Re: Caret

From: Philippe Verdy <>
Date: Thu, 15 Nov 2012 08:09:51 +0100

One of the issues caused by the two carets is that sometmes you won"t be
able to have them BOTH visible at the same time in the screen area. This
adds another complecity : how do you locate the second caret if it's not
visible ?

Do you need to design a new key to switch easily between both to make the
other blinking position visible ? Do you need to scroll the display and
where do you scroll up or down to to display enough contents ?

Note that the text input area may be very narrow, much narrower than the
text you input there, and it is not always easy to navigate here (you
already have the left/right arrow keys to position the logical insertion
point, but if you position the localical insertion point, then you have two
visual positions (one for the BEFORE context, and the other for the AFTER

Many editors don't offer the choice, and will just display the BEFORE
context (when working in insert mode), or the AFTER context (when working
in overwrite mode) using then a single caret. This does not remove for them
the possibility of displaying the associated UBA-resolved direction of this

Another solution is simply to switch between the BEFORE and AFTER context
by just pressing the same LEFT or RIGHT move function key without actually
moving the insertion point, as if there was a zero-width character
(invisible on screen and absent from the encoded text) present at the
logical insertion point :
 * the editor knows that the insertion point is at a logical position where
the resolved UBA direction is changing
 * the editor knows if it is currently showing the BEFORE direction or the
AFTER direction with a boolean state

if you type a character or press BACKSPACE or DELETE after switching the
context, the insertion or deletion performed will be exactly the same. But
when you switch between the BEFORE and the AFTER context, not only you'll
see the visual position altered but the caret will also change its form
showing the reverse direction.

But If the BEFORE text is LTR and the AFTER text is RTL (e.g. you are ar
the logical position indicated by the bar in "lati|nARABIC". and the
paragraph default forward position starts from the LEFT, then the RIGHT
arrow key will move throughout the paragraph in the FOREWARD (AFTER)
direction. Then:
 * at the current position (in Latin) both the BEFORE and AFTER contexts
are in the LTR direction, so you don't need any dual caret, the RIGHT key
will just skip immediately to the NEXT grapheme cluster, i.e. to the RIGHT.
* at this point it reaches the point where the direction will change. Two
caret positions are possible for the same logical position. However as you
have just pressed the RIGHT key, the editors knows that you are going
foreward, and should show the BEFORE context (the before context is still
in latin which is RTL, so the caret will be positioned immediately to the
right of the Latin text, showing the LTR direction on arrow head of the
caret glyph, the display will be:
* if you press the RIGHT key a second time, the editor knows that you are
already showing the BEFORE context, so it switch to display the AFTER
context and the caret is positioned to the right of the ARABIC word and it
displays the caret at the new position with the caret glyph showing an
arrow head reversed to the LEFT direction (the logical insertion point has
NOT changed, the encoded text has also NOT changed):
* if you press agains the RIGHT (FOREWARD) key a third time, the editor
knows that it is still at a direction-changing displaying logical position
but that it is currently showingf the AFTER context. In that case it can
effectively continue moving in the FOREWARD direction and will skip the
first ARABIC letter. At this point it is no longer at a direction-changing
logical position, and a single caret is enough. The display becomes:
 (if the "|" single caret shows a directional arrow head, it points to the
right : it should be made visible in this case because it shows the
FOREWARD direction is not to the RIGHT but to the LEFT, and to continue to
the foreward direction, the key to use is still the RIGHT arrow key), so
the display should better be:

Once again, I used the notation where "|" uses the glyph for a single caret
that does not needing to display the UBA-resolved direction, "^" is the
caret used to display the arrow head to the LEFT, and "_" is the caret that
display the caret with an arrow head to the RIGHT.

This operatinig mode can work in plain-text terminals with a single caret
(except that arrowheads may possibly not be displayed; but instead the
caret may still be a half-block positioned at the TOP of the character cell
oe at the bottom, when the normal caret is a thinner block usually below,
or a full block. It could work in Emacs.

In other words, you can still create a coherent editor that will display
only one caret at a time, even if it handles the dual visual positions
associated to a single logical position where the UBA-resolved direction
Received on Thu Nov 15 2012 - 01:15:10 CST

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