Commit c30b8a12 authored by Eli Zaretskii's avatar Eli Zaretskii
Browse files

xdisp.c: Add commentary about iterator geometry in R2L paragraphs.

parent 2396cbba
......@@ -217,7 +217,26 @@ along with GNU Emacs. If not, see <>. */
glyph with suitably computed width. Both the blanks and the
stretch glyph are given the face of the background of the line.
This way, the terminal-specific back-end can still draw the glyphs
left to right, even for R2L lines. */
left to right, even for R2L lines.
Note one important detail mentioned above: that the bidi reordering
engine, driven by the iterator, produces characters in R2L rows
starting at the character that will be the rightmost on display.
As far as the iterator is concerned, the geometry of such rows is
still left to right, i.e. the iterator "thinks" the first character
is at the leftmost pixel position. The iterator does not know that
PRODUCE_GLYPHS reverses the order of the glyphs that the iterator
delivers. This is important when functions from the the move_it_*
family are used to get to certain screen position or to match
screen coordinates with buffer coordinates: these functions use the
iterator geometry, which is left to right even in R2L paragraphs.
This works well with most callers of move_it_*, because they need
to get to a specific column, and columns are still numbered in the
reading order, i.e. the rightmost character in a R2L paragraph is
still column zero. But some callers do not get well with this; a
notable example is mouse clicks that need to find the character
that corresponds to certain pixel coordinates. See
buffer_posn_from_coords in dispnew.c for how this is handled. */
#include <config.h>
#include <stdio.h>
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