I pushed some changes to
https://github.com/madig/freetype2/tree/stem-darkening-autohinter. I
have that branch installed system-wide for proper dog-fooding ;)
I changed the strategy to get the standard_width of a face. Instead of
relying on the hard-to-follow workings of the style analyzers, first see
if the font defines a stdVW by itself (Postscript fonts, once I figure
out how to get at SFNT->CFF->PrivateDict without instantiating a
CFF_Decoder also OpenType/CFF fonts). If it doesn't, in the future, set
the style analyzers on it in some controlled way to determine the
standard_width the hard way. Currently the code sets the standard_width
arbitrarily like Dave suggested by looking at the weight class in the
OS/2 table (light, regular, bold: 50, 75, 110). How well this works
depends on how far off that value is from the "real" stdVW. I compared
some article with the Linux Libertine font, because it comes with
differently named .otf and .ttf fonts for quick comparison and it's
dominant stem width is approximately 79, close to the 75 of it's weight
class: http://postimg.org/gallery/32jipb0bi/0fb77538/ (Gamma correction
through modified pixman). Alternating quickly between both shows that
stem contrast is almost the same, so the default CFF darkening
parameters seem a good fit there. If you look at the developer console
though, you'll see that the font there (Liberation Mono) appears too
thick vertically (stdVW =~ 88).
Another interesting thing when comparing Adobe's CFF engine with the
emboldening autohinter: text is fairly readable at very small sizes with
Adobe's engine while I have to skew the emboldening computation below
9(pt?) because emboldening makes stuff less readable there:
http://postimg.org/image/my3w9aerp/91a87a2d/. Top is Linux Libertine
.otf -> Adobe CFF, bottom is .ttf -> autohinter.
What I now want to do:
- Find out how to access SFNT->CFF->PrivateDict.
- Find out how to to run style analyzers over the face in a controlled
way to compute real standard_widths.
Once that is done, I think the prototype should be closer to merging to
master. The last hurdle should be performance testing and optimization.
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