119 строки
5.6 KiB
C
119 строки
5.6 KiB
C
/*Daala video codec
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Copyright (c) 2001-2013 Daala project contributors. All rights reserved.
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Redistribution and use in source and binary forms, with or without
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modification, are permitted provided that the following conditions are met:
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- Redistributions of source code must retain the above copyright notice, this
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list of conditions and the following disclaimer.
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- Redistributions in binary form must reproduce the above copyright notice,
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this list of conditions and the following disclaimer in the documentation
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and/or other materials provided with the distribution.
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THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
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AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
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DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE
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FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
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SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
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CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
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OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
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OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.*/
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#if !defined(_entcode_H)
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#define _entcode_H (1)
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#include <limits.h>
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#include <stddef.h>
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#include "av1/common/odintrin.h"
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/*Set this flag 1 to enable a "reduced overhead" version of the entropy coder.
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This uses a partition function that more accurately follows the input
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probability estimates at the expense of some additional CPU cost (though
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still an order of magnitude less than a full division).
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In classic arithmetic coding, the partition function maps a value x in the
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range [0, ft] to a value in y in [0, r] with 0 < ft <= r via
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y = x*r/ft.
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Any deviation from this value increases coding inefficiency.
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To avoid divisions, we require ft <= r < 2*ft (enforcing it by shifting up
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ft if necessary), and replace that function with
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y = x + OD_MINI(x, r - ft).
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This counts values of x smaller than r - ft double compared to values larger
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than r - ft, which over-estimates the probability of symbols at the start of
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the alphabet, and under-estimates the probability of symbols at the end of
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the alphabet.
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The overall coding inefficiency assuming accurate probability models and
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independent symbols is in the 1% range, which is similar to that of CABAC.
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To reduce overhead even further, we split this into two cases:
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1) r - ft > ft - (r - ft).
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That is, we have more values of x that are double-counted than
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single-counted.
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In this case, we still double-count the first 2*r - 3*ft values of x, but
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after that we alternate between single-counting and double-counting for
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the rest.
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2) r - ft < ft - (r - ft).
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That is, we have more values of x that are single-counted than
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double-counted.
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In this case, we alternate between single-counting and double-counting for
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the first 2*(r - ft) values of x, and single-count the rest.
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For two equiprobable symbols in different places in the alphabet, this
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reduces the maximum ratio of over-estimation to under-estimation from 2:1
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for the previous partition function to either 4:3 or 3:2 (for each of the
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two cases above, respectively), assuming symbol probabilities significantly
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greater than 1/32768.
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That reduces the worst-case per-symbol overhead from 1 bit to 0.58 bits.
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The resulting function is
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e = OD_MAXI(2*r - 3*ft, 0);
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y = x + OD_MINI(x, e) + OD_MINI(OD_MAXI(x - e, 0) >> 1, r - ft).
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Here, e is a value that is greater than 0 in case 1, and 0 in case 2.
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This function is about 3 times as expensive to evaluate as the high-overhead
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version, but still an order of magnitude cheaper than a division, since it
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is composed only of very simple operations.
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Because we want to fit in 16-bit registers and must use unsigned values to do
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so, we use saturating subtraction to enforce the maximums with 0.
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Enabling this reduces the measured overhead in ectest from 0.805% to 0.621%
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(vs. 0.022% for the division-based partition function with r much greater
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than ft).
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It improves performance on ntt-short-1 by about 0.3%.*/
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#define OD_EC_REDUCED_OVERHEAD (1)
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/*OPT: od_ec_window must be at least 32 bits, but if you have fast arithmetic
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on a larger type, you can speed up the decoder by using it here.*/
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typedef uint32_t od_ec_window;
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#define OD_EC_WINDOW_SIZE ((int)sizeof(od_ec_window) * CHAR_BIT)
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/*Unsigned subtraction with unsigned saturation.
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This implementation of the macro is intentionally chosen to increase the
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number of common subexpressions in the reduced-overhead partition function.
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This matters for C code, but it would not for hardware with a saturating
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subtraction instruction.*/
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#define OD_SUBSATU(a, b) ((a)-OD_MINI(a, b))
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/*The number of bits to use for the range-coded part of unsigned integers.*/
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#define OD_EC_UINT_BITS (4)
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/*The resolution of fractional-precision bit usage measurements, i.e.,
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3 => 1/8th bits.*/
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#define OD_BITRES (3)
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extern const uint16_t OD_UNIFORM_CDFS_Q15[135];
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/*Returns a Q15 CDF for a uniform probability distribution of the given size.
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n: The size of the distribution.
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This must be at least 2, and no more than 16.*/
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#define OD_UNIFORM_CDF_Q15(n) (OD_UNIFORM_CDFS_Q15 + ((n) * ((n)-1) >> 1) - 1)
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/*See entcode.c for further documentation.*/
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OD_WARN_UNUSED_RESULT uint32_t od_ec_tell_frac(uint32_t nbits_total,
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uint32_t rng);
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#endif
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