How is current_matrix and desired_matrix be created?

How is current_matrix and desired_matrix be created?

[Tipo Jo]

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I know that window->contents is the buffer should be shown in window. The
redisplay code use current_matrix and desired_matrix as the content should
be shown.
So what's the relationship between contents and those matrices? How does
emacs create the two matrices from window->contents?

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Re: How is current_matrix and desired_matrix be created?

[tomas]

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On Mon, May 04, 2020 at 01:32:15PM +0800, Tipo Jo wrote:
> I know that window->contents is the buffer should be shown in window. The
> redisplay code use current_matrix and desired_matrix as the content should
> be shown.

(Disclaimer: I haven't read the code yet, but the very readable
comments by Gerd Moellmann)

> So what's the relationship between contents and those matrices? How does
> emacs create the two matrices from window->contents?

Current matrix is the status as is shown on screen. Buffer updates
(insert character here, change some property there, scroll, etc)
go (conceptually, see below) to desired matrix. So the differences
of both represent the work that has to be done.

At redisplay time, both are compared and some "minimal" set of
changes is calculated, which are transmitted to the "glass", as
Gerd calls that. At this point, current matrix is thrown away,
and desired matrix becomes current matrix.

The "conceptually" above hints at the fact that the desired
matrix is "sparse", i.e. where it is equal to the current matrix
it might have a "hole" (if I got things right it happens line-
wise). So at redisplay time it starts out empty (which means
"equal to current matrix").

Cheers
-- t

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Re: How is current_matrix and desired_matrix be created?

[Eli Zaretskii]

> Date: Mon, 4 May 2020 13:49:55 +0200
> From: <tomas@tuxteam.de>
> 
> Current matrix is the status as is shown on screen. Buffer updates
> (insert character here, change some property there, scroll, etc)
> go (conceptually, see below) to desired matrix. So the differences
> of both represent the work that has to be done.
> 
> At redisplay time, both are compared and some "minimal" set of
> changes is calculated, which are transmitted to the "glass", as
> Gerd calls that.

Small correction: the desired matrix is built as part of redisplay,
not before.

> At this point, current matrix is thrown away, and desired matrix
> becomes current matrix.

Small correction: the current matrix is never thrown away.  Instead,
it is updated by copying portions of the desired matrix into the
corresponding portions of the current matrix.  Then the _desired_
matrix is thrown away.

> The "conceptually" above hints at the fact that the desired
> matrix is "sparse", i.e. where it is equal to the current matrix
> it might have a "hole" (if I got things right it happens line-
> wise). So at redisplay time it starts out empty (which means
> "equal to current matrix").

Exactly, which is why the current matrix cannot be thrown away.

Re: How is current_matrix and desired_matrix be created?

[tomas]

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On Mon, May 04, 2020 at 05:55:44PM +0300, Eli Zaretskii wrote:

[...]

> Small correction [...]

Thanks for the corrections, appreciated :)

Cheers
-- t

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Re: How is current_matrix and desired_matrix be created?

[Eli Zaretskii]

> From: Tipo Jo <tiziyuanfang@gmail.com>
> Date: Mon, 4 May 2020 13:32:15 +0800
> 
> I know that window->contents is the buffer should be shown in window. The redisplay code use
> current_matrix and desired_matrix as the content should be shown. 
> So what's the relationship between contents and those matrices? How does emacs create the two matrices
> from window->contents?

Some of this is described in the large comment at the beginning of the
file xdisp.c.  I suggest to read that, and if you have follow-up
questions, ask them here.

Re: How is current_matrix and desired_matrix be created?

[Tipo Jo]

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Ok, thanks.

On Mon, May 4, 2020 at 10:51 PM Eli Zaretskii <eliz@gnu.org> wrote:

> > From: Tipo Jo <tiziyuanfang@gmail.com>
> > Date: Mon, 4 May 2020 13:32:15 +0800
> >
> > I know that window->contents is the buffer should be shown in window.
> The redisplay code use
> > current_matrix and desired_matrix as the content should be shown.
> > So what's the relationship between contents and those matrices? How does
> emacs create the two matrices
> > from window->contents?
>
> Some of this is described in the large comment at the beginning of the
> file xdisp.c.  I suggest to read that, and if you have follow-up
> questions, ask them here.
>

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