64 lines
2.1 KiB
C
64 lines
2.1 KiB
C
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// 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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