gd-format.txt

GLYPH DATA
==========

 - Glyph flag
 - Metric data
 - Outline data
 - Extra data segments

OUTLINE DATA
============

Outline data of a glyph is recorded in this manner:

 - Length of items (points and inclusions)
 - Number of VF tuples
 - Item flags
 - Static dX
 - Static dY
 - Variable deltas
 - Contour mask (optional, decided by glyph flag)

Item flag list: One byte for each glyph item (point OR inclusion)

 - Points : 0 . . . . . . .
              - --- --- ---
              S  ZT  XT  YT
              - S  : Start of a contour;
              - ZT : Point type
                  - 00 : On knot
                  - 01 : Quadratic off knot
                  - 10 : Leading cubic off-knot
                  - 11 : Trailing cubic off-knot
              - XT & YT : Coordinate type of static outline
                  - 00 : Zero
                  - 01 : Short integer
                  - 10 : Positive byte or Fraction
                  - 11 : Negative byte or Fraction
 - Includes : 1 . . . . . . .
                - --- --- ---
                G  R   XT  YT
              - G : Include a glyph rather than a fragment
              - R : Reserved (for rounding and hinting?)
              
              Including a glyph or a fragment into the font. Inclusion simply
              copies outline data of the part being included, with an extra X
              and Y shift being added into it. The X and Y shift could be
              polymorphic, and they are added to all the touched points in the
              included component.

Static dX and dY coordinate:
 - For points, depending on the XT or YT:
   - For XT/YT = 00, the data is not needed.
   - For XT/YT = 01, the data is two bytes long and represents a big-endian
     signed short.
   - For XT/YT = 10 or 11, the data is interpreted as this:
       Let c be the result of read one byte.
       if c != 0 then
           return (XT/YT == 10) ? c : -c
       else
           let f = read four bytes and decode it as a unsigned Q16.16
           return (XT/YT == 10) ? f : -f
 - For inclusion, there would always two bytes for dX, representing the ID of
   the componet being included; There could be an extra shift value, and its
   form is defined by the XT/YT.

Variable X and Y deltas:
 - Use the current GVAR mechanism. For each tuple, untouched points are
   interpolated with an IUP.
    
Contour Mask:
 - A bit field to describe whether a contour would be disabled during
   rasterization. Useful for color glyphs.