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/*
* 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
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