[ft] Strange hinting of cff font

[Boris Letocha]

Hi,

 

We have found strange hinting in attached a.cff font – some characters (eg. “lmn”) are shown 1 pixel under baseline (see in attachment) in 300 DPI and 10 pt size, without antialiazing, if you use autohinter or without hinter it looks ok.

 

Now please be kind to me in next lines, I really don’t understand hinting stuff enough.

If you look for example at “n” character it start with hstems (17,-4) -21 and (471, 451) -20.

 

Hstems are processed in this code from pshrec.c:

  /* add a new single stem (called from "T1Stem" method) */

  static FT_Error

  ps_dimension_add_t1stem( PS_Dimension  dim,

                           FT_Int        pos,

                           FT_Int        len,

                           FT_Memory     memory,

                           FT_Int       *aindex )

  {

    FT_Error  error = 0;

    FT_UInt   flags = 0;

 

 

    /* detect ghost stem */

    if ( len < 0 )

    {

      flags |= PS_HINT_FLAG_GHOST;

      if ( len == -21 )

      {

        flags |= PS_HINT_FLAG_BOTTOM;

        pos   += len;

      }

      len = 0;

    }

 

This looks correctly by specification – it contains these 2 “ghost” hstems on positions -4 and 471.

 

Problem probably is that number -4 is scaled to -10 then floored to -64 (-1 pixel) in this code in pshalgo.c (pos=-10 and len=0):

          /* Stem adjustment tries to snap stem widths to standard

           * ones.  This is important to prevent unpleasant rounding

           * artefacts.

           */

          if ( glyph->do_stem_adjust )

          {

            if ( len <= 64 )

            {

              /* the stem is less than one pixel; we will center it

               * around the nearest pixel center

               */

#if 1

              pos = FT_PIX_FLOOR( pos + ( len >> 1 ) );

#else

             /* this seems to be a bug! */

              pos = pos + FT_PIX_FLOOR( len >> 1 );

#endif

              len = 64;

            }

            else

            {

              len = psh_dimension_quantize_len( dim, len, 0 );

            }

          }

 

This #if 1 already looks suspicious, so we just tried to change it from FT_PIX_FLOOR to FT_PIX_ROUND:

#if 1

              pos = FT_PIX_ROUND( pos + ( len >> 1 ) );

#else

 

And after this change it also looks correctly. Also comment before talks about “center it around the nearest pixel center” – floor is not nearest from my understanding, but I really don’t understand this code enough.

 

Now main question it is wrong font or bug in FreeType. Do you know any other product other then FreeType to test this font in same conditions?

 

Thanks,

 

Boris Letocha

 

strangehint.png
Description: strangehint.png

a.cff
Description: a.cff