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| 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 |
| 21 extern "C" { |
| 22 #endif |
| 23 |
| 24 #define VP8_RANDOM_DITHER_FIX 8 // fixed-point precision for dithering |
| 25 #define VP8_RANDOM_TABLE_SIZE 55 |
| 26 |
| 27 typedef 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]. |
| 34 void 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. |
| 39 static 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 |
| 55 static 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_ */ |
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