Re: using Sketch as a True Type font editor

[Tamito KAJIYAMA]

Pierre HANSER <address@hidden> writes:
| 
| I realized today Sketch was quite appropriate to generate TrueType
| fonts, and so it could become the missing TrueType font editor in
| the free software world.

I'm interested in font creation with Sketch, too.

| The idea is to use Sketch to generate individual glyphs, and then
| to use Just Van Rossum's "FontTools" to transform them into a True
| Type font.

Very exciting :-)

| The difficulties:
| 
|  - maths in sketch driver to get accurate glyph descriptions
(snip)
| Questions:
| 
|   for the math part, has anybody a hint on the difficulty?

Glyphs in TrueType fonts are essentially described in terms of
quadratic B-splines.  Roughly speaking, a quadratic B-spline is
a sequence of line segments such that each segment consists of
two on-points and a off-point (control point) between them.  On
the other hand, curves drawn by Sketch are represented by cubic
Bezier curves, i.e. sequences of line segments consisting of two
on-points and two off-points between them.

Converting quadratic B-spline into cubic Bezier curves is easy,
since the former is a subclass of the latter.  However,
converting cubic Bezier curves into quadratic B-splines is not
trivial, and errors in approximation may be large.

I don't know (and would like to know) a good algorithm for
approximating cubic Bezier curves with quadratic B-splines if
such a beast is already known.

IMHO, Sketch is suitable to edit PostScript Type 1 font glyphs,
but is not for TrueType font glyphs (as of today, of course).
One approach to this problem is to introduce a new Sketch object
for drawing quadratic B-splines directly, I think.

|   has anybody ever undertaken the same task?

Although that is the opposite, an algorithm for converting
quadratic B-splines (read from TrueType font files using the
FreeType library) into cubic Bezier curves is implemented in
JapaneseText, a Sketch script of mine for drawing Japanese
characters in Sketch.  The script is available in the following
location:

  http://pseudo.grad.sccs.chukyo-u.ac.jp/~kajiyama/sketch/

Regards,

-- 
KAJIYAMA, Tamito <address@hidden>