Add utils to better quantize grayscale values to three bit indices while

tests/TextureCompressionTest.cpp

 /*
  * Copyright 2014 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkBitmap.h"
 #include "SkData.h"
 #include "SkEndian.h"
@@ skipping 237 matching lines @@
 
         REPORTER_ASSERT(reporter, kSizeToBe == latcData->size());
 
         // Make sure that it all matches a given block encoding. Since we have
         // COMPRESS_LATC_FAST defined in SkTextureCompressor_LATC.cpp, we are using
         // an approximation scheme that optimizes for speed against coverage maps.
         // That means that each palette in the encoded block is exactly the same,
         // and that the three bits saved per pixel are computed from the top three
         // bits of the luminance value.
         const uint64_t kIndexEncodingMap[8] = { 1, 7, 6, 5, 4, 3, 2, 0 };
-        const uint64_t kIndex = kIndexEncodingMap[lum >> 5];
+
+        // Quantize to three bits in the same way that we do our LATC compression:
+        // 1. Divide by two
+        // 2. Add 9
+        // 3. Divide by two
+        // 4. Approximate division by three twice
+        uint32_t quant = static_cast<uint32_t>(lum);
+        quant >>= 1; // 1
+        quant += 9;  // 2
+        quant >>= 1; // 3
+
+        uint32_t a, b, c, ar, br, cr;
+
+        // First division by three
+        a = quant >> 2;
+        ar = (quant & 0x3) << 4;
+        b = quant >> 4;
+        br = (quant & 0xF) << 2;
+        c = quant >> 6;
+        cr = (quant & 0x3F);
+        quant = (a + b + c) + ((ar + br + cr) >> 6);
+
+        // Second division by three
+        a = quant >> 2;
+        ar = (quant & 0x3) << 4;
+        b = quant >> 4;
+        br = (quant & 0xF) << 2;
+        c = quant >> 6;
+        cr = (quant & 0x3F);
+        quant = (a + b + c) + ((ar + br + cr) >> 6);
+
+        const uint64_t kIndex = kIndexEncodingMap[quant];
+
         const uint64_t kConstColorEncoding =
             SkEndian_SwapLE64(
                 255 |
                 (kIndex << 16) | (kIndex << 19) | (kIndex << 22) | (kIndex << 25) |
                 (kIndex << 28) | (kIndex << 31) | (kIndex << 34) | (kIndex << 37) |
                 (kIndex << 40) | (kIndex << 43) | (kIndex << 46) | (kIndex << 49) |
                 (kIndex << 52) | (kIndex << 55) | (kIndex << 58) | (kIndex << 61));
 
         const uint64_t* blockPtr = reinterpret_cast<const uint64_t*>(latcData->data());
         for (size_t i = 0; i < (kSizeToBe/8); ++i) {
             REPORTER_ASSERT(reporter, blockPtr[i] == kConstColorEncoding);
         }
     }
 }