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

src/utils/SkTextureCompressor_Utils.h

+/*
+* Copyright 2014 Google Inc.
+*
+* Use of this source code is governed by a BSD-style license that can be
+* found in the LICENSE file.
+*/
+
+#ifndef SkTextureCompressorUtils_DEFINED
+#define SkTextureCompressorUtils_DEFINED
+
+namespace SkTextureCompressor {
+
+    // In some compression formats used for grayscale alpha, i.e. coverage masks, three
+    // bit indices are used to represent each pixel. A compression scheme must therefore
+    // quantize the full eight bits of grayscale to three bits. The simplest way to do
+    // this is to take the top three bits of the grayscale value. However, this does not
+    // provide an accurate quantization: 192 will be quantized to 219 instead of 185. In
+    // our compression schemes, we let these three-bit indices represent the full range
+    // of grayscale values, and so when we go from three bits to eight bits, we replicate
+    // the three bits into the lower bits of the eight bit value. Below are two different
+    // techniques that offer a quality versus speed tradeoff in terms of quantization.
+#if 1
+    // Divides each byte in the 32-bit argument by three.
+    static inline uint32_t MultibyteDiv3(uint32_t x) {
+        const uint32_t a = (x >> 2) & 0x3F3F3F3F;
+        const uint32_t ar = (x & 0x03030303) << 4;
+
+        const uint32_t b = (x >> 4) & 0x0F0F0F0F;
+        const uint32_t br = (x & 0x0F0F0F0F) << 2;
+
+        const uint32_t c = (x >> 6) & 0x03030303;
+        const uint32_t cr = x & 0x3F3F3F3F;
+
+        return a + b + c + (((ar + br + cr) >> 6) & 0x03030303);
+    }
+
+    // Takes a loaded 32-bit integer of four 8-bit greyscale values and returns their
+    // quantization into 3-bit values, used by LATC and R11 EAC. Instead of taking the
+    // top three bits, the function computes the best three-bit value such that its
+    // reconstruction into an eight bit value via bit replication will yield the best
+    // results. In a 32-bit integer taking the range of values from 0-255 we would add
+    // 18 and divide by 36 (255 / 36 ~= 7). However, since we are working in constrained
+    // 8-bit space, our algorithm is the following:
+    // 1. Shift right by one to give room for overflow
+    // 2. Add 9 (18/2)
+    // 3. Divide by 18 (divide by two, then by three twice)
+    static inline uint32_t ConvertToThreeBitIndex(uint32_t x) {
+        x = (x >> 1) & 0x7F7F7F7F; // 1
+        x = x + 0x09090909;        // 2
+
+        // Need to divide by 18... so first divide by two
+        x = (x >> 1) & 0x7F7F7F7F;
+
+        // Now divide by three twice
+        x = MultibyteDiv3(x);
+        x = MultibyteDiv3(x);
+        return x;
+    }
+#else
+    // Moves the top three bits of each byte in the 32-bit argument to the least
+    // significant bits of their respective byte.
+    static inline uint32_t ConvertToThreeBitIndex(uint32_t x) {
+        return (x >> 5) & 0x07070707;
+    }
+#endif
+}
+
+#endif // SkTextureCompressorUtils_DEFINED