Correcting text baseline for tiny fonts

third_party/WebKit/Source/platform/fonts/SimpleFontData.cpp

 /*
  * Copyright (C) 2005, 2008, 2010 Apple Inc. All rights reserved.
  * Copyright (C) 2006 Alexey Proskuryakov
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  *
  * 1.  Redistributions of source code must retain the above copyright
  *     notice, this list of conditions and the following disclaimer.
@@ skipping 40 matching lines @@
 const float smallCapsFontSizeMultiplier = 0.7f;
 const float emphasisMarkFontSizeMultiplier = 0.5f;
 
 #if OS(LINUX) || OS(ANDROID)
 // This is the largest VDMX table which we'll try to load and parse.
 static const size_t maxVDMXTableSize = 1024 * 1024;  // 1 MB
 #endif
 
 SimpleFontData::SimpleFontData(const FontPlatformData& platformData,
                                PassRefPtr<CustomFontData> customData,
-                               bool isTextOrientationFallback)
+                               bool isTextOrientationFallback,
+                               bool subpixelAscentDescent)
     : m_maxCharWidth(-1),
       m_avgCharWidth(-1),
       m_platformData(platformData),
       m_isTextOrientationFallback(isTextOrientationFallback),
       m_verticalData(nullptr),
       m_hasVerticalGlyphs(false),
       m_customFontData(customData) {
-  platformInit();
+  platformInit(subpixelAscentDescent);
   platformGlyphInit();
   if (platformData.isVerticalAnyUpright() && !isTextOrientationFallback) {
     m_verticalData = platformData.verticalData();
     m_hasVerticalGlyphs =
         m_verticalData.get() && m_verticalData->hasVerticalMetrics();
   }
 }
 
 SimpleFontData::SimpleFontData(PassRefPtr<CustomFontData> customData,
                                float fontSize,
                                bool syntheticBold,
                                bool syntheticItalic)
     : m_platformData(
           FontPlatformData(fontSize, syntheticBold, syntheticItalic)),
       m_isTextOrientationFallback(false),
       m_verticalData(nullptr),
       m_hasVerticalGlyphs(false),
       m_customFontData(customData) {}
 
-void SimpleFontData::platformInit() {
+void SimpleFontData::platformInit(bool subpixelAscentDescent) {
   if (!m_platformData.size()) {
     m_fontMetrics.reset();
     m_avgCharWidth = 0;
     m_maxCharWidth = 0;
     return;
   }
 
   SkPaint::FontMetrics metrics;
 
   m_platformData.setupPaint(&m_paint);
@@ skipping 24 matching lines @@
         isVDMXValid = true;
       WTF::Partitions::fastFree(vdmxTable);
     }
   }
 #endif
 
   float ascent;
   float descent;
 
   // Beware those who step here: This code is designed to match Win32 font
-  // metrics *exactly* (except the adjustment of ascent/descent on
-  // Linux/Android).
+  // metrics *exactly* except:
+  // - the adjustment of ascent/descent on Linux/Android
+  // - metrics.fAscent and .fDesscent are not rounded to int for tiny fonts
   if (isVDMXValid) {
     ascent = vdmxAscent;
     descent = -vdmxDescent;
   } else {
-    ascent = SkScalarRoundToInt(-metrics.fAscent);
-    descent = SkScalarRoundToInt(metrics.fDescent);
+    // For tiny fonts, the rounding of fAscent and fDescent results in equal
+    // baseline for different types of text baselines (crbug.com/338908).
+    // Please see CanvasRenderingContext2D::getFontBaseline for the heuristic.
+    if (subpixelAscentDescent &&

[drott, 2016/10/18 10:42:17]

zakerinasab, did you say the same problem applies to regular text and SVG?

eae@, what do you think, would it perhaps make sense to make this the default
here using a decision function in FontCache.h for example, something like
"shouldUseSubpixelMetricsForAscentDescent(float, float) - and synchronize it
that way with the code in Canvas, instead of comments and piping through the
bool flag?

I'm also slightly worried that the bool flag is not part of
FontDescription::cacheKey. In an unlucky situation we might mix up a
SimpleFontData with subpixel baseline and without.m

+        (-metrics.fAscent < 3 || -metrics.fAscent + metrics.fDescent < 2)) {
+      ascent = -metrics.fAscent;
+      descent = metrics.fDescent;
+    } else {
+      ascent = SkScalarRoundToScalar(-metrics.fAscent);
+      descent = SkScalarRoundToScalar(metrics.fDescent);
+    }
 #if OS(LINUX) || OS(ANDROID)
     // When subpixel positioning is enabled, if the descent is rounded down, the
     // descent part of the glyph may be truncated when displayed in a 'overflow:
     // hidden' container.  To avoid that, borrow 1 unit from the ascent when
     // possible.
     // FIXME: This can be removed if sub-pixel ascent/descent is supported.
     if (platformData().getFontRenderStyle().useSubpixelPositioning &&
         descent < SkScalarToFloat(metrics.fDescent) && ascent >= 1) {
       ++descent;
       --ascent;
@@ skipping 275 matching lines @@
     if (glyphs[i]) {
       pageToFill->setGlyphDataForIndex(offset + i, glyphs[i], this);
       haveGlyphs = true;
     }
   }
 
   return haveGlyphs;
 }
 
 }  // namespace blink