1// Copyright 2013 Google Inc. All Rights Reserved. 2// 3// Use of this source code is governed by a BSD-style license 4// that can be found in the COPYING file in the root of the source 5// tree. An additional intellectual property rights grant can be found 6// in the file PATENTS. All contributing project authors may 7// be found in the AUTHORS file in the root of the source tree. 8// ----------------------------------------------------------------------------- 9// 10// Pseudo-random utilities 11// 12// Author: Skal (pascal.massimino@gmail.com) 13 14#ifndef WEBP_UTILS_RANDOM_H_ 15#define WEBP_UTILS_RANDOM_H_ 16 17#include <assert.h> 18#include "../webp/types.h" 19 20#ifdef __cplusplus 21extern "C" { 22#endif 23 24#define VP8_RANDOM_DITHER_FIX 8 // fixed-point precision for dithering 25#define VP8_RANDOM_TABLE_SIZE 55 26 27typedef struct { 28 int index1_, index2_; 29 uint32_t tab_[VP8_RANDOM_TABLE_SIZE]; 30 int amp_; 31} VP8Random; 32 33// Initializes random generator with an amplitude 'dithering' in range [0..1]. 34void VP8InitRandom(VP8Random* const rg, float dithering); 35 36// Returns a centered pseudo-random number with 'num_bits' amplitude. 37// (uses D.Knuth's Difference-based random generator). 38// 'amp' is in VP8_RANDOM_DITHER_FIX fixed-point precision. 39static WEBP_INLINE int VP8RandomBits2(VP8Random* const rg, int num_bits, 40 int amp) { 41 int diff; 42 assert(num_bits + VP8_RANDOM_DITHER_FIX <= 31); 43 diff = rg->tab_[rg->index1_] - rg->tab_[rg->index2_]; 44 if (diff < 0) diff += (1u << 31); 45 rg->tab_[rg->index1_] = diff; 46 if (++rg->index1_ == VP8_RANDOM_TABLE_SIZE) rg->index1_ = 0; 47 if (++rg->index2_ == VP8_RANDOM_TABLE_SIZE) rg->index2_ = 0; 48 // sign-extend, 0-center 49 diff = (int)((uint32_t)diff << 1) >> (32 - num_bits); 50 diff = (diff * amp) >> VP8_RANDOM_DITHER_FIX; // restrict range 51 diff += 1 << (num_bits - 1); // shift back to 0.5-center 52 return diff; 53} 54 55static WEBP_INLINE int VP8RandomBits(VP8Random* const rg, int num_bits) { 56 return VP8RandomBits2(rg, num_bits, rg->amp_); 57} 58 59#ifdef __cplusplus 60} // extern "C" 61#endif 62 63#endif /* WEBP_UTILS_RANDOM_H_ */ 64