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 _______________________________________________ Freetype-devel mailing list Freetype-devel@nongnu.org http://lists.nongnu.org/mailman/listinfo/freetype-devel
