Use geometric mean when selecting a mipmap scale

src/core/SkMipMap.cpp

 /*
  * Copyright 2013 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkMipMap.h"
 #include "SkBitmap.h"
 #include "SkColorPriv.h"
@@ skipping 308 matching lines @@
 }
 
 ///////////////////////////////////////////////////////////////////////////////
 
 bool SkMipMap::extractLevel(const SkSize& scaleSize, Level* levelPtr) const {
     if (nullptr == fLevels) {
         return false;
     }
 
     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());
 
     if (scale >= SK_Scalar1 || scale <= 0 || !SkScalarIsFinite(scale)) {
         return false;
     }
 
     SkScalar L = -SkScalarLog2(scale);
     if (!SkScalarIsFinite(L)) {
         return false;
@@ skipping 25 matching lines @@
         return nullptr;
     }
     const SkPixmap& srcPixmap = srcUnlocker.pixmap();
     // Try to catch where we might have returned nullptr for src crbug.com/492818
     if (nullptr == srcPixmap.addr()) {
         sk_throw();
     }
     return Build(srcPixmap, fact);
 }