use log2(scale) to compute mip level

BUG=skia:

Review URL: https://codereview.chromium.org/849333002

4 files changed, 37 insertions, 3 deletions

diff --git a/expectations/gm/ignored-tests.txt b/expectations/gm/ignored-tests.txt
index 7bc03a30a5..6367eb5009 100644
--- a/expectations/gm/ignored-tests.txt
+++ b/expectations/gm/ignored-tests.txt
@@ -37,6 +37,17 @@
 colortype_xfermodes
 mixed_xfermodes
 
+# reed - rebase after mipmap level improvement
+downsamplebitmap_image_medium_mandrill_512
+downsamplebitmap_image_medium_mandrill_132x132_12x12
+downsamplebitmap_image_medium_mandrill_512.png
+downsamplebitmap_image_medium_mandrill_132x132_12x12.astc
+downsamplebitmap_checkerboard_medium_512_256
+downsamplebitmap_text_medium_72.00pt
+filterindiabox
+coloremoji
+fontmgr_iter
+
 # robertphillips - skia:2995
 blurrects
 
diff --git a/include/core/SkFloatingPoint.h b/include/core/SkFloatingPoint.h
index 1003b80b4f..9fb343261b 100644
--- a/include/core/SkFloatingPoint.h
+++ b/include/core/SkFloatingPoint.h
@@ -91,6 +91,16 @@ static inline float sk_float_copysign(float x, float y) {
     #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)
diff --git a/include/core/SkScalar.h b/include/core/SkScalar.h
index 94c3ce15c0..e8c7a7fb65 100644
--- a/include/core/SkScalar.h
+++ b/include/core/SkScalar.h
@@ -58,6 +58,7 @@ typedef float SkScalar;
 #define SkScalarATan2(y, x)         (float)sk_float_atan2(y,x)
 #define SkScalarExp(x)              (float)sk_float_exp(x)
 #define SkScalarLog(x)              (float)sk_float_log(x)
+#define SkScalarLog2(x)             (float)sk_float_log2(x)
 
 #else   // SK_SCALAR_IS_DOUBLE
 
@@ -100,6 +101,7 @@ typedef double SkScalar;
 #define SkScalarATan2(y, x)         atan2(y,x)
 #define SkScalarExp(x)              exp(x)
 #define SkScalarLog(x)              log(x)
+#define SkScalarLog2(x)             log2(x)
 
 #endif
 
diff --git a/src/core/SkMipMap.cpp b/src/core/SkMipMap.cpp
index 83164b1526..2dad6d9c57 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;