use log2(scale) to compute mip level

src/core/SkMipMap.cpp

Index: src/core/SkMipMap.cpp
diff --git a/src/core/SkMipMap.cpp b/src/core/SkMipMap.cpp
index 83164b15267e0169754ddfd5d7848093ba078e6b..2dad6d9c579515be374a03966c433f9d59522ff8 100644
--- a/src/core/SkMipMap.cpp
+++ b/src/core/SkMipMap.cpp
@@ -225,8 +225,7 @@ SkMipMap* SkMipMap::Build(const SkBitmap& src, SkDiscardableFactoryProc fact) {
 
 ///////////////////////////////////////////////////////////////////////////////
 
-//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.
@@ -241,17 +240,29 @@ static SkFixed compute_level(SkScalar scale) {
     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;