[Fontconfig] Greatly improved subpixel filtering in Xft

[Anders Kaseorg]

[Doh. Forgot to send to the list.]

Keith Packard wrote:
> In any case, it does appear that the filter to be used may indeed 
> need to be specified with some kind of preference; in your 
> auto-hinted example, I preferred the sharper appearance of the 
> intra-pixel hinting to the softer (but less colored) appearance of 
> the inter-pixel hinting.

Fair enough. I think my filter is a better default though, since it is 
much better for medium-hinted text, only marginally blurrier at worst 
for full-hinted or (probably) bytecode-hinted text, and an improvement 
over grayscale in either case.

> I used an intra-pixel hinter to ensure the pixel metrics returned by 
> the library matched the actual coverage of the glyphs; inter-pixel 
> hinting will stretch the glyphs by one or more pixels in the filtered 
> dimension, which seemed less desirable to me.  We could just fix up 
> the metrics though; that seems relatively easy.

Is that really the right thing to do? Since there are separate offset 
metrics, don't width, height, x, y just bound the pixels that would be 
affected if the glyph were drawn somewhere?

>> It would make more sense to hint to pixel boundaries vertically and 
>> subpixel boundaries horizontally.

I got this working. As I expected, horizontal spacing is a bit better, 
and I don't see color fringing, although the vertical strokes in full 
hinting are somewhat blurred (about as much as medium hinting). I guess 
it's hard to filter strokes whose width isn't a multiple of a whole 
pixel. It may also be because whole-pixel hinting ends up optimizing 
most for the green channel, which the eyes are best at perceiving.

Subpixel hinting might be more useful if Xft could do subpixel 
*positioning* of glyphs. Are there any plans for that?

Anders