* xterm.c (x_underline_minimum_display_offset): New var.

(x_draw_glyph_string): Use it.
(syms_of_xterm): Declare it.
* cus-start.el (x-underline-minimum-display-offset): Give it a type.

4 files changed, 32 insertions, 1 deletions

diff --git a/lisp/ChangeLog b/lisp/ChangeLog
index 238245be904..2472e3f6112 100644
--- a/lisp/ChangeLog
+++ b/lisp/ChangeLog
@@ -1,3 +1,7 @@
+2008-06-10  David De La Harpe Golden  <david@harpegolden.net>  (tiny change)
+
+        * cus-start.el (x-underline-minimum-display-offset): Give it a type.
+
 2008-06-10  Chong Yidong  <cyd@stupidchicken.com>
 
         * mouse.el (mouse-select-font): New function.
diff --git a/lisp/cus-start.el b/lisp/cus-start.el
index 582a3f98c6e..73150ce8f7c 100644
--- a/lisp/cus-start.el
+++ b/lisp/cus-start.el
@@ -348,6 +348,7 @@ since it could result in memory overflow and make Emacs crash."
              ;; xterm.c
              (x-use-underline-position-properties display boolean "22.1")
              (x-underline-at-descent-line display boolean "22.1")
+             (x-underline-minimum-display-offset display integer "23.1")
              (x-stretch-cursor display boolean "21.1")))
       this symbol group type standard version native-p
       ;; This function turns a value
diff --git a/src/ChangeLog b/src/ChangeLog
index cfe3885b1f1..2716320139b 100644
--- a/src/ChangeLog
+++ b/src/ChangeLog
@@ -1,3 +1,9 @@
+2008-06-10  David De La Harpe Golden  <david@harpegolden.net>  (tiny change)
+
+        * xterm.c (x_underline_minimum_display_offset): New var.
+        (x_draw_glyph_string): Use it.
+        (syms_of_xterm): Declare it.
+
 2008-06-10  Chong Yidong  <cyd@stupidchicken.com>
 
         * font.c (font_parse_fcname): Accept GTK-style font names too.
diff --git a/src/xterm.c b/src/xterm.c
index fa7ffe5139e..13c2d2e4a97 100644
--- a/src/xterm.c
+++ b/src/xterm.c
@@ -183,6 +183,12 @@ int x_use_underline_position_properties;
 
 int x_underline_at_descent_line;
 
+/* Require underline to be at least this many screen pixels below baseline
+   This to avoid underline "merging" with the base of letters at small
+   font sizes, particularly when x_use_underline_position_properties is on. */
+
+int x_underline_minimum_display_offset;
+
 /* This is a chain of structures for all the X displays currently in
    use.  */
 
@@ -2744,6 +2750,8 @@ x_draw_glyph_string (s)
                   else if (s->font)
                     position = (s->font->descent + 1) / 2;
                 }
+              if (x_underline_minimum_display_offset)
+                position = max (position, eabs (x_underline_minimum_display_offset));
             }
           /* Check the sanity of thickness and position.  We should
              avoid drawing underline out of the current line area.  */
@@ -10781,7 +10789,9 @@ syms_of_xterm ()
      doc: /* *Non-nil means make use of UNDERLINE_POSITION font properties.
 A value of nil means ignore them.  If you encounter fonts with bogus
 UNDERLINE_POSITION font properties, for example 7x13 on XFree prior
-to 4.1, set this to nil.  */);
+to 4.1, set this to nil.  Variable `x-underline-minimum-display-offset' may
+be used to override the font's UNDERLINE_POSITION for small font display
+sizes.  */);
   x_use_underline_position_properties = 1;
 
   DEFVAR_BOOL ("x-underline-at-descent-line",
@@ -10792,6 +10802,16 @@ variable `x-use-underline-position-properties', which is usually at the
 baseline level.  The default value is nil.  */);
   x_underline_at_descent_line = 0;
 
+  DEFVAR_INT ("x-underline-minimum-display-offset",
+               &x_underline_minimum_display_offset,
+     doc: /* *When > 0, underline is drawn at least that many screen pixels below baseline.
+This can improve legibility of underlined text at small font sizes,
+particularly when using variable `x-use-underline-position-properties'
+with fonts that specify an UNDERLINE_POSITION relatively close to the
+baseline.  The default value is 0. */);
+  x_underline_minimum_display_offset = 0;
+
+
   DEFVAR_BOOL ("x-mouse-click-focus-ignore-position",
                &x_mouse_click_focus_ignore_position,
     doc: /* Non-nil means that a mouse click to focus a frame does not move point.