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 70 matching lines @@
     #define sk_float_acos(x)        static_cast<float>(acos(x))
     #define sk_float_asin(x)        static_cast<float>(asin(x))
 #else
     #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)

[robertphillips, 2015/01/16 01:13:37]

Map sk_float_log2 to log2f here?

[reed1, 2015/01/16 13:00:11]

Done.

 #endif
 
+// 1 / ln(2)
+#define SK_INVERSE_LN_2             1.44269504088896
+
+// log2(x) = log(x) / log(2)
+#define sk_float_log2(x)            (float)(::log(x) * SK_INVERSE_LN_2)
+
 #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