skia / skia / 7695359876b8f90225bc4be895c20f34fcdfaf2e / . / 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 |