third_party/libwebp/utils/random.h

   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_ */