[Freetype] The text layout pipeline

[Paul Pedriana]

I am trying to understand the complete text layout
pipeline. I think I have it figured out and would like
to present my view of it here. If anybody could
correct any problems with it or provide any additional
insight, I would greatly appreciate it. Sorry about
the
odd wrapping here, but my lamo free Yahoo email does
this. :(

   1. Divide text into paragraphs.
       * A paragraph is delimited by newline
         characters or is simply any self-contained
         text.

   2. Divide the paragraph into runs of same script
      and direction.
       * These runs are given a direction embedding
         level, based on Unicode TR9.
       * It's not necessary that new paragraph
         strings be created, it's only necessary to
         know the run information.
       * This step is only necessary if there is
         complex script in the paragraph.

   3. Split each of the above runs into runs of
      same font style.
       * If text is all of the same style but you need
         multiple fonts to support the characters,
         then split based on the font differences.
         Note that this may require looking ahead to
         the shaping phase to see if your font has
         alternative ways of supporting a character
         (e.g. a + ` instead of just à).
       * This step is only necessary if multiple
         styles are supported or substitution fonts
         are supported.

   4. Do shaping of text.
       * This means making a new version of the
         paragraph text which has the actual
         characters used in display of the text.
       * It will be useful or necessary to provide
         a way to map items from this string back
         to the original string.
       * Use font info to decide if you can merge
         characters (e.g. à instead of a + `) or
         need to split characters (e.g. a + `
         instead of just à).
       * Do script-specific modifications, such as
         Thai decoration substitutions and Arabic
         contextual substitutions.
       * Don't do insertions or removals of kashidas
         or other characters which would be part of
         justification of lines.
       * No reordering is done here; the text is
         shaped in logical order (not visual order).
       * This step is only necessary if there is
         complex script in the paragraph.

   5. Do line breaking.
       * Line breaking is done based on Unicode TR7,
         with a dictionary approach for Thai.
       * Line breaking is based on the shaped
         character text, not the original text.
       * Line breaking is done on the entire
         paragraph and not individual runs.
       * Line breaking is done based on the advance
         widths of the shaped character text in
         logical order (not visual order) and before
         reordering is done.
       * It seems to me that glyphs with strange
         advance widths could cause this to do
         imperfect breaking.
       * This step is only necessary if the text
         needs to wrap within some boundary.

   6. Apply justification.
       * This means inserting spaces and/or kashidas
         between words and/or characters.
       * This either generates a new set of advance
         widths and/or inserts space and/or kashida
         characters.
       * This step is only necessary if the text is
         justified (stretched to fit its bounds).

   7. Generate a display list.
       * This is based on the font's glyph metrics
         and texture (bitmap) metrics.
       * Information about font coloring or any
         other rendering or texture shading settings
         would be associated with this display list.
       * Algorithmic effects such as underlines and
         strikethroughs would be associated with this
         display list.
       * This step is only necessary if you need to
         draw the text.





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