Use pango underline metrics in RenderTextLinux.

ui/gfx/render_text_linux.cc

Index: ui/gfx/render_text_linux.cc
===================================================================
--- ui/gfx/render_text_linux.cc	(revision 140345)
+++ ui/gfx/render_text_linux.cc	(working copy)
@@ -70,6 +70,19 @@
   return subpixel_enabled;
 }
 
+// Sets underline metrics on |renderer| according to Pango font |desc|.
+void SetPangoUnderlineMetrics(PangoFontDescription *desc,
+                              internal::SkiaTextRenderer* renderer) {
+  PangoFontMetrics* metrics = GetPangoFontMetrics(desc);
+  int thickness = pango_font_metrics_get_underline_thickness(metrics);
+  // Pango returns the position "above the baseline". Change its sign to convert
+  // it to a vertical offset from the baseline.
+  int position = -pango_font_metrics_get_underline_position(metrics);
+  pango_quantize_line_geometry(&thickness, &position);

[msw, 2012/06/06 18:35:25]

Does this work ok with DIP / high dpi?

[Alexei Svitkine (slow), 2012/06/06 18:46:23]

It shouldn't cause any problems - though it will still result in the line
landing on pixel boundaries. Maybe in HighDPI mode, it's not such a big deal if
it wouldn't land on pixel boundaries (since pixels are small so blurred pixels
don't look as bad), but I don't think restricting to pixel boundaries would
causes any problems.

(It may be the case that we'll want to support non-pixel boundaries for the
underline in high DPI mode, but let's cross that bridge when we see evidence of
demand for that.)

+  renderer->SetUnderlineMetrics(PANGO_PIXELS(thickness),

[msw, 2012/06/06 18:35:25]

Should these still use pango_units_to_double as a matter of good practice to
avoid them incurring any bad rounding?

[Alexei Svitkine (slow), 2012/06/06 18:46:23]

Since pango_quantize_line_geometry() guarantees pixel boundaries, there won't be
any bad rounding here (since the units will be evenly divisible).

[msw, 2012/06/06 18:51:52]

I guess my point was why not use pango_units_to_double? What if a later change
[re]moves the pango_quantize_line_geometry call?

[Alexei Svitkine (slow), 2012/06/06 19:03:05]

I've added a comment about it.

+                                PANGO_PIXELS(position));
+}
+
 }  // namespace
 
 // TODO(xji): index saved in upper layer is utf16 index. Pango uses utf8 index.
@@ -445,6 +458,8 @@
         renderer.SetForegroundColor(styles[style].foreground);
         renderer.SetFontFamilyWithStyle(family_name, styles[style].font_style);
         renderer.DrawPosText(&pos[start], &glyphs[start], i - start);
+        if (styles[style].underline)
+          SetPangoUnderlineMetrics(desc.get(), &renderer);
         renderer.DrawDecorations(start_x, y, glyph_x - start_x, styles[style]);
 
         start = i;
@@ -461,6 +476,8 @@
     renderer.SetForegroundColor(styles[style].foreground);
     renderer.SetFontFamilyWithStyle(family_name, styles[style].font_style);
     renderer.DrawPosText(&pos[start], &glyphs[start], glyph_count - start);
+    if (styles[style].underline)
+      SetPangoUnderlineMetrics(desc.get(), &renderer);
     renderer.DrawDecorations(start_x, y, glyph_x - start_x, styles[style]);
     x = glyph_x;
   }