[C source] What is gravity and the change_gravity argument?

[C source] What is gravity and the change_gravity argument?

[Akib Azmain Turja]

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In src/termhooks.h, I found the following two members of struct
terminal:

--8<---------------cut here---------------start------------->8---
  /* This hook is called to change the size of frame F's native
   (underlying) window.  If CHANGE_GRAVITY, change to top-left-corner
   window gravity for this size change and subsequent size changes.
   Otherwise we leave the window gravity unchanged.  */
  void (*set_window_size_hook) (struct frame *f, bool change_gravity,
                                int width, int height);

  /* CHANGE_GRAVITY is 1 when calling from Fset_frame_position,
   to really change the position, and 0 when calling from
   *_make_frame_visible (in that case, XOFF and YOFF are the current
   position values).  It is -1 when calling from gui_set_frame_parameters,
   which means, do adjust for borders but don't change the gravity.  */

  void (*set_frame_offset_hook) (struct frame *f, register int xoff,
                                 register int yoff, int change_gravity);
--8<---------------cut here---------------end--------------->8---

X and PGTK uses that 'change_gravity' argument, I can't understand what
they are doing with them.  I searched the internet and found several
answers[1], but couldn't make any sense.


Footnotes:
[1]  https://stackoverflow.com/q/3482742/11819469

-- 
Akib Azmain Turja

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Re: [C source] What is gravity and the change_gravity argument?

[Eli Zaretskii]

> From: Akib Azmain Turja <akib@disroot.org>
> Date: Fri, 08 Jul 2022 18:40:16 +0600
> 
> In src/termhooks.h, I found the following two members of struct
> terminal:
> 
> --8<---------------cut here---------------start------------->8---
>   /* This hook is called to change the size of frame F's native
>    (underlying) window.  If CHANGE_GRAVITY, change to top-left-corner
>    window gravity for this size change and subsequent size changes.
>    Otherwise we leave the window gravity unchanged.  */
>   void (*set_window_size_hook) (struct frame *f, bool change_gravity,
>                                 int width, int height);
> 
>   /* CHANGE_GRAVITY is 1 when calling from Fset_frame_position,
>    to really change the position, and 0 when calling from
>    *_make_frame_visible (in that case, XOFF and YOFF are the current
>    position values).  It is -1 when calling from gui_set_frame_parameters,
>    which means, do adjust for borders but don't change the gravity.  */
> 
>   void (*set_frame_offset_hook) (struct frame *f, register int xoff,
>                                  register int yoff, int change_gravity);
> --8<---------------cut here---------------end--------------->8---
> 
> X and PGTK uses that 'change_gravity' argument, I can't understand what
> they are doing with them.  I searched the internet and found several
> answers[1], but couldn't make any sense.

Try this:

  https://tronche.com/gui/x/xlib/window/attributes/gravity.html

Re: [C source] What is gravity and the change_gravity argument?

[Po Lu]

Eli Zaretskii <eliz@gnu.org> writes:

> Try this:
>
>   https://tronche.com/gui/x/xlib/window/attributes/gravity.html

Keep in mind that resetting the gravity is only one side effect of
`change_gravity', which is some kind of misnomer.

I explained further in my reply to Akib.

Re: [C source] What is gravity and the change_gravity argument?

[Akib Azmain Turja]

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Eli Zaretskii <eliz@gnu.org> writes:

>> From: Akib Azmain Turja <akib@disroot.org>
>> Date: Fri, 08 Jul 2022 18:40:16 +0600
>> 
>> In src/termhooks.h, I found the following two members of struct
>> terminal:
>> 
>> --8<---------------cut here---------------start------------->8---
>>   /* This hook is called to change the size of frame F's native
>>    (underlying) window.  If CHANGE_GRAVITY, change to top-left-corner
>>    window gravity for this size change and subsequent size changes.
>>    Otherwise we leave the window gravity unchanged.  */
>>   void (*set_window_size_hook) (struct frame *f, bool change_gravity,
>>                                 int width, int height);
>> 
>>   /* CHANGE_GRAVITY is 1 when calling from Fset_frame_position,
>>    to really change the position, and 0 when calling from
>>    *_make_frame_visible (in that case, XOFF and YOFF are the current
>>    position values).  It is -1 when calling from gui_set_frame_parameters,
>>    which means, do adjust for borders but don't change the gravity.  */
>> 
>>   void (*set_frame_offset_hook) (struct frame *f, register int xoff,
>>                                  register int yoff, int change_gravity);
>> --8<---------------cut here---------------end--------------->8---
>> 
>> X and PGTK uses that 'change_gravity' argument, I can't understand what
>> they are doing with them.  I searched the internet and found several
>> answers[1], but couldn't make any sense.
>
> Try this:
>
>   https://tronche.com/gui/x/xlib/window/attributes/gravity.html
>

This seems like to answer I found, except the terms are different.

If I understand correctly, the following happen when gravity is
CenterGravity:

--8<---------------cut here---------------start------------->8---
+----------+              +--------+
|   some   |              |  some  |              +------+
| contents | -> resize -> |contents| -> resize -> |ontent|
|   here   |              |  here  |              +------+
+----------+              +--------+
--8<---------------cut here---------------end--------------->8---

Am I correct?

But I can't still understand why Emacs needs this.  Emacs handles frame
resizing, so setting gravity seems useless.

-- 
Akib Azmain Turja

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Re: [C source] What is gravity and the change_gravity argument?

[Po Lu]

Akib Azmain Turja <akib@disroot.org> writes:

> If I understand correctly, the following happen when gravity is
> CenterGravity:
>
> +----------+              +--------+
> |   some   |              |  some  |              +------+
> | contents | -> resize -> |contents| -> resize -> |ontent|
> |   here   |              |  here  |              +------+
> +----------+              +--------+

You're thinking of bit gravity.  Emacs sets the window gravity and
leaves the bit gravity alone.

> But I can't still understand why Emacs needs this.  Emacs handles
> frame resizing, so setting gravity seems useless.

I explained more in my other reply to you.

Re: [C source] What is gravity and the change_gravity argument?

[Po Lu]

Akib Azmain Turja <akib@disroot.org> writes:

> X and PGTK uses that 'change_gravity' argument, I can't understand what
> they are doing with them.  I searched the internet and found several
> answers[1], but couldn't make any sense.

`change_gravity' means to actually change the position of the frame's
outer window, and to reset the window gravity to NorthWestGravity.  If
it is 0, then we ignore the value of xoff and yoff, preserve the current
window gravity, and simply restore the previously set frame position.

X has two gravities: the window gravity and bit gravity.  The window
gravity defines how a window is repositioned when its parent is resized.
The bit gravity describes how the contents of the window are moved or
obscured when it is resized.

I think the values are quite self explanatory.

Re: [C source] What is gravity and the change_gravity argument?

[Akib Azmain Turja]

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Po Lu <luangruo@yahoo.com> writes:

> Akib Azmain Turja <akib@disroot.org> writes:
>
>> X and PGTK uses that 'change_gravity' argument, I can't understand what
>> they are doing with them.  I searched the internet and found several
>> answers[1], but couldn't make any sense.
>
> `change_gravity' means to actually change the position of the frame's
> outer window, and to reset the window gravity to NorthWestGravity.  If
> it is 0, then we ignore the value of xoff and yoff, preserve the current
> window gravity, and simply restore the previously set frame position.
>
> X has two gravities: the window gravity and bit gravity.  The window
> gravity defines how a window is repositioned when its parent is resized.
> The bit gravity describes how the contents of the window are moved or
> obscured when it is resized.

Thanks.

>
> I think the values are quite self explanatory.

Yes, just like directions in a map (of somewhere).

-- 
Akib Azmain Turja

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