// Copyright 2013 Google Inc. All Rights Reserved. // // Use of this source code is governed by a BSD-style license // that can be found in the COPYING file in the root of the source // tree. An additional intellectual property rights grant can be found // in the file PATENTS. All contributing project authors may // be found in the AUTHORS file in the root of the source tree. // ----------------------------------------------------------------------------- // // Pseudo-random utilities // // Author: Skal (pascal.massimino@gmail.com) #ifndef WEBP_UTILS_RANDOM_H_ #define WEBP_UTILS_RANDOM_H_ #include <assert.h> #include "webp/types.h" #ifdef __cplusplus extern "C" { #endif #define VP8_RANDOM_DITHER_FIX 8 // fixed-point precision for dithering #define VP8_RANDOM_TABLE_SIZE 55 typedef struct { int index1_, index2_; uint32_t tab_[VP8_RANDOM_TABLE_SIZE]; int amp_; } VP8Random; // Initializes random generator with an amplitude 'dithering' in range [0..1]. void VP8InitRandom(VP8Random* const rg, float dithering); // Returns a centered pseudo-random number with 'num_bits' amplitude. // (uses D.Knuth's Difference-based random generator). // 'amp' is in VP8_RANDOM_DITHER_FIX fixed-point precision. static WEBP_INLINE int VP8RandomBits2(VP8Random* const rg, int num_bits, int amp) { int diff; assert(num_bits + VP8_RANDOM_DITHER_FIX <= 31); diff = rg->tab_[rg->index1_] - rg->tab_[rg->index2_]; if (diff < 0) diff += (1u << 31); rg->tab_[rg->index1_] = diff; if (++rg->index1_ == VP8_RANDOM_TABLE_SIZE) rg->index1_ = 0; if (++rg->index2_ == VP8_RANDOM_TABLE_SIZE) rg->index2_ = 0; // sign-extend, 0-center diff = (int)((uint32_t)diff << 1) >> (32 - num_bits); diff = (diff * amp) >> VP8_RANDOM_DITHER_FIX; // restrict range diff += 1 << (num_bits - 1); // shift back to 0.5-center return diff; } static WEBP_INLINE int VP8RandomBits(VP8Random* const rg, int num_bits) { return VP8RandomBits2(rg, num_bits, rg->amp_); } #ifdef __cplusplus } // extern "C" #endif #endif /* WEBP_UTILS_RANDOM_H_ */