lyx_mirror/00README_STR_METRICS_BRANCH

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
PLEASE DO NOT DO WORK ON TOP OF THIS BRANCH.
I INTEND TO REWRITE HISTORY LATER!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

This branch is where I (jmarc) try to implement string-wise metrics
computation. The goal is to have both good metrics computation (and
font with proper kerning and ligatures) and better performance than
what we have with force_paint_single_char. Moreover there has been
some code factorization in TextMetrics, where the same row-breaking
algorithm was basically implemented 3 times.

Currently everything is supposed to work for LTR text, and RTL text
should work too except possibly metrics with Arabic and Hebrew fonts.

When KEEP_OLD_METRICS_CODE is defined in TextMetrics.cpp, the new code
is tested against the old one in getPosNearX and cursorX. This can be
helpful when looking for discrepancies between the algorithms. Note
that this only makes sense when force_paint_single_char=true, since
this enforces char-by-char metrics computation.

What is done:

* Make TextMetrics methods operate on Row objects: breakRow and
  setRowHeight instead of rowBreakPoint and rowHeight.

* Change breakRow operation to operate at strings level to compute
  metrics The list of elements is stored in the row object in visual
  ordering, not logical. This will eventually allow to get rid of the
  Bidi class.

* rename getColumnNearX to getPosNearX (and change code accordingly).
  It does not make sense to return a position relative to the start of
  row, since nobody needs this.

* Re-implement cursorX and getPosNearX using row elements.

* Implement proper string metrics computation (with cache), when
  lyxrc.force_paint_single_char is false. In this case, remove also
  useless workarounds which disable kerning and ligatures.


Next steps:

* check what happens with Arabic and/or Hebrew text. There may be some
  problems related to compose characters. I suspect that some code is
  needed in FontMetrics::width.

* investigate whether RtL strings could be drawn on a string-wise basis.

* investigate whether Row::SEPARATOR elements could be used only in
  justified text. This would speed-up painting in other cases by
  lowering the number of strings to draw.

* get lots of testing.

* Get rid of old code in cursorX and getColumnNearX; it has been
  kept for comparison purpose, guarded with KEEP_OLD_METRICS_CODE in
  order to check computations.

* Profile and see how performance can be improved.


Steps for later (aka out of the scope of this branch):

* Re-implement row painting using row elements. This is not difficult
  in principle, but the code is intricate and needs some careful
  analysis. The first thing that needs to be done is to break row
  elements with the same criteria. Currently TextMetrics::breakRow
  does not consider on-the-fly spell-checking and selection changes,
  but it is not clear to me that it is required. Moreover, this thing
  would only work if we are sure that the Row object is up-to-date
  when drawing happens. This depends on the update machinery.

  This would allow to get rid of the Bidi code.


Known bugs:

* in RtL paragraphs, the end-of-paragraph marker moves the row to the
  right (ticket #9040, already present in master).

* there are still difference in what breaks words. In particular,
  RowPainter breaks strings at: selection end, spell-checking
  extremities. This seems to be harmless.

* when clicking in the right margin, GetColumnNearX does not return
  the same value as before. I am not sure whether this is important.


Other differences in behavior (aka bug fixes):

* end of paragraph markers metrics are computed with the font of the
  actual text, not default font.

* When cursor is after a LtR separator just before a RtL chunk, the
  cursor position is computed better with the new code.