FFmpeg
 All Data Structures Namespaces Files Functions Variables Typedefs Enumerations Enumerator Macros Groups Pages
dirac_arith.h
Go to the documentation of this file.
1 /*
2  * Copyright (C) 2007 Marco Gerards <marco@gnu.org>
3  * Copyright (C) 2009 David Conrad
4  *
5  * This file is part of FFmpeg.
6  *
7  * FFmpeg is free software; you can redistribute it and/or
8  * modify it under the terms of the GNU Lesser General Public
9  * License as published by the Free Software Foundation; either
10  * version 2.1 of the License, or (at your option) any later version.
11  *
12  * FFmpeg is distributed in the hope that it will be useful,
13  * but WITHOUT ANY WARRANTY; without even the implied warranty of
14  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
15  * Lesser General Public License for more details.
16  *
17  * You should have received a copy of the GNU Lesser General Public
18  * License along with FFmpeg; if not, write to the Free Software
19  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
20  */
21 
22 /**
23  * @file
24  * Arithmetic decoder for Dirac
25  * @author Marco Gerards <marco@gnu.org>
26  */
27 
28 #ifndef AVCODEC_DIRAC_ARITH_H
29 #define AVCODEC_DIRAC_ARITH_H
30 
31 #include "libavutil/x86/asm.h"
32 #include "bytestream.h"
33 #include "get_bits.h"
34 
58 
60 };
61 
62 // Dirac resets the arith decoder between decoding various types of data,
63 // so many contexts are never used simultaneously. Thus, we can reduce
64 // the number of contexts needed by reusing them.
65 #define CTX_SB_F1 CTX_ZP_F5
66 #define CTX_SB_DATA 0
67 #define CTX_PMODE_REF1 0
68 #define CTX_PMODE_REF2 1
69 #define CTX_GLOBAL_BLOCK 2
70 #define CTX_MV_F1 CTX_ZP_F2
71 #define CTX_MV_DATA 0
72 #define CTX_DC_F1 CTX_ZP_F5
73 #define CTX_DC_DATA 0
74 
75 typedef struct {
76  unsigned low;
77  uint16_t range;
78  int16_t counter;
79 
82 
83  uint16_t contexts[DIRAC_CTX_COUNT];
84 } DiracArith;
85 
87 extern const uint16_t ff_dirac_prob[256];
88 extern int16_t ff_dirac_prob_branchless[256][2];
89 
90 static inline void renorm(DiracArith *c)
91 {
92 #if HAVE_FAST_CLZ
93  int shift = 14 - av_log2_16bit(c->range-1) + ((c->range-1)>>15);
94 
95  c->low <<= shift;
96  c->range <<= shift;
97  c->counter += shift;
98 #else
99  while (c->range <= 0x4000) {
100  c->low <<= 1;
101  c->range <<= 1;
102  c->counter++;
103  }
104 #endif
105 }
106 
107 static inline void refill(DiracArith *c)
108 {
109  int counter = c->counter;
110 
111  if (counter >= 0) {
112  int new = bytestream_get_be16(&c->bytestream);
113 
114  // the spec defines overread bits to be 1, and streams rely on this
115  if (c->bytestream > c->bytestream_end) {
116  new |= 0xff;
117  if (c->bytestream > c->bytestream_end+1)
118  new |= 0xff00;
119 
120  c->bytestream = c->bytestream_end;
121  }
122 
123  c->low += new << counter;
124  counter -= 16;
125  }
126  c->counter = counter;
127 }
128 
129 static inline int dirac_get_arith_bit(DiracArith *c, int ctx)
130 {
131  int prob_zero = c->contexts[ctx];
132  int range_times_prob, bit;
133  unsigned low = c->low;
134  int range = c->range;
135 
136  range_times_prob = (c->range * prob_zero) >> 16;
137 
138 #if HAVE_FAST_CMOV && HAVE_INLINE_ASM && HAVE_6REGS
139  low -= range_times_prob << 16;
140  range -= range_times_prob;
141  bit = 0;
142  __asm__(
143  "cmpl %5, %4 \n\t"
144  "setae %b0 \n\t"
145  "cmovb %3, %2 \n\t"
146  "cmovb %5, %1 \n\t"
147  : "+q"(bit), "+r"(range), "+r"(low)
148  : "r"(c->low), "r"(c->low>>16),
149  "r"(range_times_prob)
150  );
151 #else
152  bit = (low >> 16) >= range_times_prob;
153  if (bit) {
154  low -= range_times_prob << 16;
155  range -= range_times_prob;
156  } else {
157  range = range_times_prob;
158  }
159 #endif
160 
161  c->contexts[ctx] += ff_dirac_prob_branchless[prob_zero>>8][bit];
162  c->low = low;
163  c->range = range;
164 
165  renorm(c);
166  refill(c);
167  return bit;
168 }
169 
170 static inline int dirac_get_arith_uint(DiracArith *c, int follow_ctx, int data_ctx)
171 {
172  int ret = 1;
173  while (!dirac_get_arith_bit(c, follow_ctx)) {
174  ret <<= 1;
175  ret += dirac_get_arith_bit(c, data_ctx);
176  follow_ctx = ff_dirac_next_ctx[follow_ctx];
177  }
178  return ret-1;
179 }
180 
181 static inline int dirac_get_arith_int(DiracArith *c, int follow_ctx, int data_ctx)
182 {
183  int ret = dirac_get_arith_uint(c, follow_ctx, data_ctx);
184  if (ret && dirac_get_arith_bit(c, data_ctx+1))
185  ret = -ret;
186  return ret;
187 }
188 
190 
191 #endif /* AVCODEC_DIRAC_ARITH_H */