HiDPI support for wave style underlines

[Stephen Pegoraro <spegoraro@tutive.com>]

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Hi,

I have made an attempt at implementing scaled drawing of wave style
underlines for hidpi displays, X only for now.
This is my first contribution to emacs so any feedback would be great!
I've followed the commit and patch style from CONTRIBUTE as closely as
I could but let me know if there's any issue.

It works by determining a scale factor from x_display_info's resx and
resy members then setting the wave_height and wave_length values
accordingly. Thickness was added using XSetLineAttributes and scaled
as well. Tested on displays with various scales from 1 to 3.


Cheers,
Steve Pegoraro

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From bc71dbc78a455b46a91619bcd38af45e4e9831fb Mon Sep 17 00:00:00 2001
From: Stephen Pegoraro <spegoraro@tutive.com>
Date: Sat, 29 Jul 2017 11:12:03 +0800
Subject: [PATCH] Implement HiDPI support for wave style underlines

* src/xterm.c (x_draw_underwave): Compute height, length and thickness
based on scale factor.
(x_get_scale_factor): New function.
---
 src/xterm.c | 21 ++++++++++++++++-----
 1 file changed, 16 insertions(+), 5 deletions(-)

diff --git a/src/xterm.c b/src/xterm.c
index a214cd8103..5476c22a0b 100644
--- a/src/xterm.c
+++ b/src/xterm.c
@@ -23,9 +23,7 @@ along with GNU Emacs.  If not, see <http://www.gnu.org/licenses/>.  */
 #include <config.h>
 #include <stdio.h>
 #include <stdlib.h>
-#ifdef USE_CAIRO
 #include <math.h>
-#endif
 
 #include "lisp.h"
 #include "blockinput.h"
@@ -3475,6 +3473,15 @@ x_draw_stretch_glyph_string (struct glyph_string *s)
   s->background_filled_p = true;
 }
 
+static int x_get_scale_factor(Display *disp)
+{
+    struct x_display_info * dpyinfo = x_display_info_for_display (disp);
+    if (!dpyinfo)
+        emacs_abort ();
+
+    return floor(dpyinfo->resy / 96);
+}
+
 /*
    Draw a wavy line under S. The wave fills wave_height pixels from y0.
 
@@ -3485,11 +3492,13 @@ x_draw_stretch_glyph_string (struct glyph_string *s)
     wave_height = 3  | *   *   *   *
 
 */
-
 static void
 x_draw_underwave (struct glyph_string *s)
 {
-  int wave_height = 3, wave_length = 2;
+    /* Adjust for scale/HiDPI */
+    int scale = x_get_scale_factor (s->display);
+    int wave_height = 3 * scale, wave_length = 2 * scale, thickness = scale;
+
 #ifdef USE_CAIRO
   x_draw_horizontal_wave (s->f, s->gc, s->x, s->ybase - wave_height + 3,
 			  s->width, wave_height, wave_length);
@@ -3501,7 +3510,7 @@ x_draw_underwave (struct glyph_string *s)
   dx = wave_length;
   dy = wave_height - 1;
   x0 = s->x;
-  y0 = s->ybase - wave_height + 3;
+  y0 = s->ybase + wave_height / 2;
   width = s->width;
   xmax = x0 + width;
 
@@ -3535,6 +3544,8 @@ x_draw_underwave (struct glyph_string *s)
 
   while (x1 <= xmax)
     {
+      XSetLineAttributes (s->display, s->gc, thickness, LineSolid, CapButt,
+                          JoinRound);
       XDrawLine (s->display, FRAME_X_DRAWABLE (s->f), s->gc, x1, y1, x2, y2);
       x1  = x2, y1 = y2;
       x2 += dx, y2 = y0 + odd*dy;
-- 
2.13.3