use log2(scale) to compute mip level

include/core/SkFloatingPoint.h

 
 /*
  * Copyright 2006 The Android Open Source Project
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 
 #ifndef SkFloatingPoint_DEFINED
@@ skipping 73 matching lines @@
     #define sk_float_acos(x)        acosf(x)
     #define sk_float_asin(x)        asinf(x)
 #endif
     #define sk_float_atan2(y,x)     atan2f(y,x)
     #define sk_float_abs(x)         fabsf(x)
     #define sk_float_mod(x,y)       fmodf(x,y)
     #define sk_float_exp(x)         expf(x)
     #define sk_float_log(x)         logf(x)
 #endif
 
+// can't find log2f on android, but maybe that just a tool bug?
+#ifdef SK_BUILD_FOR_ANDROID
+    static inline float sk_float_log2(float x) {
+        const double inv_ln_2 = 1.44269504088896;
+        return (float)(log(x) * inv_ln_2);
+    }
+#else
+    #define sk_float_log2(x)        log2f(x)
+#endif
+
 #ifdef SK_BUILD_FOR_WIN
     #define sk_float_isfinite(x)    _finite(x)
     #define sk_float_isnan(x)       _isnan(x)
     static inline int sk_float_isinf(float x) {
         int32_t bits = SkFloat2Bits(x);
         return (bits << 1) == (0xFF << 24);
     }
 #else
     #define sk_float_isfinite(x)    isfinite(x)
     #define sk_float_isnan(x)       isnan(x)
@@ skipping 61 matching lines @@
     float estimate = *SkTCast<float*>(&i);
 
     // One step of Newton's method to refine.
     const float estimate_sq = estimate*estimate;
     estimate *= (1.5f-0.5f*x*estimate_sq);
     return estimate;
 #endif
 }
 
 #endif