decoder_synchro.c

VLC Player Source Code

/*****************************************************************************
 * decoder_synchro.c : frame dropping routines
 *****************************************************************************
 * Copyright (C) 1999-2005 the VideoLAN team
 * $Id$
 *
 * Authors: Christophe Massiot <[e-mail]>
 *          Samuel Hocevar <[e-mail]>
 *          Jean-Marc Dressler <[e-mail]>
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston MA 02110-1301, USA.
 *****************************************************************************/

/*
 * DISCUSSION : How to Write an efficient Frame-Dropping Algorithm
 * ==========
 *
 * This implementation is based on mathematical and statistical
 * developments. Older implementations used an enslavement, considering
 * that if we're late when reading an I picture, we will decode one frame
 * less. It had a tendancy to derive, and wasn't responsive enough, which
 * would have caused trouble with the stream control stuff.
 *
 * 1. Structure of a picture stream
 *    =============================
 * Between 2 I's, we have for instance :
 *    I   B   P   B   P   B   P   B   P   B   P   B   I
 *    t0  t1  t2  t3  t4  t5  t6  t7  t8  t9  t10 t11 t12
 * Please bear in mind that B's and IP's will be inverted when displaying
 * (decoding order != presentation order). Thus, t1 < t0.
 *
 * 2. Definitions
 *    ===========
 * t[0..12]     : Presentation timestamps of pictures 0..12.
 * t            : Current timestamp, at the moment of the decoding.
 * T            : Picture period, T = 1/frame_rate.
 * tau[I,P,B]   : Mean time to decode an [I,P,B] picture.
 * tauYUV       : Mean time to render a picture (given by the video_output).
 * tau´[I,P,B] = 2 * tau[I,P,B] + tauYUV
 *              : Mean time + typical difference (estimated to tau/2, that
 *                needs to be confirmed) + render time.
 * DELTA        : A given error margin.
 *
 * 3. General considerations
 *    ======================
 * We define three types of machines :
 *      14T > tauI : machines capable of decoding all I pictures
 *      2T > tauP  : machines capable of decoding all P pictures
 *      T > tauB   : machines capable of decoding all B pictures
 *
 * 4. Decoding of an I picture
 *    ========================
 * On fast machines, we decode all I's.
 * Otherwise :
 * We can decode an I picture if we simply have enough time to decode it
 * before displaying :
 *      t0 - t > tau´I + DELTA
 *
 * 5. Decoding of a P picture
 *    =======================
 * On fast machines, we decode all P's.
 * Otherwise :
 * First criterion : have time to decode it.
 *      t2 - t > tau´P + DELTA
 *
 * Second criterion : it shouldn't prevent us from displaying the forthcoming
 * I picture, which is more important.
 *      t12 - t > tau´P + tau´I + DELTA
 *
 * 6. Decoding of a B picture
 *    =======================
 * On fast machines, we decode all B's. Otherwise :
 *      t1 - t > tau´B + DELTA
 * Since the next displayed I or P is already decoded, we don't have to
 * worry about it.
 *
 * I hope you will have a pleasant flight and do not forget your life
 * jacket.
 *                                                  --Meuuh (2000-12-29)
 */

/*****************************************************************************
 * Preamble
 *****************************************************************************/
#ifdef HAVE_CONFIG_H
# include "config.h"
#endif

#include <vlc_common.h>
#include <vlc_input.h>
#include <vlc_codec.h>
#include <vlc_codec_synchro.h>

/*
 * Local prototypes
 */

#define MAX_PIC_AVERAGE         8

struct decoder_synchro_t
{
    /* */
    decoder_t       *p_dec;

    /* */
    int             i_frame_rate;
    int             i_current_rate;
    bool      b_no_skip;
    bool      b_quiet;

    /* date of the beginning of the decoding of the current picture */
    mtime_t         decoding_start;

    /* stream properties */
    unsigned int    i_n_p, i_n_b;

    /* decoding values */
    mtime_t         p_tau[4];                  /* average decoding durations */
    unsigned int    pi_meaningful[4];            /* number of durations read */

    /* render_time filled by SynchroChoose() */
    int i_render_time;

    /* stream context */
    int             i_nb_ref;                /* Number of reference pictures */
    int             i_dec_nb_ref;      /* Number of reference pictures we'll *
                                        * have if we decode the current pic  */
    int             i_trash_nb_ref;    /* Number of reference pictures we'll *
                                        * have if we trash the current pic   */
    unsigned int    i_eta_p, i_eta_b;
    mtime_t         backward_pts, current_pts;
    int             i_current_period;   /* period to add to the next picture */
    int             i_backward_period;  /* period to add after the next
                                         * reference picture
                                         * (backward_period * period / 2) */

    /* statistics */
    unsigned int    i_trashed_pic, i_not_chosen_pic, i_pic;
};

/* Error margins */
#define DELTA                   (int)(0.075*CLOCK_FREQ)
#define MAX_VALID_TAU           (int)(0.3*CLOCK_FREQ)

#define DEFAULT_NB_P            5
#define DEFAULT_NB_B            1

/*****************************************************************************
 * decoder_SynchroInit : You know what ?
 *****************************************************************************/
decoder_synchro_t * decoder_SynchroInit( decoder_t *p_dec, int i_frame_rate )
{
    decoder_synchro_t * p_synchro = malloc( sizeof(*p_synchro) );
    if ( p_synchro == NULL )
        return NULL;
    memset( p_synchro, 0, sizeof(*p_synchro) );

    p_synchro->p_dec = p_dec;
    p_synchro->b_no_skip = !config_GetInt( p_dec, "skip-frames" );
    p_synchro->b_quiet = config_GetInt( p_dec, "quiet-synchro" );

    /* We use a fake stream pattern, which is often right. */
    p_synchro->i_n_p = p_synchro->i_eta_p = DEFAULT_NB_P;
    p_synchro->i_n_b = p_synchro->i_eta_b = DEFAULT_NB_B;
    memset( p_synchro->p_tau, 0, 4 * sizeof(mtime_t) );
    memset( p_synchro->pi_meaningful, 0, 4 * sizeof(unsigned int) );
    p_synchro->i_nb_ref = 0;
    p_synchro->i_trash_nb_ref = p_synchro->i_dec_nb_ref = 0;
    p_synchro->current_pts = mdate() + DEFAULT_PTS_DELAY;
    p_synchro->backward_pts = 0;
    p_synchro->i_current_period = p_synchro->i_backward_period = 0;
    p_synchro->i_trashed_pic = p_synchro->i_not_chosen_pic =
        p_synchro->i_pic = 0;

    p_synchro->i_frame_rate = i_frame_rate;

    return p_synchro;
}

/*****************************************************************************
 * decoder_SynchroRelease : You know what ?
 *****************************************************************************/
void decoder_SynchroRelease( decoder_synchro_t * p_synchro )
{
    free( p_synchro );
}

/*****************************************************************************
 * decoder_SynchroReset : Reset the reference picture counter
 *****************************************************************************/
void decoder_SynchroReset( decoder_synchro_t * p_synchro )
{
    p_synchro->i_nb_ref = 0;
    p_synchro->i_trash_nb_ref = p_synchro->i_dec_nb_ref = 0;
}

/*****************************************************************************
 * decoder_SynchroChoose : Decide whether we will decode a picture or not
 *****************************************************************************/
bool decoder_SynchroChoose( decoder_synchro_t * p_synchro, int i_coding_type,
                               int i_render_time, bool b_low_delay )
{
#define TAU_PRIME( coding_type )    (p_synchro->p_tau[(coding_type)] \
                                    + (p_synchro->p_tau[(coding_type)] >> 1) \
                                    + p_synchro->i_render_time)
#define S (*p_synchro)
    mtime_t         now, period;
    mtime_t         pts = 0;
    bool      b_decode = 0;

    if ( p_synchro->b_no_skip )
        return 1;

    now = mdate();
    period = 1000000 * 1001 / p_synchro->i_frame_rate
                     * p_synchro->i_current_rate / INPUT_RATE_DEFAULT;

    p_synchro->i_render_time = i_render_time;

    switch( i_coding_type )
    {
    case I_CODING_TYPE:
        if( b_low_delay )
        {
            pts = S.current_pts;
        }
        else if( S.backward_pts )
        {
            pts = S.backward_pts;
        }
        else
        {
            /* displaying order : B B P B B I
             *                      ^       ^
             *                      |       +- current picture
             *                      +- current PTS
             */
            pts = S.current_pts + period * (S.i_n_b + 2);
        }

        if( (1 + S.i_n_p * (S.i_n_b + 1)) * period >
                S.p_tau[I_CODING_TYPE] )
        {
            b_decode = 1;
        }
        else
        {
            b_decode = (pts - now) > (TAU_PRIME(I_CODING_TYPE) + DELTA);
        }
        if( !b_decode && !p_synchro->b_quiet )
        {
            msg_Warn( p_synchro->p_dec,
                      "synchro trashing I (%"PRId64")", pts - now );
        }
        break;

    case P_CODING_TYPE:
        if( b_low_delay )
        {
            pts = S.current_pts;
        }
        else if( S.backward_pts )
        {
            pts = S.backward_pts;
        }
        else
        {
            pts = S.current_pts + period * (S.i_n_b + 1);
        }

        if( p_synchro->i_nb_ref < 1 )
        {
            b_decode = 0;
        }
        else if( (1 + S.i_n_p * (S.i_n_b + 1)) * period >
                S.p_tau[I_CODING_TYPE] )
        {
            if( (S.i_n_b + 1) * period > S.p_tau[P_CODING_TYPE] )
            {
                /* Security in case we're _really_ late */
                b_decode = (pts - now > 0);
            }
            else