Use geometric mean when selecting a mipmap scale

src/core/SkMipMap.cpp

Index: src/core/SkMipMap.cpp
diff --git a/src/core/SkMipMap.cpp b/src/core/SkMipMap.cpp
index 7266dc6b21192c16019c0277ea4387244033d209..a3a51245eed5223cb1e1e4ef49fb9a466e2760cc 100644
--- a/src/core/SkMipMap.cpp
+++ b/src/core/SkMipMap.cpp
@@ -326,8 +326,16 @@ bool SkMipMap::extractLevel(const SkSize& scaleSize, Level* levelPtr) const {
     }
 
     SkASSERT(scaleSize.width() >= 0 && scaleSize.height() >= 0);
+
+#ifdef SK_SUPPORT_LEGACY_ANISOTROPIC_MIPMAPS
     // Use the smallest scale to match the GPU impl.
-    const SkScalar scale = SkTMin(scaleSize.width(), scaleSize.height());
+    const SkScalar scale = (scaleSize.width() + scaleSize.height()) / 2;
+#else
+    // For an anisotropic scale S(scaleX, scaleY), the ideal mipmap scale M(k, k)
+    // minimizes the distance d(S, M).  This is equivalent to picking M as the
+    // projection of S on the diagonal line.
+    const SkScalar scale = (scaleSize.width() + scaleSize.height()) / 2;
+#endif
 
     if (scale >= SK_Scalar1 || scale <= 0 || !SkScalarIsFinite(scale)) {

[f(malita), 2016/02/16 17:31:59]

Q: what should we do when the average > 1?  We're currently degrading to kLow,
which may be OK.  Alternatives:

a) clamp to 1 and continue using kMedium
b) don't drop from kHigh to kMed in this case (means making the HQ controller
aware of this heuristic)

         return false;