Use pango underline metrics in RenderTextLinux.

ui/gfx/render_text_linux.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "ui/gfx/render_text_linux.h"
 
 #include <fontconfig/fontconfig.h>
 #include <pango/pangocairo.h>
 #include <algorithm>
 #include <string>
@@ skipping 52 matching lines @@
   int fc_rgba = FC_RGBA_RGB;
   FcPatternGetInteger(match, FC_RGBA, 0, &fc_rgba);
   FcPatternDestroy(pattern);
   FcPatternDestroy(match);
 
   already_queried = true;
   subpixel_enabled = (fc_rgba != FC_RGBA_NONE);
   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.
 // Since caret_pos is used internally, we could save utf8 index for caret_pos
 // to avoid conversion.
 
 RenderTextLinux::RenderTextLinux()
     : layout_(NULL),
       current_line_(NULL),
       log_attrs_(NULL),
@@ skipping 355 matching lines @@
       DCHECK_GE(style, 0);
       DCHECK_LT(style, static_cast<int>(styles.size()));
       if (!IndexInRange(style_ranges_utf8[style], glyph_byte_index)) {
         // TODO(asvitkine): For cases like "fi", where "fi" is a single glyph
         //                  but can span multiple styles, Pango splits the
         //                  styles evenly over the glyph. We can do this too by
         //                  clipping and drawing the glyph several times.
         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;
         start_x = glyph_x;
         // Loop to find the next style, in case the glyph spans multiple styles.
         do {
           style += style_increment;
         } while (style >= 0 && style < static_cast<int>(styles.size()) &&
                  !IndexInRange(style_ranges_utf8[style], glyph_byte_index));
       }
     }
 
     // Draw the remaining glyphs.
     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;
   }
 }
 
 GSList* RenderTextLinux::GetRunContainingCaret(
     const SelectionModel& caret) const {
   size_t position = TextIndexToLayoutIndex(caret.caret_pos());
   LogicalCursorDirection affinity = caret.caret_affinity();
   GSList* run = current_line_->runs;
@@ skipping 67 matching lines @@
   return bounds;
 }
 
 std::vector<Rect> RenderTextLinux::GetSelectionBounds() {
   if (selection_visual_bounds_.empty())
     selection_visual_bounds_ = CalculateSubstringBounds(selection());
   return selection_visual_bounds_;
 }
 
 }  // namespace gfx