SkMipMap::ComputeLevelSize should only cover SkMipMap's levels.

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 352 matching lines @@
         default:
             // TODO: We could build miplevels for kIndex8 if the levels were in 8888.
             //       Means using more ram, but the quality would be fine.
             return nullptr;
     }
 
     if (src.width() <= 1 && src.height() <= 1) {
         return nullptr;
     }
     // whip through our loop to compute the exact size needed
-    size_t  size = 0;
+    size_t size = 0;
     int countLevels = ComputeLevelCount(src.width(), src.height());
-    for (int currentMipLevel = countLevels; currentMipLevel > 0; currentMipLevel--) {
+    for (int currentMipLevel = countLevels; currentMipLevel >= 0; currentMipLevel--) {
         SkISize mipSize = ComputeLevelSize(src.width(), src.height(), currentMipLevel);
         size += SkColorTypeMinRowBytes(ct, mipSize.fWidth) * mipSize.fHeight;
     }
 
     size_t storageSize = SkMipMap::AllocLevelsSize(countLevels, size);
     if (0 == storageSize) {
         return nullptr;
     }
 
     SkMipMap* mipmap;
@@ skipping 109 matching lines @@
 
     return mipLevelCount;
 }
 
 SkISize SkMipMap::ComputeLevelSize(int baseWidth, int baseHeight, int level) {
     if (baseWidth < 1 || baseHeight < 1) {
         return SkISize::Make(0, 0);
     }
 
     int maxLevelCount = ComputeLevelCount(baseWidth, baseHeight);
-    if (level > maxLevelCount || level < 0) {
+    if (level >= maxLevelCount || level < 0) {
         return SkISize::Make(0, 0);
     }
-    if (level == 0) {
-        return SkISize::Make(baseWidth, baseHeight);
-    }
-
     // OpenGL's spec requires that each mipmap level have height/width equal to
     // max(1, floor(original_height / 2^i)
     // (or original_width) where i is the mipmap level.
 
-    int width = SkTMax(1, baseWidth >> level);
-    int height = SkTMax(1, baseHeight >> level);
+    // SkMipMap does not include the base mip level.
+    // For example, it contains levels 1-x instead of 0-x.
+    // This is because the image used to create SkMipMap is the base level.
+    // So subtract 1 from the mip level to get the index stored by SkMipMap.
+    int width = SkTMax(1, baseWidth >> (level + 1));
+    int height = SkTMax(1, baseHeight >> (level + 1));
 
     return SkISize::Make(width, height);
 }
 
 ///////////////////////////////////////////////////////////////////////////////
 
 bool SkMipMap::extractLevel(const SkSize& scaleSize, Level* levelPtr) const {
     if (nullptr == fLevels) {
         return false;
     }
@@ skipping 64 matching lines @@
         return false;
     }
     if (index > fCount - 1) {
         return false;
     }
     if (levelPtr) {
         *levelPtr = fLevels[index];
     }
     return true;
 }