Replace subpixel font size heuristics with using OpenType gasp table

third_party/WebKit/Source/platform/fonts/win/FontPlatformDataWin.cpp

 /*
  * Copyright (C) 2006, 2007 Apple Computer, Inc.
  * Copyright (c) 2006, 2007, 2008, 2009, 2012 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
@@ skipping 43 matching lines @@
   uint32_t textFlags = paintTextFlags();
   uint32_t flags = paint->getFlags();
   static const uint32_t textFlagsMask =
       SkPaint::kAntiAlias_Flag | SkPaint::kLCDRenderText_Flag |
       SkPaint::kEmbeddedBitmapText_Flag | SkPaint::kSubpixelText_Flag;
   flags &= ~textFlagsMask;
 
   if (ts <= kMaxSizeForEmbeddedBitmap)
     flags |= SkPaint::kEmbeddedBitmapText_Flag;
 
-  if (ts >= m_minSizeForAntiAlias) {
-    // Disable subpixel text for certain older fonts at smaller sizes as
-    // they tend to get quite blurry at non-integer sizes and positions.
-    // For high-DPI this workaround isn't required.
-    if ((ts >= m_minSizeForSubpixel ||
-         FontCache::fontCache()->deviceScaleFactor() >= 1.5)
+  // Subpixel text positioning leads to uneven spacing without font

[eae, 2017/03/17 16:53:16]

This explanation is a little confusing at first as only describes a problem and
not the solution. How about starting with "Only use sub-pixel positioning if
anti aliasing is enabled."?

+  // smoothing. Subpixel test placement coordinates would be passed to Skia,
+  // which only has non-antialiased glyphs to draw, so they necessarily get
+  // clamped at pixel positions, which leads to uneven spacing, either too close
+  // or too far away from adjacent glyphs.
+  if (textFlags & SkPaint::kAntiAlias_Flag)
+    flags |= SkPaint::kSubpixelText_Flag;
 
-        // Subpixel text positioning looks pretty bad without font
-        // smoothing. Disable it unless some type of font smoothing is used.
-        // As most tests run without font smoothing we enable it for tests
-        // to ensure we get good test coverage matching the more common
-        // smoothing enabled behavior.
-        && ((textFlags & SkPaint::kAntiAlias_Flag) ||
-            LayoutTestSupport::isRunningLayoutTest()))
-      flags |= SkPaint::kSubpixelText_Flag;
-
-    SkASSERT(!(textFlags & ~textFlagsMask));
-    flags |= textFlags;
-  }
+  SkASSERT(!(textFlags & ~textFlagsMask));
+  flags |= textFlags;
 
   paint->setFlags(flags);
 }
 
 static bool isWebFont(const String& familyName) {
   // Web-fonts have artifical names constructed to always be:
   // 1. 24 characters, followed by a '\0'
   // 2. the last two characters are '=='
   return familyName.length() == 24 && '=' == familyName[22] &&
          '=' == familyName[23];
@@ skipping 21 matching lines @@
     textFlags |= SkPaint::kAntiAlias_Flag;
 
   return textFlags;
 }
 
 void FontPlatformData::querySystemForRenderStyle() {
   m_paintTextFlags = computePaintTextFlags(fontFamilyName());
 }
 
 }  // namespace blink