Support HiDPI displays 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.

Copyright-paperwork-exempt: yes

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 +3473,21 @@ x_draw_stretch_glyph_string (struct glyph_string *s)
   s->background_filled_p = true;
 }
 
+static void
+x_get_scale_factor(Display *disp, int *scale_x, int *scale_y)
+{
+  const int base_res = 96;
+  struct x_display_info * dpyinfo = x_display_info_for_display (disp);
+
+  *scale_x = *scale_y = 1;
+
+  if (dpyinfo)
+    {
+      *scale_x = floor (dpyinfo->resx / base_res);
+      *scale_y = floor (dpyinfo->resy / base_res);
+    }
+}
+
 /*
    Draw a wavy line under S. The wave fills wave_height pixels from y0.
 
@@ -3485,11 +3498,16 @@ 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, scale_y;
+
+  x_get_scale_factor (s->display, &scale_x, &scale_y);
+
+  int wave_height = 3 * scale_y, wave_length = 2 * scale_x, thickness = scale_y;
+
 #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 +3519,7 @@ 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 +3553,8 @@ 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;