Re: Pixel-based display functions

[Eli Zaretskii <eliz@gnu.org>]

> From: Lars Ingebrigtsen <larsi@gnus.org>
> Cc: Eli Zaretskii <eliz@gnu.org>,  emacs-devel@gnu.org
> Date: Sat, 31 Jan 2015 17:59:31 +1100
> 
> When redisplay happens it's kinda too late to make layout decisions.
> Unless you extend the display engine to allow constraint-based layouts.
> That'd be cool, but I didn't think that was in the offing?

AFAIR, no one has ever presented a set of requirements for supporting
such layouts.  If you or someone else can do that, it might be good to
have that in TODO.

> +---------------+----+----------------+-----------------------+
> |R1C1           |R1C2|RA1C2 With Space|R1C2                   |
> +---------------+----+----------------+-----------------------+
> |R2C1 and R2C2 in one|RC4-with-a-really-long-unbreakable-thing|
> |simple box          |                                        |
> +--------------------+----------------------------------------+

How to decipher the RnCm notation here? "row n, column m"?  If so, why
are there two instances of R1C2, and what does RA1C2 mean?

Also what is the order of the texts in a buffer, i.e. is any of the
RnCm a continuation of text in some other cell, or are they all
independent strings that don't exits as single contiguous text in any
buffer?

> where each of the two main columns are said to be 50% of the width, but
> where one of the elements can't be filled, so you have to extend that
> column and compress the other columns to make things fit.

I'm not sure I understand how the layout is specified on the "document
language" (whatever that is) level.  IOW, what is known about the
layout when this text is about to be rendered?