tag_t horizontal_features[] = {
>  };
>  
>  /* Note:
> - * Technically speaking, vrt2 and vert are mutually exclusive.
> - * According to the spec, valt and vpal are also mutually exclusive.
> - * But we apply them all for now.
> + * Use vert by default. Layout engines are responsible for separating
> + * vertical/horizontal runs.
>   */
>  static hb_tag_t vertical_features[] = {
> -  HB_TAG('v','a','l','t'),
>    HB_TAG('v','e','r','t'),
> -  HB_TAG('v','k','r','n'),
> -  HB_TAG('v','p','a','l'),
> -  HB_TAG('v','r','t','2'),
>  };
>  
>  
> 
> 
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-- 
behdad
http://behdad.org/