[Devel] Hinting metrics

[David Chester]

Hello,

In the grand scheme of things, I think uniformly increasing kerning values 
would do more harm than good.

A better way to improve kerning would be to keep track of errors due to 
rounding, pass them to the next level up (e.g. Xft), and then make 
adjustments at that time if necessary.  For instance, if the right boundary 
of a character is rounded left, and the left boundary of the subsequent 
character is rounded right, and further, if the magnitude of the errors taken 
together exceeds half a pixel, then this should be corrected.  Under the 
conditions just described, glyphs would be kerned too closely, and a pixel's 
worth of white space should added between them.  

Here's a graphic showing three examples: the output from an unmodified 
freetype, the previously proposed solution which would increasing kerning 
values across the board, and finally, my solution as described above:

http://www.cs.mcgill.ca/~dchest/kern.png

It seems to me that since freetype doesn't deal with layout, corrections like 
this must be left to a higher-level library or application.  Is this correct?

As it is now though, the rounding errors are not recorded, so they cannot be 
passed on.  I've attached a patch to the autohinter which implements one 
possible way to pass along the information; and a second patch to ftstring 
(from the ft2demos package) which puts the information to use, making the 
necessary corrections during the layout process.  Of course for this to be 
used in real applications, a similar patch would be needed for Xft.

I'd be interested to hear thoughts on the implementation.

David Chester

freetype-kern.diff
Description: Text Data

ft2demos-kern.diff
Description: Text Data