/src/libwebp/src/utils/random_utils.h

Source
// Copyright 2013 Google Inc. All Rights Reserved.
//
// Use of this source code is governed by a BSD-style license
// that can be found in the COPYING file in the root of the source
// tree. An additional intellectual property rights grant can be found
// in the file PATENTS. All contributing project authors may
// be found in the AUTHORS file in the root of the source tree.
// -----------------------------------------------------------------------------
//
// Pseudo-random utilities
//
// Author: Skal (pascal.massimino@gmail.com)

#ifndef WEBP_UTILS_RANDOM_UTILS_H_
#define WEBP_UTILS_RANDOM_UTILS_H_

#include <assert.h>

#include "src/utils/bounds_safety.h"
#include "src/webp/types.h"

WEBP_ASSUME_UNSAFE_INDEXABLE_ABI

#ifdef __cplusplus
extern "C" {
#endif

#define VP8_RANDOM_DITHER_FIX 8  // fixed-point precision for dithering
#define VP8_RANDOM_TABLE_SIZE 55

typedef struct {
  int index1, index2;
  uint32_t tab[VP8_RANDOM_TABLE_SIZE];
  int amp;
} VP8Random;

// Initializes random generator with an amplitude 'dithering' in range [0..1].
void VP8InitRandom(VP8Random* const rg, float dithering);

// Returns a centered pseudo-random number with 'num_bits' amplitude.
// (uses D.Knuth's Difference-based random generator).
// 'amp' is in VP8_RANDOM_DITHER_FIX fixed-point precision.
static WEBP_INLINE int VP8RandomBits2(VP8Random* const rg, int num_bits,
                                      int amp) {
  int diff;
  assert(num_bits + VP8_RANDOM_DITHER_FIX <= 31);
  diff = rg->tab[rg->index1] - rg->tab[rg->index2];
  if (diff < 0) diff += (1u << 31);
  rg->tab[rg->index1] = diff;
  if (++rg->index1 == VP8_RANDOM_TABLE_SIZE) rg->index1 = 0;
  if (++rg->index2 == VP8_RANDOM_TABLE_SIZE) rg->index2 = 0;
  // sign-extend, 0-center
  diff = (int)((uint32_t)diff << 1) >> (32 - num_bits);
  diff = (diff * amp) >> VP8_RANDOM_DITHER_FIX;  // restrict range
  diff += 1 << (num_bits - 1);                   // shift back to 0.5-center
  return diff;
}

static WEBP_INLINE int VP8RandomBits(VP8Random* const rg, int num_bits) {
  return VP8RandomBits2(rg, num_bits, rg->amp);
}

#ifdef __cplusplus
}  // extern "C"
#endif

#endif  // WEBP_UTILS_RANDOM_UTILS_H_

Coverage Report

Created: 2026-01-20 07:37

Line	Count	Source
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_UTILS_H_
15		#define WEBP_UTILS_RANDOM_UTILS_H_
16
17		#include <assert.h>
18
19		#include "src/utils/bounds_safety.h"
20		#include "src/webp/types.h"
21
22		WEBP_ASSUME_UNSAFE_INDEXABLE_ABI
23
24		#ifdef __cplusplus
25		extern "C" {
26		#endif
27
28	0	#define VP8_RANDOM_DITHER_FIX 8 // fixed-point precision for dithering
29	0	#define VP8_RANDOM_TABLE_SIZE 55
30
31		typedef struct {
32		int index1, index2;
33		uint32_t tab[VP8_RANDOM_TABLE_SIZE];
34		int amp;
35		} VP8Random;
36
37		// Initializes random generator with an amplitude 'dithering' in range [0..1].
38		void VP8InitRandom(VP8Random* const rg, float dithering);
39
40		// Returns a centered pseudo-random number with 'num_bits' amplitude.
41		// (uses D.Knuth's Difference-based random generator).
42		// 'amp' is in VP8_RANDOM_DITHER_FIX fixed-point precision.
43		static WEBP_INLINE int VP8RandomBits2(VP8Random* const rg, int num_bits,
44	0	int amp) {
45	0	int diff;
46	0	assert(num_bits + VP8_RANDOM_DITHER_FIX <= 31);
47	0	diff = rg->tab[rg->index1] - rg->tab[rg->index2];
48	0	if (diff < 0) diff += (1u << 31);
49	0	rg->tab[rg->index1] = diff;
50	0	if (++rg->index1 == VP8_RANDOM_TABLE_SIZE) rg->index1 = 0;
51	0	if (++rg->index2 == VP8_RANDOM_TABLE_SIZE) rg->index2 = 0;
52		// sign-extend, 0-center
53	0	diff = (int)((uint32_t)diff << 1) >> (32 - num_bits);
54	0	diff = (diff * amp) >> VP8_RANDOM_DITHER_FIX; // restrict range
55	0	diff += 1 << (num_bits - 1); // shift back to 0.5-center
56	0	return diff;
57	0	} Unexecuted instantiation: muxedit.c:VP8RandomBits2 Unexecuted instantiation: muxinternal.c:VP8RandomBits2 Unexecuted instantiation: muxread.c:VP8RandomBits2 Unexecuted instantiation: buffer_dec.c:VP8RandomBits2 Unexecuted instantiation: idec_dec.c:VP8RandomBits2 Unexecuted instantiation: io_dec.c:VP8RandomBits2 Unexecuted instantiation: tree_dec.c:VP8RandomBits2 Unexecuted instantiation: vp8_dec.c:VP8RandomBits2 Unexecuted instantiation: webp_dec.c:VP8RandomBits2 Unexecuted instantiation: dec.c:VP8RandomBits2 Unexecuted instantiation: dec_sse2.c:VP8RandomBits2 Unexecuted instantiation: dec_sse41.c:VP8RandomBits2 Unexecuted instantiation: picture_csp_enc.c:VP8RandomBits2 Unexecuted instantiation: random_utils.c:VP8RandomBits2 Unexecuted instantiation: alpha_dec.c:VP8RandomBits2 Unexecuted instantiation: frame_dec.c:VP8RandomBits2 Unexecuted instantiation: quant_dec.c:VP8RandomBits2
58
59	0	static WEBP_INLINE int VP8RandomBits(VP8Random* const rg, int num_bits) {
60	0	return VP8RandomBits2(rg, num_bits, rg->amp);
61	0	} Unexecuted instantiation: muxedit.c:VP8RandomBits Unexecuted instantiation: muxinternal.c:VP8RandomBits Unexecuted instantiation: muxread.c:VP8RandomBits Unexecuted instantiation: buffer_dec.c:VP8RandomBits Unexecuted instantiation: idec_dec.c:VP8RandomBits Unexecuted instantiation: io_dec.c:VP8RandomBits Unexecuted instantiation: tree_dec.c:VP8RandomBits Unexecuted instantiation: vp8_dec.c:VP8RandomBits Unexecuted instantiation: webp_dec.c:VP8RandomBits Unexecuted instantiation: dec.c:VP8RandomBits Unexecuted instantiation: dec_sse2.c:VP8RandomBits Unexecuted instantiation: dec_sse41.c:VP8RandomBits Unexecuted instantiation: picture_csp_enc.c:VP8RandomBits Unexecuted instantiation: random_utils.c:VP8RandomBits Unexecuted instantiation: alpha_dec.c:VP8RandomBits Unexecuted instantiation: frame_dec.c:VP8RandomBits Unexecuted instantiation: quant_dec.c:VP8RandomBits
62
63		#ifdef __cplusplus
64		} // extern "C"
65		#endif
66
67		#endif // WEBP_UTILS_RANDOM_UTILS_H_