uvg266/src/rate_control.c

/*****************************************************************************
 * This file is part of Kvazaar HEVC encoder.
 *
 * Copyright (C) 2013-2015 Tampere University of Technology and others (see
 * COPYING file).
 *
 * Kvazaar is free software: you can redistribute it and/or modify it under
 * the terms of the GNU Lesser General Public License as published by the
 * Free Software Foundation; either version 2.1 of the License, or (at your
 * option) any later version.
 *
 * Kvazaar 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 Lesser General Public License for
 * more details.
 *
 * You should have received a copy of the GNU General Public License along
 * with Kvazaar.  If not, see <http://www.gnu.org/licenses/>.
 ****************************************************************************/

#include "rate_control.h"

#include <math.h>

static const int SMOOTHING_WINDOW = 40;

/**
 * \brief Update alpha and beta parameters.
 * \param state the main encoder state
 *
 * Sets global->rc_alpha and global->rc_beta of the encoder state.
 */
static void update_rc_parameters(encoder_state_t * state)
{
  const encoder_control_t * const encoder = state->encoder_control;

  const double pixels_per_picture = encoder->in.width * encoder->in.height;
  const double bpp = state->stats_bitstream_length * 8 / pixels_per_picture;
  const double log_bpp = log(bpp);

  const double alpha_old = state->global->rc_alpha;
  const double beta_old = state->global->rc_beta;
  // lambda computed from real bpp
  const double lambda_comp = CLIP(0.1, 10000, alpha_old * pow(bpp, beta_old));
  // lambda used in encoding
  const double lambda_real = state->global->cur_lambda_cost;
  const double lambda_log_ratio = log(lambda_real) - log(lambda_comp);

  const double alpha = alpha_old + 0.1 * lambda_log_ratio * alpha_old;
  state->global->rc_alpha = CLIP(0.05, 20, alpha);

  const double beta = beta_old + 0.05 * lambda_log_ratio * CLIP(-5, 1, log_bpp);
  state->global->rc_beta = CLIP(-3, -0.1, beta);
}

/**
 * \brief Allocate bits for the current GOP.
 * \param state the main encoder state
 *
 * If GOPs are not used, allocates bits for a single picture.
 *
 * Sets the cur_gop_target_bits of the encoder state.
 */
static void gop_allocate_bits(encoder_state_t * const state)
{
  const encoder_control_t * const encoder = state->encoder_control;

  // At this point, total_bits_coded of the current state contains the
  // number of bits written encoder->owf frames before the current frame.
  int bits_coded = state->global->total_bits_coded;
  int pictures_coded = MAX(0, state->global->frame - encoder->owf);

  int gop_offset = (state->global->gop_offset - encoder->owf) % MAX(1, encoder->cfg->gop_len);
  // Only take fully coded GOPs into account.
  if (encoder->cfg->gop_len > 0 && gop_offset != encoder->cfg->gop_len - 1) {
    // Subtract number of bits in the partially coded GOP.
    bits_coded -= state->global->cur_gop_bits_coded;
    // Subtract number of pictures in the partially coded GOP.
    pictures_coded -= gop_offset + 1;
  }

  double gop_target_bits =
    (encoder->target_avg_bppic * (pictures_coded + SMOOTHING_WINDOW) - bits_coded)
    * MAX(1, encoder->cfg->gop_len) / SMOOTHING_WINDOW;
  state->global->cur_gop_target_bits = MAX(200, gop_target_bits);
}

/**
 * Allocate bits for the current picture.
 * \param state the main encoder state
 * \return target number of bits
 */
static double pic_allocate_bits(const encoder_state_t * const state)
{
  const encoder_control_t * const encoder = state->encoder_control;

  if (encoder->cfg->gop_len <= 0) {
    return state->global->cur_gop_target_bits;
  }

  const double pic_weight = encoder->gop_layer_weights[
    encoder->cfg->gop[state->global->gop_offset].layer - 1];
  double pic_target_bits = state->global->cur_gop_target_bits * pic_weight;
  return MAX(100, pic_target_bits);
}

/**
 * \brief Select a lambda value for encoding the next picture
 * \param state the main encoder state
 * \return lambda for the next picture
 *
 * Rate control must be enabled (i.e. cfg->target_bitrate > 0) when this
 * function is called.
 */
double kvz_select_picture_lambda(encoder_state_t * const state)
{
  const encoder_control_t * const encoder = state->encoder_control;

  assert(encoder->cfg->target_bitrate > 0);

  if (state->global->frame > encoder->owf) {
    // At least one frame has been written.
    update_rc_parameters(state);
  }

  if (encoder->cfg->gop_len == 0 || state->global->gop_offset == 0) {
    // A new GOP begins at this frame.
    gop_allocate_bits(state);
  } else {
    state->global->cur_gop_target_bits =
      state->previous_encoder_state->global->cur_gop_target_bits;
  }

  // TODO: take the picture headers into account
  const double target_bits_current_picture = pic_allocate_bits(state);
  const double target_bits_per_pixel =
    target_bits_current_picture / encoder->in.pixels_per_pic;
  const double lambda =
    state->global->rc_alpha * pow(target_bits_per_pixel, state->global->rc_beta);
  return CLIP(0.1, 10000, lambda);
}

int8_t kvz_lambda_to_QP(const double lambda)
{
  const int8_t qp = 4.2005 * log(lambda) + 13.7223 + 0.5;
  return CLIP(0, 51, qp);
}

/**
 * \brief Select a lambda value according to current QP value
 * \param state the main encoder state
 * \return lambda for the next picture
 *
 * This function should be used to select lambda when rate control is
 * disabled.
 */
double kvz_select_picture_lambda_from_qp(encoder_state_t const * const state)
{
  const int gop_len = state->encoder_control->cfg->gop_len;
  const int intra_period = state->encoder_control->cfg->intra_period;
  const int keyframe_period = gop_len > 0 ? gop_len : intra_period;
  
  double lambda = pow(2.0, (state->global->QP - 12) / 3.0);

  if (state->global->slicetype == KVZ_SLICE_I) {
    lambda *= 0.57;
    
    // Reduce lambda for I-frames according to the number of references.
    if (keyframe_period == 0) {
      lambda *= 0.5;
    } else {
      lambda *= 1.0 - CLIP(0.0, 0.5, 0.05 * (keyframe_period - 1));
    }
  } else if (gop_len > 0) {
    lambda *= state->global->QP_factor;
  } else {
    lambda *= 0.4624;
  }

  // Increase lambda if not key-frame.
  if (keyframe_period > 0 && state->global->poc % keyframe_period != 0) {
    lambda *= CLIP(2.0, 4.0, (state->global->QP - 12) / 6.0);
  }
  
  return lambda;
}