Re: [ft-devel] [Patch] CJK autofit/autohint blue zones

[JustFillBug]

On 2011-01-14, JustFillBug <address@hidden> wrote:
> Two things.
>
> 1. There are always some chars that have higher bottom lines either by
>    design or by mistake. Chars like 日, 士 might be designed to have a
>    higher bottom in certain fonts. Which cannot be corrected by auto
>    hinting.
>
> 2. Parameters used by the auto hinter are choosen by experience. A set
>    of parameters perfect for one font at certain scaling size might not
>    be very good for another font or at another scaling size.
>
>    The patch inherited blue zone threshold parameters of latin script.
>    Which is 1/40 of EM for the zone width, and max out at 1/2 pixel.
>
>    Since ideographs are more sensitive to bottom line alignment at
>    smaller font size than latin glyphs, we can relax those parameters
>    some more.
>

And the effect of Blue zone management is two fold.

1. to align similar bounding edges to a unified value. to straight up
   bottom line.

2. to unify sharp bottom/top and flat bottom/top at small font size.
   This's important in practicle text run when flat and sharp bottoms
   interleave randomly.

The test sample used all the flat bottom chars to show blue zone effect,
because non-stem edges are not well detected when there are a small
curve on the edges. As a result, effect no. 2 of blue zone management
cannot be shown in the test sample.

The algorithm used in freetype's auto hinter is documented at 

    http://freetype.sourceforge.net/autohinting/hinter.html