IDC's versus Egyptian format controls

[Richard Wordingham via Unicode]

On Fri, 16 Feb 2018 11:10:29 -0800
Ken Whistler via Unicode <unicode at unicode.org> wrote:

> On 2/16/2018 11:00 AM, Asmus Freytag via Unicode wrote:
> 
> On 2/16/2018 8:00 AM, Richard Wordingham via Unicode wrote:
> >> That doesn't square well with, "An implementation *may* render a
> >> valid Ideographic Description Sequence either by rendering the
> >> individual characters separately or by parsing the Ideographic
> >> Description Sequence and drawing the ideograph so described." (TUS
> >> 10.0 p704, in Section 18.2)  
> 
> Emphasis on the "may". In point of fact, no widespread layout engine
> or set of fonts does parse IDS'es to turn them into single ideographs
> for display. That would be a highly specialized display.

And doing it reasonably well could be a lot of work.  However, I don't
see any good reason to discourage fonts from doing it by default, which
is what is now being proposed.

> > Should we ask t make the default behavior (visible IDS characters) 
> > more explicit?  
> 
> Ask away.

> > I don't mind allowing the other as an option (it's kind of the
> > reverse of the "show invisible"
> > mode, which we also allow, but for which we do have a clear
> > default).  

If that analogy is to be enforced, that strikes me as a major change
to the allowed meaning of the IDCs.  A default form should be the
natural form for reading, and it has already been stated that
visible IDCs are not intuitive.  And I thought I was joking when I
suggested that Unicode was deliberately designed to stifle innovation. 

Now, one could suggest that IDCs should be retained as sutures in
parsed IDSes.  However, even that is a change in the character
identity. Having visible IDCs is rather like making every Devanagari
virama visble. It's an admission that the font cannot cope.   For
IDSes, it is not unreasonable for a font to lack the ability to parse
them.

Richard.