Re: Patch to vertically center line content when using line-spacing variable

[Jesse Medeiros <jessenzr@gmail.com>]

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>     Jesse> diff --git a/doc/lispref/frames.texi b/doc/lispref/frames.texi
>     Jesse> index 618ea16fcf..75a6725f03 100644
>     Jesse> --- a/doc/lispref/frames.texi
>     Jesse> +++ b/doc/lispref/frames.texi
>     Jesse> @@ -1857,6 +1857,11 @@ Layout Parameters
>     Jesse>  Additional space to leave below each text line, in pixels (a positive
>     Jesse>  integer).  @xref{Line Height}, for more information.
>
>     Jesse> +@vindex line-spacing-vertical-center@r{, a frame parameter}
>     Jesse> +@item line-spacing-vertical-center
>     Jesse> +If non-nil, centers the line content vertically when using
>     Jesse> +using the @code{line-spacing} variable.
>     Jesse> +
>
> I donʼt think you implemented this as a frame parameter, which means the
> documentation should go in "@node Line Height" in display.texi, or you
> could implement the frame parameter as well. [1]
>
>     Jesse>    if (extra_line_spacing > 0)
>     Jesse>      {
>     Jesse> -      it->descent += extra_line_spacing;
>     Jesse> +      if (! BVAR (XBUFFER (it->w->contents), line_spacing_vertical_center))
>
> I think this currently works because Qnil == 0, for now, but you should
> probably use NILP.

I've worked the on these issues you raised. Hopefully it's all ok now.

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From 9bc5cbb727e5dcac9f774028858e2608d1a23033 Mon Sep 17 00:00:00 2001
From: Jesse Nazario <jessenzr@gmail.com>
Date: Sun, 8 Sep 2019 20:06:15 -0300
Subject: [PATCH] Center lines vertically with line-spacing-vertical-center

When using line-spacing, the new variable line-spacing-vertical-center
can be set to non-nil to center the line content vertically.
---
 doc/lispref/display.texi |  5 +++++
 etc/NEWS                 |  4 ++++
 src/buffer.c             | 14 ++++++++++++++
 src/buffer.h             |  4 ++++
 src/xdisp.c              | 10 +++++++++-
 5 files changed, 36 insertions(+), 1 deletion(-)

diff --git a/doc/lispref/display.texi b/doc/lispref/display.texi
index fd6820897f..875c50c6ef 100644
--- a/doc/lispref/display.texi
+++ b/doc/lispref/display.texi
@@ -2195,6 +2195,11 @@ Line Height
 number of pixels put below lines.  A floating-point number specifies
 the spacing relative to the frame's default line height.
 
+  The @code{line-spacing} parameter creates the space by putting some
+margin only below the line. If you wish to center the line vertically
+instead, you can set the variable @code{line-spacing-vertical-center}
+as non-nin.
+
 @vindex line-spacing
   You can specify the line spacing for all lines in a buffer via the
 buffer-local @code{line-spacing} variable.  An integer specifies
diff --git a/etc/NEWS b/etc/NEWS
index cb8b6fcac1..3209f2535f 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -445,6 +445,10 @@ RGB triplets with a single hexadecimal digit per component.
 ---
 ** The toolbar now shows the equivalent key binding in its tooltips.
 
+** New variable line-spacing-vertical-center.
+This variable, if non-nil, centers the line content vertically when
+using the 'line-spacing' variable.
+
 \f
 * Editing Changes in Emacs 27.1
 
diff --git a/src/buffer.c b/src/buffer.c
index 77e8b6bb77..854b3924c0 100644
--- a/src/buffer.c
+++ b/src/buffer.c
@@ -219,6 +219,11 @@ bset_extra_line_spacing (struct buffer *b, Lisp_Object val)
   b->extra_line_spacing_ = val;
 }
 static void
+bset_line_spacing_vertical_center (struct buffer *b, Lisp_Object val)
+{
+  b->line_spacing_vertical_center_ = val;
+}
+static void
 bset_file_format (struct buffer *b, Lisp_Object val)
 {
   b->file_format_ = val;
@@ -962,6 +967,8 @@ reset_buffer (register struct buffer *b)
     (b, BVAR (&buffer_defaults, enable_multibyte_characters));
   bset_cursor_type (b, BVAR (&buffer_defaults, cursor_type));
   bset_extra_line_spacing (b, BVAR (&buffer_defaults, extra_line_spacing));
+  bset_line_spacing_vertical_center (b, BVAR (&buffer_defaults,
+                                              line_spacing_vertical_center));
 
   b->display_error_modiff = 0;
 }
@@ -5196,6 +5203,7 @@ init_buffer_once (void)
   XSETFASTINT (BVAR (&buffer_local_flags, header_line_format), idx); ++idx;
   XSETFASTINT (BVAR (&buffer_local_flags, cursor_type), idx); ++idx;
   XSETFASTINT (BVAR (&buffer_local_flags, extra_line_spacing), idx); ++idx;
+  XSETFASTINT (BVAR (&buffer_local_flags, line_spacing_vertical_center), idx); ++idx;
   XSETFASTINT (BVAR (&buffer_local_flags, cursor_in_non_selected_windows), idx); ++idx;
 
   /* buffer_local_flags contains no pointers, so it's safe to treat it
@@ -5265,6 +5273,7 @@ init_buffer_once (void)
   bset_bidi_paragraph_separate_re (&buffer_defaults, Qnil);
   bset_cursor_type (&buffer_defaults, Qt);
   bset_extra_line_spacing (&buffer_defaults, Qnil);
+  bset_line_spacing_vertical_center (&buffer_defaults, Qnil);
   bset_cursor_in_non_selected_windows (&buffer_defaults, Qt);
 
   bset_enable_multibyte_characters (&buffer_defaults, Qt);
@@ -6254,6 +6263,11 @@ from (abs POSITION).  If POSITION is positive, point was at the front
 If value is a floating point number, it specifies the spacing relative
 to the default frame line height.  A value of nil means add no extra space.  */);
 
+  DEFVAR_PER_BUFFER ("line-spacing-vertical-center",
+		     &BVAR (current_buffer, line_spacing_vertical_center), Qnil,
+                     doc: /* Non-nil means center the line content vertically
+when using `line-spacing' variable.  */);
+
   DEFVAR_PER_BUFFER ("cursor-in-non-selected-windows",
 		     &BVAR (current_buffer, cursor_in_non_selected_windows), Qnil,
 		     doc: /* Non-nil means show a cursor in non-selected windows.
diff --git a/src/buffer.h b/src/buffer.h
index 280d4e9098..d7cb27305e 100644
--- a/src/buffer.h
+++ b/src/buffer.h
@@ -549,6 +549,10 @@ #define BVAR(buf, field) ((buf)->field ## _)
      in the display of this buffer.  */
   Lisp_Object extra_line_spacing_;
 
+  /* Non-nil means center the line content vertically. To be used
+     along with `line-spacing'.  */
+  Lisp_Object line_spacing_vertical_center_;
+
   /* Cursor type to display in non-selected windows.
      t means to use hollow box cursor.
      See `cursor-type' for other values.  */
diff --git a/src/xdisp.c b/src/xdisp.c
index 95895ec3ac..e2c7309f32 100644
--- a/src/xdisp.c
+++ b/src/xdisp.c
@@ -29307,7 +29307,15 @@ gui_produce_glyphs (struct it *it)
 
   if (extra_line_spacing > 0)
     {
-      it->descent += extra_line_spacing;
+      if (! NILP (BVAR (XBUFFER (it->w->contents),
+                        line_spacing_vertical_center)))
+        it->descent += extra_line_spacing;
+      else
+        {
+          int spacing = extra_line_spacing / 2;
+          it->ascent += spacing;
+          it->descent += spacing;
+        }
       if (extra_line_spacing > it->max_extra_line_spacing)
 	it->max_extra_line_spacing = extra_line_spacing;
     }
-- 
2.23.0