OverStrike control character

[Richard Wordingham]

On Tue, 16 Jun 2020 16:43:16 +0000
Abraham Gross via Unicode <unicode at unicode.org> wrote:

> > What are these "pixels" to which you refer?  Fonts these days are
> > defined in terms of strokes, not pixels. 
> 
> even though fonts are vectors, they still get rendered onto a raster
> screen. but the point was that they get overlayed and centered
> horizontally.
> 
> > Consider <l, OVERSTRIKE, m> and <m, OVERSTRIKE, l> in a proportional
> > width font.  Are you expecting the rendering system to position the
> > 'l' using the knowledge that it will be overstruck? Overstriking is
> > designed for a teletype with fixed width characters.  
> 
> You can think of the knowledge of being overstruck like the knowledge
> fonts have of characters being combined to with diacritics – Fonts
> can specify an anchor point where the diacritics will go. Except with
> overstrike, the anchor will always be the center. The overstrike
> character is sorta like a ZWJ (zero width joiner) that turns the next
> character into a "diacritic". (hope this explanation makes sense)

You miss the problem.  There is an issue of advance width.  Font
writers by and large don't seem very fond of making 'i' with a
circumflex wider than one without.  Some bite the bullet - there is
at least one Arabic font where adding vowel marks changes the
consonant skeleton. Your equivalence calls for <l, OVERSTRIKE, m> and
<m, OVERSTRIKE, l> to have the same advance width.

Richard.