2 * Copyright 2014 Google Inc.
4 * Use of this source code is governed by a BSD-style license that can be
5 * found in the LICENSE file.
8 #ifndef SkTextureCompressorUtils_DEFINED
9 #define SkTextureCompressorUtils_DEFINED
11 namespace SkTextureCompressor {
13 // In some compression formats used for grayscale alpha, i.e. coverage masks, three
14 // bit indices are used to represent each pixel. A compression scheme must therefore
15 // quantize the full eight bits of grayscale to three bits. The simplest way to do
16 // this is to take the top three bits of the grayscale value. However, this does not
17 // provide an accurate quantization: 192 will be quantized to 219 instead of 185. In
18 // our compression schemes, we let these three-bit indices represent the full range
19 // of grayscale values, and so when we go from three bits to eight bits, we replicate
20 // the three bits into the lower bits of the eight bit value. Below are two different
21 // techniques that offer a quality versus speed tradeoff in terms of quantization.
23 // Divides each byte in the 32-bit argument by three.
24 static inline uint32_t MultibyteDiv3(uint32_t x) {
25 const uint32_t a = (x >> 2) & 0x3F3F3F3F;
26 const uint32_t ar = (x & 0x03030303) << 4;
28 const uint32_t b = (x >> 4) & 0x0F0F0F0F;
29 const uint32_t br = (x & 0x0F0F0F0F) << 2;
31 const uint32_t c = (x >> 6) & 0x03030303;
32 const uint32_t cr = x & 0x3F3F3F3F;
34 return a + b + c + (((ar + br + cr) >> 6) & 0x03030303);
37 // Takes a loaded 32-bit integer of four 8-bit greyscale values and returns their
38 // quantization into 3-bit values, used by LATC and R11 EAC. Instead of taking the
39 // top three bits, the function computes the best three-bit value such that its
40 // reconstruction into an eight bit value via bit replication will yield the best
41 // results. In a 32-bit integer taking the range of values from 0-255 we would add
42 // 18 and divide by 36 (255 / 36 ~= 7). However, since we are working in constrained
43 // 8-bit space, our algorithm is the following:
44 // 1. Shift right by one to give room for overflow
46 // 3. Divide by 18 (divide by two, then by three twice)
47 static inline uint32_t ConvertToThreeBitIndex(uint32_t x) {
48 x = (x >> 1) & 0x7F7F7F7F; // 1
49 x = x + 0x09090909; // 2
51 // Need to divide by 18... so first divide by two
52 x = (x >> 1) & 0x7F7F7F7F;
54 // Now divide by three twice
60 // Moves the top three bits of each byte in the 32-bit argument to the least
61 // significant bits of their respective byte.
62 static inline uint32_t ConvertToThreeBitIndex(uint32_t x) {
63 return (x >> 5) & 0x07070707;
68 #endif // SkTextureCompressorUtils_DEFINED