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