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[truetype] Make `IUP' gvar deltas do the same as Apple (#50832).

When points are not touched by gvar interpolation deltas, FreeType
gave a slightly different result than Apple's CoreText.

The OpenType working group will update the specification to document
the following behaviour: If the two points with deltas to the `left'
and `right' of the untouched point have the same coordinate, then
the inferred delta for the untouched point should be zero.

* src/truetype/ttgxvar.c (tt_delta_interpolate): Implement new
behaviour.

git/fs: mount .git/fs: mount/attach disallowed

--- a/ChangeLog

+++ b/ChangeLog

@@ -1,3 +1,18 @@

+2017-05-02  Behdad Esfahbod  <[email protected]>

+

+	[truetype] Make `IUP' gvar deltas do the same as Apple (#50832).

+

+	When points are not touched by gvar interpolation deltas, FreeType

+	gave a slightly different result than Apple's CoreText.

+

+	The OpenType working group will update the specification to document

+	the following behaviour: If the two points with deltas to the `left'

+	and `right' of the untouched point have the same coordinate, then

+	the inferred delta for the untouched point should be zero.

+

+	* src/truetype/ttgxvar.c (tt_delta_interpolate): Implement new

+	behaviour.

+

 2017-05-02  Werner Lemberg  <[email protected]>

 	[autofit] Remove `slight' auto-hint mode again.

--- a/src/truetype/ttgxvar.c

+++ b/src/truetype/ttgxvar.c

@@ -3081,25 +3081,12 @@

       d1   = out1 - in1;

       d2   = out2 - in2;

-      if ( out1 == out2 || in1 == in2 )

+      /* If the reference points have the same coordinate but different */

+      /* delta, inferred delta is zero.  Otherwise interpolate.         */

+      if ( in1 != in2 || out1 == out2 )

       {

-        for ( p = p1; p <= p2; p++ )

-        {

-          out = in_points[p].x;

-

-          if ( out <= in1 )

-            out += d1;

-          else if ( out >= in2 )

-            out += d2;

-          else

-            out = out1;

-

-          out_points[p].x = out;

-        }

-      }

-      else

-      {

-        FT_Fixed  scale = FT_DivFix( out2 - out1, in2 - in1 );

+        FT_Fixed  scale = in1 != in2 ? FT_DivFix( out2 - out1, in2 - in1 )

+                                     : 0;

         for ( p = p1; p <= p2; p++ )