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

SkMipMap only deals with the levels it generates.
That is to day, it deals with mipmap levels 1-x, not 0-x.
Other functions reflect thing when indexing.
They go from 0 to x-1 (giving the index into SkMipMap's contents).

ComputeLevelSize should also follow that same indexing.

BUG=578304
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2042843005

Review-Url: https://codereview.chromium.org/2042843005

3 files changed, 48 insertions, 27 deletions

diff --git a/src/core/SkMipMap.cpp b/src/core/SkMipMap.cpp
index e6f4bad6d2..355c1edebb 100644
--- a/src/core/SkMipMap.cpp
+++ b/src/core/SkMipMap.cpp
@@ -370,9 +370,9 @@ SkMipMap* SkMipMap::Build(const SkPixmap& src, SkDiscardableFactoryProc fact) {
         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;
     }
@@ -502,19 +502,19 @@ SkISize SkMipMap::ComputeLevelSize(int baseWidth, int baseHeight, int level) {
     }
 
     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);
 }
diff --git a/src/core/SkMipMap.h b/src/core/SkMipMap.h
index a230a23392..7c798ef833 100644
--- a/src/core/SkMipMap.h
+++ b/src/core/SkMipMap.h
@@ -18,15 +18,26 @@ class SkDiscardableMemory;
 
 typedef SkDiscardableMemory* (*SkDiscardableFactoryProc)(size_t bytes);
 
+/*
+ * SkMipMap will generate mipmap levels when given a base mipmap level image.
+ *
+ * Any function which deals with mipmap levels indices will start with index 0
+ * being the first mipmap level which was generated. Said another way, it does
+ * not include the base level in its range.
+ */
 class SkMipMap : public SkCachedData {
 public:
     static SkMipMap* Build(const SkPixmap& src, SkDiscardableFactoryProc);
     static SkMipMap* Build(const SkBitmap& src, SkDiscardableFactoryProc);
 
     // Determines how many levels a SkMipMap will have without creating that mipmap.
+    // This does not include the base mipmap level that the user provided when
+    // creating the SkMipMap.
     static int ComputeLevelCount(int baseWidth, int baseHeight);
 
     // Determines the size of a given mipmap level.
+    // |level| is an index into the generated mipmap levels. It does not include
+    // the base level. So index 0 represents mipmap level 1.
     static SkISize ComputeLevelSize(int baseWidth, int baseHeight, int level);
 
     struct Level {
@@ -35,7 +46,13 @@ public:
     };
 
     bool extractLevel(const SkSize& scale, Level*) const;
+
+    // countLevels returns the number of mipmap levels generated (which does not
+    // include the base mipmap level).
     int countLevels() const;
+
+    // |index| is an index into the generated mipmap levels. It does not include
+    // the base level. So index 0 represents mipmap level 1.
     bool getLevel(int index, Level*) const;
 
 protected:
diff --git a/tests/MipMapTest.cpp b/tests/MipMapTest.cpp
index e8d02b878d..596e995868 100644
--- a/tests/MipMapTest.cpp
+++ b/tests/MipMapTest.cpp
@@ -64,11 +64,15 @@ static void test_mipmap_generation(int width, int height, int expectedMipLevelCo
 
     const int mipLevelCount = mm->countLevels();
     REPORTER_ASSERT(reporter, mipLevelCount == expectedMipLevelCount);
+    REPORTER_ASSERT(reporter, mipLevelCount == SkMipMap::ComputeLevelCount(width, height));
     for (int i = 0; i < mipLevelCount; ++i) {
         SkMipMap::Level level;
         REPORTER_ASSERT(reporter, mm->getLevel(i, &level));
         // Make sure the mipmaps contain valid data and that the sizes are correct
         REPORTER_ASSERT(reporter, level.fPixmap.addr());
+        SkISize size = SkMipMap::ComputeLevelSize(width, height, i);
+        REPORTER_ASSERT(reporter, level.fPixmap.width() == size.width());
+        REPORTER_ASSERT(reporter, level.fPixmap.height() == size.height());
 
         // + 1 because SkMipMap does not include the base mipmap level.
         int twoToTheMipLevel = 1 << (i + 1);
@@ -161,34 +165,34 @@ struct LevelSizeScenario {
 DEF_TEST(MipMap_ComputeLevelSize, reporter) {
     const LevelSizeScenario tests[] = {
         // Test mipmaps with negative sizes
-        {-100, 100, 1, SkISize::Make(0, 0)},
-        {100, -100, 1, SkISize::Make(0, 0)},
-        {-100, -100, 1, SkISize::Make(0, 0)},
+        {-100, 100, 0, SkISize::Make(0, 0)},
+        {100, -100, 0, SkISize::Make(0, 0)},
+        {-100, -100, 0, SkISize::Make(0, 0)},
 
         // Test mipmaps with 0, 1, 2 as dimensions
         // (SkMipMap::Build requires a min size of 1)
         //
         // 0
-        {0, 100, 1, SkISize::Make(0, 0)},
-        {100, 0, 1, SkISize::Make(0, 0)},
-        {0, 0, 1, SkISize::Make(0, 0)},
+        {0, 100, 0, SkISize::Make(0, 0)},
+        {100, 0, 0, SkISize::Make(0, 0)},
+        {0, 0, 0, SkISize::Make(0, 0)},
         // 1
 
-        {1, 100, 1, SkISize::Make(1, 50)},
-        {100, 1, 1, SkISize::Make(50, 1)},
-        {1, 1, 1, SkISize::Make(0, 0)},
+        {1, 100, 0, SkISize::Make(1, 50)},
+        {100, 1, 0, SkISize::Make(50, 1)},
+        {1, 1, 0, SkISize::Make(0, 0)},
         // 2
-        {2, 100, 1, SkISize::Make(1, 50)},
-        {100, 2, 2, SkISize::Make(25, 1)},
-        {2, 2, 1, SkISize::Make(1, 1)},
+        {2, 100, 0, SkISize::Make(1, 50)},
+        {100, 2, 1, SkISize::Make(25, 1)},
+        {2, 2, 0, SkISize::Make(1, 1)},
 
         // Test a handful of cases
-        {63, 63, 3, SkISize::Make(7, 7)},
-        {64, 64, 3, SkISize::Make(8, 8)},
-        {127, 127, 3, SkISize::Make(15, 15)},
-        {64, 129, 4, SkISize::Make(4, 8)},
-        {255, 32, 7, SkISize::Make(1, 1)},
-        {500, 1000, 2, SkISize::Make(125, 250)},
+        {63, 63, 2, SkISize::Make(7, 7)},
+        {64, 64, 2, SkISize::Make(8, 8)},
+        {127, 127, 2, SkISize::Make(15, 15)},
+        {64, 129, 3, SkISize::Make(4, 8)},
+        {255, 32, 6, SkISize::Make(1, 1)},
+        {500, 1000, 1, SkISize::Make(125, 250)},
     };
 
     for (auto& currentTest : tests) {