[ft-devel] hinting & halftoning

[another gol]

Hello,

I'm using Freetype along with a custom rasterizer (based on Antigrain's) in my app.
I have a few questions as well as some thoughts I'd like to share.


1. my rasterizer doesn't use horizontal hinting at all, only vertical. Again, this was proved to work well in Antigrain's test rasterizer. Even though there's a tiny bit of clarity that's lost with no horizontal hinting (& I mean none, not 1/3 pixel), text gets much clearer to read when inter-character spacing is fully preserved. It's combining 3-pixel wide cached masks with blended subpixelling.
One problem that I originally thought to be linked to this [Freetype being told to rasterize a very wide character so that horizontal hinting is visibly bypassed, but later I found out that Freetype's own sub-pixel mode had the same problem] is that vertical hinting changes a font's aspect ratio. It's quite normal afterall, at small sizes a character can be +-1 pixel tall, which corresponds to up to 20% of the character's size for small sizes. I wanted to compensate for this, but I'm not sure it even is possible. Afterall, it's not the WHOLE character that changes in size, but some bars in it. Still, let's assume that we'd take the size of a lowercase or an uppercase, could it be compared to the original, unsnapped height, & a compensation factor be computed from that?
But is that original height even available? I had a lot of troubles with font metrics, especially for text centering & alignment, it's still not quite over (I was using the font's bounding box for best results because the font's height seemed quite unreliable, & now I realize with Windows 8 that Segoe UI got its bounding box changed while its metrics didn't change at all, only because they added some glyphs).
This is not a big problem, only slightly annoting. I was used to the aspect ratio of the great Segoe UI as I was using it at small sizes, & on a high-PPI screen it got wider ("as designed") & not so cool anymore. Of course if I managed to do the compensation, it's the small size that'd get wider, but I'd just compensate with my own aspect ratio (which is no problem with no horizontal hinting).


2. hinting. I quite like Freetype's autohinting, even for fonts that are supposed to have a great hinting actually. I prefer the way it only snaps 1 edge when the thickness is over 1 pixel, it looks better & there's no jump in thickness.
However, for symbols, the autohinter isn't always reliable. For example, if you wanna paint a checkbox that's a composite of a circle & a disk in 2 separate glyphs, you can't have any kind of autohinting (or hinting at all) here, as the disk has to look properly centered inside the circle.
Problem is, how to combine Freetype's autohinting with the font's own hinting inside the same font? There doesn't seem to be a hinting mode that would mean "Autohinting when there is no hinting instruction for a glyph, otherwise use the glyph instructions".
Does Freetype provide info about a glyph's hinting instructions so that I could check it & change the hinting method accordingly?

What also makes me wanna use the autohinter more is that apps that offer hinting out there seem quite rare. I'm using FontForge & having troubles with it.


3. halftoning.
It's a bit sad that, while a common font caching nowadays is based on alpha masks, we can't get halftoning from fonts. Some will say that it's an heresy, I'd say that wanting a multicolor font might be an heresy (even though you could still change its hue/brightness/saturation which is still useful enough), there's absolutely nothing against halftoning in a font glyph.
The problem is the implementation. I've read about support for this in some font format (can't remember) that's not widely supported. I know that halftoning can be faked using a halftoning grid, which is even less of a problem when you can predict the maximum size your glyphs will have on screen. But this method is very.. hinting unfriendly, so not really an option for me (even less with the autohinter enabled).
So I was wondering what kind of hack would work.. one idea would be to build cached glyphs out of several glyphs. That is, one character would be the opaque one and the next one would be the half-transparent one, & both would be merged into the same cache. But there's still the problem of marking a symbol glyph as having a twin somewhere (possibly the kerning table could be used for this, & the spacing could be used to define a transparency level), and autohinting is still a problem (but not internal hinting!).

It's a problem on different sides: the font editor that'd have to provide some way to mark a path as transparent, one that survives in the TTF that is (could be path direction? It'd be limiting, but better than nothing). It would also be a problem of retrieving the path from Freetype itself, as I'm using Antigrain to do the actual rasterizing.


There are 2 reasons I'm "going vectorial", it's that I can't imagine PC desktop monitors will lag behind tablets for so long, it's "only" for legacy reasons that we're not seeing high-density PC desktop monitors yet. And also because today's style is vectorial, both Windows 8 & the new iOS showed that flat was great.

Any help or comments? Thanks