Re: autokerning (was [ft-devel] New FreeType proofing tool: ftdiff)

[Behdad Esfahbod]

On Wed, 2007-04-04 at 00:32 +0200, David Turner wrote:
> Hi Behdad,
> 
> > 
> > Hi David,
> > 
> > I'm not sure if I understand what these lsb/rsb stuff is.  Is there
> > anything here that cairo/pango should be doing that it's not already?
> >
> 
> I'll try to summarize the issue as succintly as I can. Basically, 
> rsb/lsb_delta
> correspond to the displacement of the right/left edges of a glyph due to
> auto-hinting, and they can be used to adjust inter-character spacing.
> 
> - the rsb (right side bearing) is the distance between the right-most edge
>   of a glyph and its advance.
> 
> - the lsb (left side bearing) is the distance between the left-most edge
>   of a glyph and its origin.
> 
> - the space between the right-most and left-most edges of two consecutive
>   glyphs, respectively, is always computed as rsb(glyph1) + lsb(glyph2)
> 
> - Imagine that you have a pair of glyphs to display, like "po"
> 
>   Consider unhinted rendering, and let's assume that the scaled rsb of "p" is
>   0.3 pixels, and the scaled lsb of "o" is 0.45 pixels. This means that the
>   right edge of the "p" and the left edge of the "o" should be spaced by
>   0.75 pixels, which is pretty close to 1
> 
>   Consider auto-hinting, which tends to round these distances. Now, the rsb
>   of "p" is 0, and the lsb of "o" is also 0. Both auto-hinted glyphs will
>   appear "stuck" to each other
> 
> 
> the values of rsb_delta and lsb_delta simply tell us how the rsb/lsb changed
> due to the auto-hinting; we can compute the old inter-character space as:
> 
>     old_space = (rsb-rsb_delta) + (lsb-lsb_delta) 
>               = (rsb+lsb) - (rsb_delta+lsb_delta)
>               = new_space - (rsb_delta+lsb_delta)
> 
> so the sum of rsb_delta and lsb_delta corresponds to the inter-character space
> distortion that is due to the hinting.
> 
>     (rsb_delta+lsb_delta) = new_space - old_space
> 
> If we find that it is bigger than a half pixel, we simply substract 1 to the
> new space in order to reduce the spacing error while preserving the
> grid-aligned rendering
>  
> Similarly, if the "error" is smaller than -1/2 pixel, we increase the new 
> space
> by 1.
> 
>     error = (rsb_delta(glyph1) + lsb_delta(glyph2))
>     if (error >= 32)
>       origin -= 64;
>     else if (error <= -32)
>       origin += 64;    
> 
> the result is text that looks much better, as demonstrated by the ftdiff
> program. Note that you can mix this with scaled but unrounded kerning for
> maximal effect.
> 
> The issue is that the lsb/rsb_delta values depend on the hinting mode, and
> current character size and need to be cached in order to get good
> performance (you can hardly recompute them quickly on each pair)
> 
> So it means changing the caching scheme of any text layout library that wants
> to use it. Either store the values with the glyph images themselves, or in
> another table.

Thanks a lot David.

So this is something not currently implemented in our text rendering
stack IIUC.

I'll be thinking about it.


> Owen Taylor wrote a paper on this subject, with nice graphics explaining the
> issues. I can't find it anymore. I'm putting him in CC: in case he has it
> somewhere...

Yeah, excellent stuff:

  http://people.redhat.com/otaylor/grid-fitting/

While looking for it, came across someone named Taylor Owen...


Cheers,
behdad


> Hope this helps,
> 
> - David
> 
> > cheers
> > behdad
> > 
> > 
> > > An alternative would be to hack FT_Get_Kerning to return "adjusted" 
> > > values, but
> > > this would require implementing a rather complicated caching scheme 
> > > within the
> > > engine if we don't want performance to suck horribly. Moreover, this is 
> > > likely
> > > to break some libraries...
> > > 
> > > - David
> > 
> > -- 
> > behdad
> > http://behdad.org/
> > 
> > "Those who would give up Essential Liberty to purchase a little
> >  Temporary Safety, deserve neither Liberty nor Safety."
> >         -- Benjamin Franklin, 1759
> > 
> > 
> > 
-- 
behdad
http://behdad.org/

"Those who would give up Essential Liberty to purchase a little
 Temporary Safety, deserve neither Liberty nor Safety."
        -- Benjamin Franklin, 1759