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..aaf9fdb055066b2b31e2d967cf1e1659b31c8c31 100644
--- a/src/core/SkMipMap.cpp
+++ b/src/core/SkMipMap.cpp
@@ -326,6 +326,16 @@ bool SkMipMap::extractLevel(const SkSize& scaleSize, Level* levelPtr) const {
     }
 
     SkASSERT(scaleSize.width() >= 0 && scaleSize.height() >= 0);
+
+#ifndef SK_SUPPORT_LEGACY_ANISOTROPIC_MIPMAPS
+    const SkScalar kMaxScaleRatio = 0.25f;
+    if (SkScalarAbs(scaleSize.width() / scaleSize.height() - 1) > kMaxScaleRatio) {

[reed1, 2016/02/16 16:41:31]

- The minus-1 is a little confusing

I would have thought it would look like this

ratio = width / height;

const kTooBig = 4.0f;
const kTooSmall = 1 / kTooBig;

if (ratio > kTooBig || ratio < kTooSmall) {
    return false;
}

+        // Mipmaps produce bad results for extremely anisotropic scales
+        // (https://bugs.chromium.org/p/skia/issues/detail?id=4863). Fall back to lerp.
+        return false;
+    }
+#endif
+
     // Use the smallest scale to match the GPU impl.
     const SkScalar scale = SkTMin(scaleSize.width(), scaleSize.height());