use log2(scale) to compute mip level

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 207 matching lines @@
         srcBM = dstBM;
         addr += height * rowBytes;
     }
     SkASSERT(addr == baseAddr + size);
 
     return mipmap;
 }
 
 ///////////////////////////////////////////////////////////////////////////////
 
-//static int gCounter;
-
+#ifdef SK_SUPPORT_LEGACY_MIPLEVELCHOICE
 static SkFixed compute_level(SkScalar scale) {
     SkScalar inv = SkScalarAbs(SkScalarInvert(scale));
     if (inv > 32767) {  // Watch out for SkFixed overflow.
         inv = 32767;
     }
     SkFixed s = SkScalarToFixed(inv);
 
     if (s < SK_Fixed1) {
         return 0;
     }
     int clz = SkCLZ(s);
     SkASSERT(clz >= 1 && clz <= 15);
     return SkIntToFixed(15 - clz) + ((unsigned)(s << (clz + 1)) >> 16);
 }
+#endif
 
 bool SkMipMap::extractLevel(SkScalar scale, Level* levelPtr) const {
     if (NULL == fLevels) {
         return false;
     }
 
-    if (scale >= SK_Scalar1) {
+    if (scale >= SK_Scalar1 || scale <= 0 || !SkScalarIsFinite(scale)) {
         return false;
     }
 
+#ifdef SK_SUPPORT_LEGACY_MIPLEVELCHOICE
     int level = compute_level(scale) >> 16;
+#else
+    SkScalar L = -SkScalarLog2(scale);
+    if (!SkScalarIsFinite(L)) {
+        return false;
+    }
+    SkASSERT(L >= 0);
+    int level = SkScalarRoundToInt(L);
+//    SkDebugf("mipmap scale=%g L=%g level=%d\n", scale, L, level);
+#endif
+
     SkASSERT(level >= 0);
     if (level <= 0) {
         return false;
     }
 
     if (level > fCount) {
         level = fCount;
     }
     if (levelPtr) {
         *levelPtr = fLevels[level - 1];
     }
     return true;
 }