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From: Werner LEMBERG
Subject: [ft-devel] Problem with FT_LOAD_IGNORE_GLOBAL_ADVANCE_WIDTH
Date: Sun, 30 Jan 2011 14:41:11 +0100 (CET)


I got a bug report about a subsetted TrueType font with a buggy `post'

    <formatType value="3.0"/>
    <italicAngle value="0.0"/>
    <underlinePosition value="-217"/>
    <underlineThickness value="150"/>
    <isFixedPitch value="1091"/>
    <minMemType42 value="70648888"/>
    <maxMemType42 value="71304253"/>
    <minMemType1 value="72025145"/>
    <maxMemType1 value="603979776"/>

In particular, the `isFixedPitch' value is incorrect (it should be
zero).  If you use the default load flags, FreeType trusts the value
in the `post' table and consequently uses `advanceWidthMax' as the
advance width for all glyphs.  This particular font, however, distorts
the shape of some glyphs in dependence on the horizontal phantom
points, causing really bad rendering results.

A quick fix is to add FT_LOAD_IGNORE_GLOBAL_ADVANCE_WIDTH to the load
flags.  However, this is a hack, not a solution.  In function
`compile_glyph_metrics' (in file ttgload.c) we have this:

  if ( face->postscript.isFixedPitch                                     &&
       ( loader->load_flags & FT_LOAD_IGNORE_GLOBAL_ADVANCE_WIDTH ) == 0 )
    advance = face->horizontal.advance_Width_Max;

I think we need some heuristics to improve that, given that the
`isFixedPitch' field is obviously not reliable.

Any suggestions?  Some ideas come to my mind:

  . Complete `advance_Width_Max' completely and always use the values
    from the `hdmx' table.

  . Recompute `advance_Width_Max' and use the original value only if
    it is greater than the actual widths.  [I don't think this is a
    good idea.]


PS: I've now implemented key `A' in ftview which allows, similar to
    `ftdiff', to toggle this flag.

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