[DirectWrite] Limit noSubpixelForSmallSizeFont to low-DPI

Limit the noSubpixelForSmallSizeFont functionality that disables
subpixel text for older fonts at sizes below 16px to low-DPI devices.
At or above a 150% DPI subpixel text is enabled for all fonts assuming
anti-aliasing is enabled.

As the font code isn't aware of the DPI or device pixel ratio it needs
to be supplied by Chrome though WebFontRendering.

Chrome side change is in https://codereview.chromium.org/561503002/

R=cpu@chromium.org, dglazkov@chromium.org
BUG=405445

Review URL: https://codereview.chromium.org/553163002

git-svn-id: svn://svn.chromium.org/blink/trunk@181688 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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

@@ -69,6 +69,7 @@ static FontPlatformDataCache* gFontPlatformDataCache = 0;
 bool FontCache::s_useDirectWrite = false;
 IDWriteFactory* FontCache::s_directWriteFactory = 0;
 bool FontCache::s_useSubpixelPositioning = false;
+float FontCache::s_deviceScaleFactor = 1.0;
 #endif // OS(WIN)
 
 FontCache* FontCache::fontCache()

third_party/WebKit/Source/platform/fonts/FontCache.h

@@ -104,8 +104,10 @@ public:
     bool useSubpixelPositioning() const { return s_useSubpixelPositioning; }
     SkFontMgr* fontManager() { return m_fontManager.get(); }
     static bool useDirectWrite() { return s_useDirectWrite; }
+    static float deviceScaleFactor() { return s_deviceScaleFactor; }
     static void setUseDirectWrite(bool useDirectWrite) { s_useDirectWrite = useDirectWrite; }
     static void setDirectWriteFactory(IDWriteFactory* factory) { s_directWriteFactory = factory; }
+    static void setDeviceScaleFactor(float deviceScaleFactor) { s_deviceScaleFactor = deviceScaleFactor; }
     static void setUseSubpixelPositioning(bool useSubpixelPositioning) { s_useSubpixelPositioning = useSubpixelPositioning; }
     static void addSideloadedFontForTesting(SkTypeface*);
 #endif
@@ -162,6 +164,7 @@ private:
     OwnPtr<SkFontMgr> m_fontManager;
     static bool s_useDirectWrite;
     static IDWriteFactory* s_directWriteFactory;
+    static float s_deviceScaleFactor;
     static bool s_useSubpixelPositioning;
     static HashMap<String, RefPtr<SkTypeface> >* s_sideloadedFonts;
 #endif

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

@@ -64,13 +64,20 @@ void FontPlatformData::setupPaint(SkPaint* paint, GraphicsContext* context) cons
 
     if (ts >= m_minSizeForAntiAlias) {
 
-        // 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.
-        if (m_useSubpixelPositioning && ts >= m_minSizeForSubpixel
-            && ((textFlags & SkPaint::kAntiAlias_Flag) || LayoutTestSupport::isRunningLayoutTest()))
+        if (m_useSubpixelPositioning
+            // 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.
+            && (ts >= m_minSizeForSubpixel
+                || FontCache::fontCache()->deviceScaleFactor() >= 1.5)
+
+            // 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;
 
         // Only set painting flags when we're actually painting.

third_party/WebKit/Source/web/win/WebFontRendering.cpp

@@ -21,6 +21,12 @@ void WebFontRendering::setDirectWriteFactory(IDWriteFactory* factory)
     blink::FontCache::setDirectWriteFactory(factory);
 }
 
+// static
+void WebFontRendering::setDeviceScaleFactor(float deviceScaleFactor)
+{
+    blink::FontCache::setDeviceScaleFactor(deviceScaleFactor);
+}
+
 // static
 void WebFontRendering::setUseSubpixelPositioning(bool useSubpixelPositioning)
 {

third_party/WebKit/public/web/win/WebFontRendering.h

@@ -16,6 +16,7 @@ class WebFontRendering {
 public:
     BLINK_EXPORT static void setUseDirectWrite(bool);
     BLINK_EXPORT static void setDirectWriteFactory(IDWriteFactory*);
+    BLINK_EXPORT static void setDeviceScaleFactor(float);
     BLINK_EXPORT static void setUseSubpixelPositioning(bool);
     BLINK_EXPORT static void addSideloadedFontForTesting(SkTypeface*);
 };