1，函数功能

本函数主要完成一些编码策略的配置，比如bit的设置，编码模式的选择，原图像没有不编码，申请参考帧列表，疑惑的地方在于show_frame这个属性是不是解码相关，函数在denoise_and_encode进入下一层编码

2，代码

/*该函数涉及编码策略相关的功能，如比特分配、编码速度配置，参考帧列表
*此后调用denoise_and_encode()完成编码，
*更新编码后的信息（update_rc_counts()）
*/
int av1_encode_strategy(AV1_COMP *const cpi, size_t *const size,
                        uint8_t *const dest, unsigned int *frame_flags,
                        int64_t *const time_stamp, int64_t *const time_end,
                        const aom_rational64_t *const timestamp_ratio,
                        int flush) {
  const AV1EncoderConfig *const oxcf = &cpi->oxcf;
  AV1_COMMON *const cm = &cpi->common; 

  EncodeFrameInput frame_input;
  EncodeFrameParams frame_params;
  EncodeFrameResults frame_results;
  memset(&frame_input, 0, sizeof(frame_input));
  memset(&frame_params, 0, sizeof(frame_params));
  memset(&frame_results, 0, sizeof(frame_results));

  // TODO(sarahparker) finish bit allocation for one pass pyramid
  /*为第一次编码配置bit*/
  if (oxcf->pass == 0 && oxcf->rc_mode != AOM_Q)
    cpi->oxcf.gf_max_pyr_height = USE_ALTREF_FOR_ONE_PASS;

  if (oxcf->pass == 0 || oxcf->pass == 2) {
    check_show_existing_frame(cpi, &frame_params);
    frame_params.show_existing_frame &= allow_show_existing(cpi, *frame_flags);
  } else {
    frame_params.show_existing_frame = 0;
  }

  int temporal_filtered = 0;
  struct lookahead_entry *source = NULL;
  struct lookahead_entry *last_source = NULL;
  FRAME_UPDATE_TYPE frame_update_type;
  if (frame_params.show_existing_frame) {
    source = av1_lookahead_pop(cpi->lookahead, flush);
    frame_update_type = LF_UPDATE;
  } else {//根据当前帧的类型来决定是否需要temporal filter时空滤波
    source = choose_frame_source(cpi, &temporal_filtered, &flush, &last_source,
                                 &frame_update_type, &frame_params);
  }

  /*从gf_group获得frame_update_type*/
  if (oxcf->pass != 1) {
    frame_update_type = get_frame_update_type(cpi);
  }
  /*原图像不在不编码*/
  if (source == NULL) {  // If no source was found, we can't encode a frame.
#if !CONFIG_REALTIME_ONLY
    if (flush && oxcf->pass == 1 && !cpi->twopass.first_pass_done) {
      av1_end_first_pass(cpi); /* get last stats packet */
      cpi->twopass.first_pass_done = 1;
    }
#endif
    return -1;
  }

  frame_input.source = temporal_filtered ? &cpi->alt_ref_buffer : &source->img;
  frame_input.last_source = last_source != NULL ? &last_source->img : NULL;
  frame_input.ts_duration = source->ts_end - source->ts_start;

  *time_stamp = source->ts_start;
  *time_end = source->ts_end;
  if (source->ts_start < cpi->first_time_stamp_ever) {
    cpi->first_time_stamp_ever = source->ts_start;
    cpi->last_end_time_stamp_seen = source->ts_start;
  }

  av1_apply_encoding_flags(cpi, source->flags);
  if (!frame_params.show_existing_frame)
    *frame_flags = (source->flags & AOM_EFLAG_FORCE_KF) ? FRAMEFLAGS_KEY : 0;

  const int is_overlay = frame_params.show_existing_frame &&
                         (frame_update_type == OVERLAY_UPDATE ||
                          frame_update_type == INTNL_OVERLAY_UPDATE);
  if (frame_params.show_frame || is_overlay) {//Shown frames好像解码相关？
    // Shown frames and arf-overlay frames need frame-rate considering
    adjust_frame_rate(cpi, source);
  }

  if (frame_params.show_existing_frame) {
    // show_existing_frame implies this frame is shown!
    frame_params.show_frame = 1;
  } else {
    if (cpi->film_grain_table) {
      cm->cur_frame->film_grain_params_present = aom_film_grain_table_lookup(
          cpi->film_grain_table, *time_stamp, *time_end, 0 /* =erase */,
          &cm->film_grain_params);
    } else {
      cm->cur_frame->film_grain_params_present =
          cm->seq_params.film_grain_params_present;
    }
    // only one operating point supported now
    const int64_t pts64 = ticks_to_timebase_units(timestamp_ratio, *time_stamp);
    if (pts64 < 0 || pts64 > UINT32_MAX) return AOM_CODEC_ERROR;
    cpi->common.frame_presentation_time = (uint32_t)pts64;
  }

#if CONFIG_REALTIME_ONLY
  av1_get_one_pass_rt_params(cpi, &frame_params, *frame_flags);
  frame_update_type = get_frame_update_type(cpi);
#else
  //编码模式选择
  if (oxcf->pass == 0 && oxcf->mode == REALTIME && oxcf->lag_in_frames == 0) {
    av1_get_one_pass_rt_params(cpi, &frame_params, *frame_flags);
    frame_update_type = get_frame_update_type(cpi);
  } else if (oxcf->pass != 1 &&
             (!frame_params.show_existing_frame || is_overlay)) {
    // GF_GROUP needs updating for arf overlays as well as non-show-existing
    av1_get_second_pass_params(cpi, &frame_params, *frame_flags);
    frame_update_type = get_frame_update_type(cpi);
  }
#endif

  if (frame_params.show_existing_frame &&
      frame_params.frame_type != KEY_FRAME) {
    // Force show-existing frames to be INTER, except forward keyframes
    frame_params.frame_type = INTER_FRAME;
  }

  // TODO(david.turner@argondesign.com): Move all the encode strategy
  // (largely near av1_get_compressed_data) in here
  // TODO(david.turner@argondesign.com): Change all the encode strategy to
  // modify frame_params instead of cm or cpi.

  // Per-frame encode speed.  In theory this can vary, but things may have been
  // written assuming speed-level will not change within a sequence, so this
  // parameter should be used with caution.
  frame_params.speed = oxcf->speed;

  // Work out some encoding parameters specific to the pass:
  if (cpi->oxcf.pass == 0 && cpi->oxcf.aq_mode == CYCLIC_REFRESH_AQ) {
    av1_cyclic_refresh_update_parameters(cpi);
  } else if (oxcf->pass == 1) {
    cpi->td.mb.e_mbd.lossless[0] = is_lossless_requested(&cpi->oxcf);
    const int kf_requested = (cm->current_frame.frame_number == 0 ||
                              (*frame_flags & FRAMEFLAGS_KEY));
    if (kf_requested && frame_update_type != OVERLAY_UPDATE &&
        frame_update_type != INTNL_OVERLAY_UPDATE) {
      frame_params.frame_type = KEY_FRAME;
    } else {
      frame_params.frame_type = INTER_FRAME;
    }
  } else if (oxcf->pass == 2) {
#if CONFIG_MISMATCH_DEBUG
    mismatch_move_frame_idx_w();
#endif
#if TXCOEFF_COST_TIMER
    cm->txcoeff_cost_timer = 0;
    cm->txcoeff_cost_count = 0;
#endif
  }

  if (oxcf->pass == 0 || oxcf->pass == 2) set_ext_overrides(cpi, &frame_params);

  // Shown keyframes and S frames refresh all reference buffers
  const int force_refresh_all =
      ((frame_params.frame_type == KEY_FRAME && frame_params.show_frame) ||
       frame_params.frame_type == S_FRAME) &&
      !frame_params.show_existing_frame;
  //若是关键帧或者s帧就刷新参考帧列表
  av1_configure_buffer_updates(cpi, &frame_params, frame_update_type,
                               force_refresh_all);

  if (oxcf->pass == 0 || oxcf->pass == 2) {
    if (!cpi->ext_refresh_frame_flags_pending)
      av1_get_ref_frames(cpi, frame_update_type, &cpi->ref_buffer_stack);

    // Work out which reference frame slots may be used.找可用参考帧
    frame_params.ref_frame_flags = get_ref_frame_flags(cpi);//获取参考帧标志值

    frame_params.primary_ref_frame =
        choose_primary_ref_frame(cpi, &frame_params);//获取关键参考帧
    frame_params.order_offset = get_order_offset(&cpi->gf_group, &frame_params);//获取参考帧偏移量

    frame_params.refresh_frame_flags = av1_get_refresh_frame_flags(//刷新参考帧标志位
        cpi, &frame_params, frame_update_type, &cpi->ref_buffer_stack);
  }

  // The way frame_params->remapped_ref_idx is setup is a placeholder.
  // Currently, reference buffer assignment is done by update_ref_frame_map()
  // which is called by high-level strategy AFTER encoding a frame.  It modifies
  // cm->remapped_ref_idx.  If you want to use an alternative method to
  // determine reference buffer assignment, just put your assignments into
  // frame_params->remapped_ref_idx here and they will be used when encoding
  // this frame.  If frame_params->remapped_ref_idx is setup independently of
  // cm->remapped_ref_idx then update_ref_frame_map() will have no effect.
  /*frame_params->remapped_ref_idx的设置方式是占位符。
  目前，参考缓冲区的分配是通过update_ref_frame_map()来完成的，在对帧进行编码之后，高级策略会调用它。
  它修改cm->remapped_ref_idx。
  如果您想使用另一种方法来确定引用缓冲区分配，只需将您的分配放入frame_params->remapped_ref_idx中，
  它们将在对其帧进行编码时使用。如果frame_params->remapped_ref_idx是独立于cm->remapped_ref_idx设置的，
  则update_ref_frame_map（）将无效。*/
  /*通过remapped_ref_idx建立参考帧buffer*/
  memcpy(frame_params.remapped_ref_idx, cm->remapped_ref_idx,
         REF_FRAMES * sizeof(*cm->remapped_ref_idx));

  cpi->td.mb.e_mbd.delta_qindex = 0;
#if ENABLE_KF_TPL
  if (oxcf->lag_in_frames > 0 && oxcf->pass != 1 &&
      frame_params.frame_type == KEY_FRAME && frame_params.show_frame) {
    av1_configure_buffer_updates(cpi, &frame_params, frame_update_type, 0);
    av1_set_frame_size(cpi, cm->width, cm->height);
    av1_tpl_setup_stats(cpi, &frame_params, &frame_input);
  }
#endif  // ENABLE_KF_TPL

  if (!frame_params.show_existing_frame) {
    cm->using_qmatrix = cpi->oxcf.using_qm;
    cm->min_qmlevel = cpi->oxcf.qm_minlevel;
    cm->max_qmlevel = cpi->oxcf.qm_maxlevel;
    if (oxcf->lag_in_frames > 0 && oxcf->pass != 1) {
      if (cpi->gf_group.index == 1 && cpi->oxcf.enable_tpl_model) {
        av1_configure_buffer_updates(cpi, &frame_params, frame_update_type, 0);
        av1_set_frame_size(cpi, cm->width, cm->height);
        av1_tpl_setup_stats(cpi, &frame_params, &frame_input);
        assert(cpi->num_gf_group_show_frames == 1);
      }
    }
  }

#if TEMPORAL_FILTER_KEY_FRAME
  //进入下一层编码
  if (denoise_and_encode(cpi, dest, &frame_input, &frame_params, &frame_results,
                         &temporal_filtered) != AOM_CODEC_OK) {
    return AOM_CODEC_ERROR;
  }
#else   // !TEMPORAL_FILTER_KEY_FRAME
  cpi->pack_bitstream = 1;
  if (av1_encode(cpi, dest, &frame_input, &frame_params, &frame_results) !=
      AOM_CODEC_OK) {
    return AOM_CODEC_ERROR;
  }
#endif  // TEMPORAL_FILTER_KEY_FRAME
  if (oxcf->pass != 1) cpi->num_gf_group_show_frames += frame_params.show_frame;

  if (oxcf->pass == 0 || oxcf->pass == 2) {
    // First pass doesn't modify reference buffer assignment or produce frame
    // flags
    update_frame_flags(cpi, frame_flags);
    int ref_map_index =
        av1_get_refresh_ref_frame_map(cm->current_frame.refresh_frame_flags);
    av1_update_ref_frame_map(cpi, frame_update_type, ref_map_index,
                             &cpi->ref_buffer_stack);
  }

#if !CONFIG_REALTIME_ONLY
  if (oxcf->pass != 1) {
#if TXCOEFF_COST_TIMER
    cm->cum_txcoeff_cost_timer += cm->txcoeff_cost_timer;
    fprintf(stderr,
            "\ntxb coeff cost block number: %ld, frame time: %ld, cum time %ld "
            "in us\n",
            cm->txcoeff_cost_count, cm->txcoeff_cost_timer,
            cm->cum_txcoeff_cost_timer);
#endif
    av1_twopass_postencode_update(cpi);
  }
#endif  // !CONFIG_REALTIME_ONLY

  if (oxcf->pass != 1) {
    update_fb_of_context_type(cpi, &frame_params, cpi->fb_of_context_type);
    set_additional_frame_flags(cm, frame_flags);
    update_rc_counts(cpi);
  }

  // Unpack frame_results:
  *size = frame_results.size;

  // Leave a signal for a higher level caller about if this frame is droppable
  if (*size > 0) {
    cpi->droppable = is_frame_droppable(cpi);
  }

  return AOM_CODEC_OK;