Re: [ft] What do kerning values mean in RTL text?

[David Turner]

Paul Pedriana a écrit :
> I'm trying to understand how kerning is done in RTL text and can't
> find any documentation (on the Internet nor in Freetype) that
> addresses this.
>
> Questions:
>   - Do kerning pairs in RTL refer to two sequential glyphs in logical
> order or visual order?
>
At least for TrueType and Type 1 fonts, it only refers to visual order.
Nelson just wrote
something about TeX which apparently follows the opposite convention,
but I can guarantee
you that it is not followed by other industry-standard formats.
>   - If there is a string with the logical order is CAT and a visual
> order of TAC,
>      do kerning adjustment pairings apply for CA (logical order) or
> for AC (visual order)?
>
you'll need to apply the kernings for (T,A) then (A,C)

>   - Given that the RTL layout alrogithm
> (http://freetype.sourceforge.net/freetype2/docs/glyphs/glyphs-5.html)
>      specifies that advance width of C (from our CAT above) is applied
> before the positioning
>      and drawing of C, how and when and in what way is a kerning
> adjustment applied?
>
if you write text left-to-right, independent of its logical order, you're
going to do the following:

  prev = -1;
  xpos = 0;
  for ( n=0; n < num_visual_glyphs; n++ )
  {
    gindex = visual_glyphs[n].index;

    if ( prev > 0 )
       xpos += kerning( prev, gindex );

    drawGlyph( gindex, xpos );
    xpos += visual_glyphs[n].advance;

    prev = gindex;
  }

when writing right-to-left text, you can do the following, which results
in the same glyph positions, justified to the right:

  prev = -1;
  xpos = width;
  for ( n=num_visual_glyphs-1; n >= 0; n-- )
  {
     gindex = visual_glyphs[n].index;

     xpos   -= visual_glyphs[n].advance;
     if ( prev > 0 )
         xpos -= kerning( gindex, prev );

     drawGlyph( gindex, xpos );
     prev = gindex;
   }

to better understand why, here's an example with two glyphs,
named 'L' (for left) and 'R' (for right):

left-to-right text: 
     L.x = 0
     R.x = L.advance + kerning(L,R)
     xpos.final = L.advance + R.advance + kerning(L,R)

     xpos.final - xpos.start = L.advance + R.advance + kerning(L,R)

right-to-left text:
     R.x = width - R.advance
     L.x = width - R.advance - L.advance - kerning(L,R)
     xpos.final = L.x
     xpos.final - xpos.start = -(L.advance + R.advance + kerning(L,R))

     L.x - xpos.final = 0
     R.x - xpos.final = L.advance + kerning(L,R)

Hope this helps,

- David

> If I can get this answered, I would like to document this and
> contribute the documentation to the Freetype project, as this is
> something that appears to have flummoxed a lot of people. Every
> description of kerning I can find discusses LTR kerning and
> unfortunately seems to leave RTL kerning as an "exercise for the reader".
>
> Thanks.
>
>
>
>
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