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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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Chromium Code Reviews
Issue 2651883004:
libwebp-0.6.0-rc1 (Closed)
