Correcting text baseline for tiny fonts

third_party/WebKit/Source/modules/canvas2d/CanvasRenderingContext2D.cpp

 /*
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011, 2012 Apple Inc.
  * All rights reserved.
  * Copyright (C) 2008, 2010 Nokia Corporation and/or its subsidiary(-ies)
  * Copyright (C) 2007 Alp Toker <alp@atoker.com>
  * Copyright (C) 2008 Eric Seidel <eric@webkit.org>
  * Copyright (C) 2008 Dirk Schulze <krit@webkit.org>
  * Copyright (C) 2010 Torch Mobile (Beijing) Co. Ltd. All rights reserved.
  * Copyright (C) 2012, 2013 Intel Corporation. All rights reserved.
  * Copyright (C) 2013 Adobe Systems Incorporated. All rights reserved.
@@ skipping 815 matching lines @@
 
   // Currently, SkPictureImageFilter does not support subpixel text
   // anti-aliasing, which is expected when !creationAttributes().alpha(), so we
   // need to fall out of display list mode when drawing text to an opaque
   // canvas. crbug.com/583809
   if (!creationAttributes().alpha() && !isAccelerated())
     canvas()->disableDeferral(
         DisableDeferralReasonSubPixelTextAntiAliasingSupport);
 
   const Font& font = accessFont();
+  font.getFontDescription().setSubpixelAscentDescent(true);
   const SimpleFontData* fontData = font.primaryFont();
   DCHECK(fontData);
   if (!fontData)
     return;
   const FontMetrics& fontMetrics = fontData->getFontMetrics();
 
   // FIXME: Need to turn off font smoothing.
 
   const ComputedStyle* computedStyle = 0;
   TextDirection direction =
@@ skipping 62 matching lines @@
       textRunPaintInfo.bounds, paintType);
 }
 
 const Font& CanvasRenderingContext2D::accessFont() {
   if (!state().hasRealizedFont())
     setFont(state().unparsedFont());
   canvas()->document().canvasFontCache()->willUseCurrentFont();
   return state().font();
 }
 
-int CanvasRenderingContext2D::getFontBaseline(
+float CanvasRenderingContext2D::getFontBaseline(
     const FontMetrics& fontMetrics) const {
+  // If the font is so tiny that the lroundf operations result in two
+  // different types of text baselines to return the same baseline, use
+  // floating point metrics (crbug.com/338908).
+  // If you changed the heuristic here, for consistency please also change it
+  // in SimpleFontData::platformInit().
+  bool useFloatAscentDescent =
+      fontMetrics.ascent() < 3 || fontMetrics.height() < 2;
   switch (state().getTextBaseline()) {
     case TopTextBaseline:
-      return fontMetrics.ascent();
+      return useFloatAscentDescent ? fontMetrics.floatAscent()
+                                   : fontMetrics.ascent();
     case HangingTextBaseline:
       // According to
       // http://wiki.apache.org/xmlgraphics-fop/LineLayout/AlignmentHandling
       // "FOP (Formatting Objects Processor) puts the hanging baseline at 80% of
       // the ascender height"
-      return (fontMetrics.ascent() * 4) / 5;
+      return useFloatAscentDescent ? (fontMetrics.floatAscent() * 4.0) / 5.0
+                                   : (fontMetrics.ascent() * 4) / 5;
     case BottomTextBaseline:
     case IdeographicTextBaseline:
-      return -fontMetrics.descent();
+      return useFloatAscentDescent ? -fontMetrics.floatDescent()
+                                   : -fontMetrics.descent();
     case MiddleTextBaseline:
-      return -fontMetrics.descent() + fontMetrics.height() / 2;
+      return useFloatAscentDescent
+                 ? -fontMetrics.floatDescent() + fontMetrics.floatHeight() / 2.0
+                 : -fontMetrics.descent() + fontMetrics.height() / 2;
     case AlphabeticTextBaseline:
     default:
       // Do nothing.
       break;
   }
   return 0;
 }
 
 void CanvasRenderingContext2D::setIsHidden(bool hidden) {
   if (canvas()->hasImageBuffer())
@@ skipping 201 matching lines @@
   }
   return true;
 }
 
 void CanvasRenderingContext2D::resetUsageTracking() {
   UsageCounters newCounters;
   m_usageCounters = newCounters;
 }
 
 }  // namespace blink