Bug 818169 - Add a pref to configure resolution of low precision buffer. r=bgirard

gfx/layers/basic/BasicTiledThebesLayer.cpp

@@ -238,6 +238,20 @@ BasicTiledLayerBuffer::ValidateTile(BasicTiledLayerTile aTile,
   return aTile;
 }
 
+BasicTiledThebesLayer::BasicTiledThebesLayer(BasicShadowLayerManager* const aManager)
+  : ThebesLayer(aManager, static_cast<BasicImplData*>(this))
+  , mLastScrollOffset(0, 0)
+  , mFirstPaint(true)
+{
+  MOZ_COUNT_CTOR(BasicTiledThebesLayer);
+  mLowPrecisionTiledBuffer.SetResolution(gfxPlatform::GetLowPrecisionResolution());
+}
+
+BasicTiledThebesLayer::~BasicTiledThebesLayer()
+{
+  MOZ_COUNT_DTOR(BasicTiledThebesLayer);
+}
+
 void
 BasicTiledThebesLayer::FillSpecificAttributes(SpecificLayerAttributes& aAttrs)
 {

gfx/layers/basic/BasicTiledThebesLayer.h

@@ -12,8 +12,6 @@
 #include "BasicImplData.h"
 #include <algorithm>
 
-#define LOW_PRECISION_RESOLUTION 0.25
-
 namespace mozilla {
 namespace layers {
 
@@ -176,20 +174,8 @@ class BasicTiledThebesLayer : public ThebesLayer,
   typedef ThebesLayer Base;
 
 public:
-  BasicTiledThebesLayer(BasicShadowLayerManager* const aManager)
-    : ThebesLayer(aManager, static_cast<BasicImplData*>(this))
-    , mLastScrollOffset(0, 0)
-    , mFirstPaint(true)
-  {
-    MOZ_COUNT_CTOR(BasicTiledThebesLayer);
-    mLowPrecisionTiledBuffer.SetResolution(LOW_PRECISION_RESOLUTION);
-  }
-
-  ~BasicTiledThebesLayer()
-  {
-    MOZ_COUNT_DTOR(BasicTiledThebesLayer);
-  }
-
+  BasicTiledThebesLayer(BasicShadowLayerManager* const aManager);
+  ~BasicTiledThebesLayer();
 
   // Thebes Layer
   virtual Layer* AsLayer() { return this; }

gfx/thebes/gfxPlatform.cpp

@@ -1262,6 +1262,24 @@ gfxPlatform::UseLowPrecisionBuffer()
     return sUseLowPrecisionBuffer;
 }
 
+float
+gfxPlatform::GetLowPrecisionResolution()
+{
+    static float sLowPrecisionResolution;
+    static bool sLowPrecisionResolutionPrefCached = false;
+
+    if (!sLowPrecisionResolutionPrefCached) {
+        int32_t lowPrecisionResolution = 250;
+        sLowPrecisionResolutionPrefCached = true;
+        mozilla::Preferences::AddIntVarCache(&lowPrecisionResolution,
+                                             "layers.low-precision-resolution",
+                                             250);
+        sLowPrecisionResolution = lowPrecisionResolution / 1000.f;
+    }
+
+    return sLowPrecisionResolution;
+}
+
 bool
 gfxPlatform::UseReusableTileStore()
 {

gfx/thebes/gfxPlatform.h

@@ -409,6 +409,9 @@ public:
     // it into a buffer at a lower precision. Requires tiled buffers.
     static bool UseLowPrecisionBuffer();
 
+    // Retrieve the resolution that a low precision buffer should render at.
+    static float GetLowPrecisionResolution();
+
     // Retain some invalid tiles when the valid region of a layer changes and
     // excludes previously valid tiles.
     static bool UseReusableTileStore();

mobile/android/app/mobile.js

@@ -524,6 +524,7 @@ pref("layers.offmainthreadcomposition.enabled", true);
 pref("layers.async-video.enabled", true);
 pref("layers.progressive-paint", true);
 pref("layers.low-precision-buffer", true);
+pref("layers.low-precision-resolution", 250);
 
 pref("notification.feature.enabled", true);