GNU bug report logs - #52129
29.0.50; Wish: Extend `:align-to center' to work on lines in buffer

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Message #5 received at submit <at> debbugs.gnu.org (full text, mbox):

From: Arthur Miller <arthur.miller <at> live.com>
To: bug-gnu-emacs <at> gnu.org
Subject: 29.0.50; Wish: Extend `:align-to center' to work on lines in buffer
Date: Fri, 26 Nov 2021 21:01:11 +0100

[Message part 1 (text/plain, inline)]

I am not sure if the title is descriptive enough, or even correct, but as seen
from attached code and image, I am trying to align two strings in a center of
each strings line.

It works now when string-pixel-width correctly calculates pixel widths. However,
I would prefer if I could just use :align-to center as a spec for a string as
property. Currently we have to use space as a spacer as we used to use
gif-spacers back in time.

I am not sure about how semantics would work, there are several cases to
consider, but generally if a string or a stretch of text in a string were given
text property 'display '(:align-to center), then Emacs would align that stretch
of text in the middle of the line that text occupies.

There would be cases where text dooes not fit the line, where there are multiple
such substring in same line maybe etc.

I don't know if it is possible or not, I am not familar with the display code,
nor am I sure about the syntax of proposal, but it would be nice and probably
useful to have some more declarative approach then what I had to do procedurally
in the attached example.

[aligned-clock.png (image/png, attachment)]

[emacs-vision-clock.el (text/plain, attachment)]

[Message part 4 (text/plain, inline)]

In GNU Emacs 29.0.50 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.30, cairo version 1.17.4)
 of 2021-11-26 built on pascal
Repository revision: c32e8bdc23214793ffcb065ba8478570679c1f0a
Repository branch: alpha-patch
Windowing system distributor 'The X.Org Foundation', version 11.0.12101001
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Message #8 received at 52129 <at> debbugs.gnu.org (full text, mbox):

From: Eli Zaretskii <eliz <at> gnu.org>
To: Arthur Miller <arthur.miller <at> live.com>
Cc: 52129 <at> debbugs.gnu.org
Subject: Re: bug#52129: 29.0.50;
 Wish: Extend `:align-to center' to work on lines in buffer
Date: Sat, 27 Nov 2021 07:59:33 +0200

> From: Arthur Miller <arthur.miller <at> live.com>
> Date: Fri, 26 Nov 2021 21:01:11 +0100
> 
> I am not sure if the title is descriptive enough, or even correct, but as seen
> from attached code and image, I am trying to align two strings in a center of
> each strings line.
> 
> It works now when string-pixel-width correctly calculates pixel widths. However,
> I would prefer if I could just use :align-to center as a spec for a string as
> property. Currently we have to use space as a spacer as we used to use
> gif-spacers back in time.
> 
> I am not sure about how semantics would work, there are several cases to
> consider, but generally if a string or a stretch of text in a string were given
> text property 'display '(:align-to center), then Emacs would align that stretch
> of text in the middle of the line that text occupies.

I don't understand what exactly are you asking for.  Is your problem
that you have to prepend some character to the string and put the
:align-to spec on that character?  Because AFAIU that simple measure
should achieve your goal.  If that's not what you want, or if what I
described doesn't work, please show some simple code to explain your
requests.

Message #11 received at 52129 <at> debbugs.gnu.org (full text, mbox):

From: Arthur Miller <arthur.miller <at> live.com>
To: Eli Zaretskii <eliz <at> gnu.org>
Cc: 52129 <at> debbugs.gnu.org
Subject: Re: bug#52129: 29.0.50; Wish: Extend `:align-to center' to work on
 lines in buffer
Date: Sat, 27 Nov 2021 09:47:58 +0100

Eli Zaretskii <eliz <at> gnu.org> writes:

>> From: Arthur Miller <arthur.miller <at> live.com>
>> Date: Fri, 26 Nov 2021 21:01:11 +0100
>> 
>> I am not sure if the title is descriptive enough, or even correct, but as seen
>> from attached code and image, I am trying to align two strings in a center of
>> each strings line.
>> 
>> It works now when string-pixel-width correctly calculates pixel widths. However,
>> I would prefer if I could just use :align-to center as a spec for a string as
>> property. Currently we have to use space as a spacer as we used to use
>> gif-spacers back in time.
>> 
>> I am not sure about how semantics would work, there are several cases to
>> consider, but generally if a string or a stretch of text in a string were given
>> text property 'display '(:align-to center), then Emacs would align that stretch
>> of text in the middle of the line that text occupies.
>
> I don't understand what exactly are you asking for.  Is your problem
> that you have to prepend some character to the string and put the
> :align-to spec on that character?  Because AFAIU that simple measure
> should achieve your goal.

You mean something like this:

#+begin_src emacs-lisp
(defun evc--time ()
  (propertize
   (time-stamp-string " %H:%M") 'face evc--time-face 'display '(space :align-to center)))

(defun evc--date ()
  (propertize
   (concat
    " "
    (capitalize (time-stamp-string "%A")) ". "
    (capitalize (time-stamp-string "%B %d"))) 'face evc--date-face 'display '(space :align-to center)))

(defun evc--update ()
  (let ((time (evc--time))
        (date (evc--date)))
    (when (frame-live-p evc--frame)
      (select-frame evc--frame))
    (with-current-buffer evc--buffer
      (erase-buffer)
      (insert time "\n" date))))
#+end_src

That "simple measure" does not achieve the goal of aligning involved strings to
center; or I don't know understand how you mean.

This does:

#+begin_src emacs-lisp
(defun evc--time ()
  (propertize
   (time-stamp-string " %H:%M") 'face evc--time-face))

(defun evc--date ()
  (propertize
   (concat
    (capitalize (time-stamp-string "%A")) ". "
    (capitalize (time-stamp-string "%B %d"))) 'face evc--date-face))

(let* ((tlen (* 0.5 (string-pixel-width time)))
             (dlen (* 0.5 (string-pixel-width date)))
             (spct (propertize " " 'display `(space :align-to (- center (,tlen)))))
             (spcd (propertize " " 'display `(space :align-to (- center (,dlen))))))
        (insert spct time "\n" spcd date))
#+end_src

>                         please show some simple code to explain your
> requests.

I hope above illustrates that it is not just to prepend a space, you have to
calculate how much it should extend to. This has to be done for each line.

Also, even if it would be just to prepend a spacer character, the goal is still
to align text in center of a line, so why can't we just renderer to do it for
us, instead of having such procedural way of telling it what to do. Why can we
not just tell:

(defun evc--time ()
  (propertize
   (time-stamp-string " %H:%M") 'face evc--time-face 'display '(line :align-to center)))

or something similar.

Also what if the text is longer then visual line? Would it be possible for the
rendering engine to center part of the tring with 'aligne-to center' property,
and cull extensive part on sides that are outside. That is not acheavable with a spacer.

Message #14 received at 52129 <at> debbugs.gnu.org (full text, mbox):

From: Eli Zaretskii <eliz <at> gnu.org>
To: Arthur Miller <arthur.miller <at> live.com>
Cc: 52129 <at> debbugs.gnu.org
Subject: Re: bug#52129: 29.0.50; Wish: Extend `:align-to center' to work on
 lines in buffer
Date: Sat, 27 Nov 2021 11:15:16 +0200

> From: Arthur Miller <arthur.miller <at> live.com>
> Cc: 52129 <at> debbugs.gnu.org
> Date: Sat, 27 Nov 2021 09:47:58 +0100
> 
> Eli Zaretskii <eliz <at> gnu.org> writes:
> 
> > I don't understand what exactly are you asking for.  Is your problem
> > that you have to prepend some character to the string and put the
> > :align-to spec on that character?  Because AFAIU that simple measure
> > should achieve your goal.
> 
> You mean something like this:
> 
> #+begin_src emacs-lisp
> (defun evc--time ()
>   (propertize
>    (time-stamp-string " %H:%M") 'face evc--time-face 'display '(space :align-to center)))
> 
> (defun evc--date ()
>   (propertize
>    (concat
>     " "
>     (capitalize (time-stamp-string "%A")) ". "
>     (capitalize (time-stamp-string "%B %d"))) 'face evc--date-face 'display '(space :align-to center)))

No, I mean to put this property:

    'display '(space :align-to center)

on a space character (or any other character, which will not be
displayed) that is prepended to the time-stamp-string you want to
display.

> (let* ((tlen (* 0.5 (string-pixel-width time)))
>              (dlen (* 0.5 (string-pixel-width date)))
>              (spct (propertize " " 'display `(space :align-to (- center (,tlen)))))
>              (spcd (propertize " " 'display `(space :align-to (- center (,dlen))))))
>         (insert spct time "\n" spcd date))
> 
> I hope above illustrates that it is not just to prepend a space, you have to
> calculate how much it should extend to.

So you want to center the string, not its first character?  OK, but
why is the above a problem?  It's simple enough code, I think.

> Also, even if it would be just to prepend a spacer character, the goal is still
> to align text in center of a line, so why can't we just renderer to do it for
> us, instead of having such procedural way of telling it what to do. Why can we
> not just tell:
> 
> (defun evc--time ()
>   (propertize
>    (time-stamp-string " %H:%M") 'face evc--time-face 'display '(line :align-to center)))
> 
> or something similar.

Because the Emacs display code examines buffer text one character at a
time, left to right, and produces the glyphs for display for each
character before it goes to the next.  You are asking the display
engine to know the display width of your string before it performs the
layout calculations for that string, and that cannot work.  The
display code cannot even assume that it will traverse all the
characters of a line, it could be forced to stop before it reaches the
end of the line, in which case it cannot know that width even
post-factum.

> Also what if the text is longer then visual line? Would it be possible for the
> rendering engine to center part of the tring with 'aligne-to center' property,
> and cull extensive part on sides that are outside. That is not acheavable with a spacer.

You want the display engine to decide where to break such a long line?
Or do you want to decide that up front and indicate that via the
portion of the text on which you put this hypothetical property?  IOW,
what exactly is the meaning of "cull" here?

Message #17 received at 52129 <at> debbugs.gnu.org (full text, mbox):

From: Arthur Miller <arthur.miller <at> live.com>
To: Eli Zaretskii <eliz <at> gnu.org>
Cc: 52129 <at> debbugs.gnu.org
Subject: Re: bug#52129: 29.0.50; Wish: Extend `:align-to center' to work on
 lines in buffer
Date: Sat, 27 Nov 2021 12:06:19 +0100

Eli Zaretskii <eliz <at> gnu.org> writes:

>> From: Arthur Miller <arthur.miller <at> live.com>
>> Cc: 52129 <at> debbugs.gnu.org
>> Date: Sat, 27 Nov 2021 09:47:58 +0100
>> 
>> Eli Zaretskii <eliz <at> gnu.org> writes:
>> 
>> > I don't understand what exactly are you asking for.  Is your problem
>> > that you have to prepend some character to the string and put the
>> > :align-to spec on that character?  Because AFAIU that simple measure
>> > should achieve your goal.
>> 
>> You mean something like this:
>> 
>> #+begin_src emacs-lisp
>> (defun evc--time ()
>>   (propertize
>>    (time-stamp-string " %H:%M") 'face evc--time-face 'display '(space :align-to center)))
>> 
>> (defun evc--date ()
>>   (propertize
>>    (concat
>>     " "
>>     (capitalize (time-stamp-string "%A")) ". "
>>     (capitalize (time-stamp-string "%B %d"))) 'face evc--date-face 'display '(space :align-to center)))
>
> No, I mean to put this property:
>
>     'display '(space :align-to center)
>
> on a space character (or any other character, which will not be
> displayed) that is prepended to the time-stamp-string you want to
> display.

Aha, like this?:

(defun evc--time ()
  (concat (propertize " " 'display '(space :align-to center))
          (propertize
           (time-stamp-string " %H:%M") 'face evc--time-face)))

That still seems to be dependanble on when the time string is inserted? Because
when I test to insert like this:

(defun evc--update ()
  (let ((time (evc--time))
        (date (evc--date)))
    (when (frame-live-p evc--frame)
      (select-frame evc--frame))
    (with-current-buffer evc--buffer
      (erase-buffer)
      (insert time "\n" date)
      (goto-char 1))))

the string is not centered correctly; there is ~70 pixels difference between
left and right margins.

Maybe if I inserted longer string first, fitted frame to buffer, and than
inserted time, but I don't think it is relevant if you say it is not possible to
do it due to renderer only looking at one char at a time.

> Because the Emacs display code examines buffer text one character at a
> time, left to right, and produces the glyphs for display for each
> character before it goes to the next.  You are asking the display
> engine to know the display width of your string before it performs the
> layout calculations for that string, and that cannot work.  The
> display code cannot even assume that it will traverse all the
> characters of a line, it could be forced to stop before it reaches the
> end of the line, in which case it cannot know that width even
> post-factum.
>
>> Also what if the text is longer then visual line? Would it be possible for the
>> rendering engine to center part of the tring with 'aligne-to center' property,
>> and cull extensive part on sides that are outside. That is not acheavable with a spacer.
>
> You want the display engine to decide where to break such a long line?
> Or do you want to decide that up front and indicate that via the
> portion of the text on which you put this hypothetical property?  IOW,

Meant mostly that display engine would keep marked portion of the string
centered on the display, for example if there is some more important of the
string or so, but at the second thought I don't see much use of such
feature.

Anyway, I understand now when you say that renderer only looks at one char at a
time.

Is there any way to make it aware of surrounding context, like entire line? Or
would that be too demanding computationally? I guess there is a reason why it is
made one char at a time.

Message #20 received at 52129 <at> debbugs.gnu.org (full text, mbox):

From: Eli Zaretskii <eliz <at> gnu.org>
To: Arthur Miller <arthur.miller <at> live.com>
Cc: 52129 <at> debbugs.gnu.org
Subject: Re: bug#52129: 29.0.50; Wish: Extend `:align-to center' to work on
 lines in buffer
Date: Sat, 27 Nov 2021 13:25:13 +0200

> From: Arthur Miller <arthur.miller <at> live.com>
> Cc: 52129 <at> debbugs.gnu.org
> Date: Sat, 27 Nov 2021 12:06:19 +0100
> 
> > No, I mean to put this property:
> >
> >     'display '(space :align-to center)
> >
> > on a space character (or any other character, which will not be
> > displayed) that is prepended to the time-stamp-string you want to
> > display.
> 
> Aha, like this?:
> 
> (defun evc--time ()
>   (concat (propertize " " 'display '(space :align-to center))
>           (propertize
>            (time-stamp-string " %H:%M") 'face evc--time-face)))
> 
> That still seems to be dependanble on when the time string is inserted? Because
> when I test to insert like this:
> 
> (defun evc--update ()
>   (let ((time (evc--time))
>         (date (evc--date)))
>     (when (frame-live-p evc--frame)
>       (select-frame evc--frame))
>     (with-current-buffer evc--buffer
>       (erase-buffer)
>       (insert time "\n" date)
>       (goto-char 1))))
> 
> the string is not centered correctly; there is ~70 pixels difference between
> left and right margins.

That's because it centers the first character of the string, not its
middle character.

> Is there any way to make it aware of surrounding context, like entire line? Or
> would that be too demanding computationally?

I don't think I understand what you mean by "surrounding context", and
what would it mean for it to "be aware" of it?

> I guess there is a reason why it is made one char at a time.

Simplicity and speed, I guess.  But that's not for me to answer: I
wasn't there when this was designed.

Message #23 received at 52129 <at> debbugs.gnu.org (full text, mbox):

From: Arthur Miller <arthur.miller <at> live.com>
To: Eli Zaretskii <eliz <at> gnu.org>
Cc: 52129 <at> debbugs.gnu.org
Subject: Re: bug#52129: 29.0.50; Wish: Extend `:align-to center' to work on
 lines in buffer
Date: Sun, 28 Nov 2021 00:40:30 +0100

Eli Zaretskii <eliz <at> gnu.org> writes:

>> From: Arthur Miller <arthur.miller <at> live.com>
>> Cc: 52129 <at> debbugs.gnu.org
>> Date: Sat, 27 Nov 2021 12:06:19 +0100
>> 
>> > No, I mean to put this property:
>> >
>> >     'display '(space :align-to center)
>> >
>> > on a space character (or any other character, which will not be
>> > displayed) that is prepended to the time-stamp-string you want to
>> > display.
>> 
>> Aha, like this?:
>> 
>> (defun evc--time ()
>>   (concat (propertize " " 'display '(space :align-to center))
>>           (propertize
>>            (time-stamp-string " %H:%M") 'face evc--time-face)))
>> 
>> That still seems to be dependanble on when the time string is inserted? Because
>> when I test to insert like this:
>> 
>> (defun evc--update ()
>>   (let ((time (evc--time))
>>         (date (evc--date)))
>>     (when (frame-live-p evc--frame)
>>       (select-frame evc--frame))
>>     (with-current-buffer evc--buffer
>>       (erase-buffer)
>>       (insert time "\n" date)
>>       (goto-char 1))))
>> 
>> the string is not centered correctly; there is ~70 pixels difference between
>> left and right margins.
>
> That's because it centers the first character of the string, not its
> middle character.

That too; so you see, it is not as simple to get it right by just prepending a
space with :align-to center property.

>> Is there any way to make it aware of surrounding context, like entire line? Or
>> would that be too demanding computationally?
>
> I don't think I understand what you mean by "surrounding context", and
> what would it mean for it to "be aware" of it?

With context I meant simply the line, or at least part of the line to be
displayed. To be aware of it = to take it into consideration when displaying
line, so things like suggested would be possible.

Message #26 received at 52129 <at> debbugs.gnu.org (full text, mbox):

From: Eli Zaretskii <eliz <at> gnu.org>
To: Arthur Miller <arthur.miller <at> live.com>
Cc: 52129 <at> debbugs.gnu.org
Subject: Re: bug#52129: 29.0.50; Wish: Extend `:align-to center' to work on
 lines in buffer
Date: Sun, 28 Nov 2021 09:48:48 +0200

> From: Arthur Miller <arthur.miller <at> live.com>
> Cc: 52129 <at> debbugs.gnu.org
> Date: Sun, 28 Nov 2021 00:40:30 +0100
> 
> > That's because it centers the first character of the string, not its
> > middle character.
> 
> That too; so you see, it is not as simple to get it right by just prepending a
> space with :align-to center property.

We've been through this: you need to dynamically compute the pixel
coordinate to align-to, as function of the string width.

> >> Is there any way to make it aware of surrounding context, like entire line? Or
> >> would that be too demanding computationally?
> >
> > I don't think I understand what you mean by "surrounding context", and
> > what would it mean for it to "be aware" of it?
> 
> With context I meant simply the line, or at least part of the line to be
> displayed. To be aware of it = to take it into consideration when displaying
> line, so things like suggested would be possible.

The display engine considers the line as a whole only when the entire
screen line has been laid out, i.e. we get to a newline or the X
coordinate is at the right edge of the window.  Before that, there's
no information about the "line context".

Message #29 received at 52129 <at> debbugs.gnu.org (full text, mbox):

From: Arthur Miller <arthur.miller <at> live.com>
To: Eli Zaretskii <eliz <at> gnu.org>
Cc: 52129 <at> debbugs.gnu.org
Subject: Re: bug#52129: 29.0.50; Wish: Extend `:align-to center' to work on
 lines in buffer
Date: Sun, 28 Nov 2021 11:38:18 +0100

Eli Zaretskii <eliz <at> gnu.org> writes:

>> From: Arthur Miller <arthur.miller <at> live.com>
>> Cc: 52129 <at> debbugs.gnu.org
>> Date: Sun, 28 Nov 2021 00:40:30 +0100
>> 
>> > That's because it centers the first character of the string, not its
>> > middle character.
>> 
>> That too; so you see, it is not as simple to get it right by just prepending a
>> space with :align-to center property.
>
> We've been through this: you need to dynamically compute the pixel
> coordinate to align-to, as function of the string width.

Yes, I know. I just reflected further to say it wasn't "just" to add align-to
property but it needs more computations :).

But if you say it is one character at a time, and it is not much to do about it,
then it is not much more to be done about that.

>> >> Is there any way to make it aware of surrounding context, like entire line? Or
>> >> would that be too demanding computationally?
>> >
>> > I don't think I understand what you mean by "surrounding context", and
>> > what would it mean for it to "be aware" of it?
>> 
>> With context I meant simply the line, or at least part of the line to be
>> displayed. To be aware of it = to take it into consideration when displaying
>> line, so things like suggested would be possible.
>
> The display engine considers the line as a whole only when the entire
> screen line has been laid out, i.e. we get to a newline or the X
> coordinate is at the right edge of the window.  Before that, there's
> no information about the "line context".

So it would in principle mean, to render line twice, if such property would be
implemented, and that could be very expensive computation, so it is off the radar.

Message #32 received at 52129 <at> debbugs.gnu.org (full text, mbox):

From: Lars Ingebrigtsen <larsi <at> gnus.org>
To: Arthur Miller <arthur.miller <at> live.com>
Cc: Eli Zaretskii <eliz <at> gnu.org>, 52129 <at> debbugs.gnu.org
Subject: Re: bug#52129: 29.0.50; Wish: Extend `:align-to center' to work on
 lines in buffer
Date: Mon, 29 Nov 2021 16:59:59 +0100

Arthur Miller <arthur.miller <at> live.com> writes:

> So it would in principle mean, to render line twice, if such property would be
> implemented, and that could be very expensive computation, so it is
> off the radar.

Yes, I think so, so I'm closing this bug report.

-- 
(domestic pets only, the antidote for overdose, milk.)
   bloggy blog: http://lars.ingebrigtsen.no

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