uvg266/src/inter.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 "inter.h"

#include <stdlib.h>
#include <string.h>
#include <limits.h>

#include "encoder.h"
#include "imagelist.h"
#include "strategies/generic/picture-generic.h"
#include "strategies/strategies-ipol.h"
#include "videoframe.h"


typedef struct {
  const cu_info_t *a[2];
  const cu_info_t *b[3];
  const cu_info_t *c3;
  const cu_info_t *h;
} merge_candidates_t;


static void inter_recon_frac_luma(const encoder_state_t * const state,
                                  const kvz_picture * const ref,
                                  int32_t xpos,
                                  int32_t ypos,
                                  int32_t block_width,
                                  int32_t block_height,
                                  const int16_t mv_param[2],
                                  lcu_t *lcu)
{
  int mv_frac_x = (mv_param[0] & 3);
  int mv_frac_y = (mv_param[1] & 3);

 #define FILTER_SIZE_Y 8 //Luma filter size

  // Fractional luma 1/4-pel
  kvz_extended_block src = {0, 0, 0, 0};

  // Fractional luma
  kvz_get_extended_block(xpos,
                         ypos,
                         mv_param[0] >> 2,
                         mv_param[1] >> 2,
                         state->tile->offset_x,
                         state->tile->offset_y,
                         ref->y,
                         ref->width,
                         ref->height,
                         FILTER_SIZE_Y,
                         block_width,
                         block_height,
                         &src);
  kvz_sample_quarterpel_luma(state->encoder_control,
                                     src.orig_topleft,
                                     src.stride,
                                     block_width,
                                     block_height,
                                     lcu->rec.y + (ypos%LCU_WIDTH)*LCU_WIDTH + (xpos%LCU_WIDTH),
                                     LCU_WIDTH,
                                     mv_frac_x,
                                     mv_frac_y,
                                     mv_param);

  if (src.malloc_used) free(src.buffer);
}

static void inter_recon_14bit_frac_luma(const encoder_state_t * const state,
                                        const kvz_picture * const ref,
                                        int32_t xpos,
                                        int32_t ypos,
                                        int32_t block_width,
                                        int32_t block_height,
                                        const int16_t mv_param[2],
                                        hi_prec_buf_t *hi_prec_out)
{
  int mv_frac_x = (mv_param[0] & 3);
  int mv_frac_y = (mv_param[1] & 3);

#define FILTER_SIZE_Y 8 //Luma filter size

  // Fractional luma 1/4-pel
  kvz_extended_block src = { 0, 0, 0, 0 };

  // Fractional luma
  kvz_get_extended_block(xpos,
                         ypos,
                         mv_param[0] >> 2,
                         mv_param[1] >> 2,
                         state->tile->offset_x,
                         state->tile->offset_y,
                         ref->y,
                         ref->width,
                         ref->height,
                         FILTER_SIZE_Y,
                         block_width,
                         block_height,
                         &src);
  kvz_sample_14bit_quarterpel_luma(state->encoder_control,
                                           src.orig_topleft,
                                           src.stride,
                                           block_width,
                                           block_height,
                                           hi_prec_out->y + (ypos%LCU_WIDTH)*LCU_WIDTH + (xpos%LCU_WIDTH),
                                           LCU_WIDTH,
                                           mv_frac_x,
                                           mv_frac_y,
                                           mv_param);

  if (src.malloc_used) free(src.buffer);
}

static void inter_recon_frac_chroma(const encoder_state_t * const state,
                                    const kvz_picture * const ref,
                                    int32_t xpos,
                                    int32_t ypos,
                                    int32_t block_width,
                                    int32_t block_height,
                                    const int16_t mv_param[2],
                                    lcu_t *lcu)
{
  int mv_frac_x = (mv_param[0] & 7);
  int mv_frac_y = (mv_param[1] & 7);

  // Translate to chroma
  xpos >>= 1;
  ypos >>= 1;
  block_width >>= 1;
  block_height >>= 1;

#define FILTER_SIZE_C 4 //Chroma filter size

  // Fractional chroma 1/8-pel
  kvz_extended_block src_u = { 0, 0, 0, 0 };
  kvz_extended_block src_v = { 0, 0, 0, 0 };

  //Fractional chroma U
  kvz_get_extended_block(xpos, ypos,
                         (mv_param[0] >> 2) >> 1,
                         (mv_param[1] >> 2) >> 1,
                         state->tile->offset_x >> 1,
                         state->tile->offset_y >> 1,
                         ref->u,
                         ref->width >> 1,
                         ref->height >> 1,
                         FILTER_SIZE_C,
                         block_width,
                         block_height,
                         &src_u);
  kvz_sample_octpel_chroma(state->encoder_control, src_u.orig_topleft, src_u.stride, block_width,
    block_height, lcu->rec.u + (ypos % LCU_WIDTH_C)*LCU_WIDTH_C + (xpos % LCU_WIDTH_C), LCU_WIDTH_C, mv_frac_x, mv_frac_y, mv_param);

  //Fractional chroma V
  kvz_get_extended_block(xpos, ypos,
                         (mv_param[0] >> 2) >> 1,
                         (mv_param[1] >> 2) >> 1,
                         state->tile->offset_x >> 1,
                         state->tile->offset_y >> 1,
                         ref->v,
                         ref->width >> 1,
                         ref->height >> 1,
                         FILTER_SIZE_C,
                         block_width,
                         block_height,
                         &src_v);
  kvz_sample_octpel_chroma(state->encoder_control, src_v.orig_topleft, src_v.stride, block_width,
    block_height, lcu->rec.v + (ypos  % LCU_WIDTH_C)*LCU_WIDTH_C + (xpos % LCU_WIDTH_C), LCU_WIDTH_C, mv_frac_x, mv_frac_y, mv_param);

  if (src_u.malloc_used) free(src_u.buffer);
  if (src_v.malloc_used) free(src_v.buffer);
}

static void inter_recon_14bit_frac_chroma(const encoder_state_t * const state,
                                          const kvz_picture * const ref,
                                          int32_t xpos,
                                          int32_t ypos,
                                          int32_t block_width,
                                          int32_t block_height,
                                          const int16_t mv_param[2],
                                          hi_prec_buf_t *hi_prec_out)
{
  int mv_frac_x = (mv_param[0] & 7);
  int mv_frac_y = (mv_param[1] & 7);

  // Translate to chroma
  xpos >>= 1;
  ypos >>= 1;
  block_width >>= 1;
  block_height >>= 1;

#define FILTER_SIZE_C 4 //Chroma filter size

  // Fractional chroma 1/8-pel
  kvz_extended_block src_u = { 0, 0, 0, 0 };
  kvz_extended_block src_v = { 0, 0, 0, 0 };

  //Fractional chroma U
  kvz_get_extended_block(xpos,
                         ypos,
                         (mv_param[0] >> 2) >> 1,
                         (mv_param[1] >> 2) >> 1,
                         state->tile->offset_x >> 1,
                         state->tile->offset_y >> 1,
                         ref->u,
                         ref->width >> 1,
                         ref->height >> 1,
                         FILTER_SIZE_C,
                         block_width,
                         block_height,
                         &src_u);
  kvz_sample_14bit_octpel_chroma(state->encoder_control,
                                         src_u.orig_topleft,
                                         src_u.stride,
                                         block_width,
                                         block_height,
                                         hi_prec_out->u + (ypos % LCU_WIDTH_C)*LCU_WIDTH_C + (xpos % LCU_WIDTH_C),
                                         LCU_WIDTH_C,
                                         mv_frac_x,
                                         mv_frac_y,
                                         mv_param);

  //Fractional chroma V
  kvz_get_extended_block(xpos,
                         ypos,
                         (mv_param[0] >> 2) >> 1,
                         (mv_param[1] >> 2) >> 1,
                         state->tile->offset_x >> 1,
                         state->tile->offset_y >> 1,
                         ref->v,
                         ref->width >> 1,
                         ref->height >> 1,
                         FILTER_SIZE_C,
                         block_width,
                         block_height,
                         &src_v);
  kvz_sample_14bit_octpel_chroma(state->encoder_control,
                                         src_v.orig_topleft,
                                         src_v.stride,
                                         block_width,
                                         block_height,
                                         hi_prec_out->v + (ypos  % LCU_WIDTH_C)*LCU_WIDTH_C + (xpos % LCU_WIDTH_C),
                                         LCU_WIDTH_C,
                                         mv_frac_x,
                                         mv_frac_y,
                                         mv_param);

  if (src_u.malloc_used) free(src_u.buffer);
  if (src_v.malloc_used) free(src_v.buffer);
}


/**
* \brief Copy from frame with extended border.
*
* \param ref_buf      pointer to the start of ref buffer
* \param ref_stride   stride of ref buffer
* \param ref_width    width of frame
* \param ref_height   height of frame
* \param rec_buf      pointer to the start of pu in rec buffer
* \param rec_stride   stride of rec buffer
* \param width        width of copied block
* \param height       height of copied block
* \param mv_in_frame  coordinates of copied block in frame coordinates
*/
static void inter_cp_with_ext_border(const kvz_pixel *ref_buf, int ref_stride,
                                     int ref_width, int ref_height,
                                     kvz_pixel *rec_buf, int rec_stride,
                                     int width, int height,
                                     const vector2d_t *mv_in_frame)
{
  for (int y = mv_in_frame->y; y < mv_in_frame->y + height; ++y) {
    for (int x = mv_in_frame->x; x < mv_in_frame->x + width; ++x) {
      vector2d_t in_frame = {
        CLIP(0, ref_width - 1, x),
        CLIP(0, ref_height - 1, y),
      };
      vector2d_t in_pu = {
        x - mv_in_frame->x,
        y - mv_in_frame->y,
      };
      int pu_index = in_pu.y * rec_stride + in_pu.x;
      int frame_index = in_frame.y * ref_stride + in_frame.x;
      rec_buf[pu_index] = ref_buf[frame_index];
    }
  }
}


/**
 * \brief Reconstruct inter block
 *
 * \param state         encoder state
 * \param ref           picture to copy the data from
 * \param xpos          block x position
 * \param ypos          block y position
 * \param width         block width
 * \param height        block height
 * \param mv_param      motion vector
 * \param lcu           destination lcu
 * \param hi_prec_out   destination of high precision output (null if not needed)
*/
void kvz_inter_recon_lcu(const encoder_state_t * const state,
                         const kvz_picture * const ref,
                         int32_t xpos,
                         int32_t ypos,
                         int32_t width,
                         int32_t height,
                         const int16_t mv_param[2],
                         lcu_t *lcu,
                         hi_prec_buf_t *hi_prec_out)
{
  const vector2d_t pu_in_tile = { xpos, ypos };
  const vector2d_t pu_in_lcu = { xpos % LCU_WIDTH, ypos % LCU_WIDTH };

  const vector2d_t mv_in_pu = { mv_param[0] >> 2, mv_param[1] >> 2 };
  const vector2d_t mv_in_frame = {
    mv_in_pu.x + pu_in_tile.x + state->tile->offset_x,
    mv_in_pu.y + pu_in_tile.y + state->tile->offset_y
  };

  const bool mv_is_outside_frame = mv_in_frame.x < 0 ||
      mv_in_frame.y < 0 ||
      mv_in_frame.x + width > ref->width ||
      mv_in_frame.y + height > ref->height;

  // With 420, odd coordinates need interpolation.
  const int8_t fractional_chroma = (mv_in_pu.x & 1) || (mv_in_pu.y & 1);
  const int8_t fractional_luma = ((mv_param[0] & 3) || (mv_param[1] & 3));

  // Generate prediction for luma.
  if (fractional_luma) {
    // With a fractional MV, do interpolation.
    if (state->encoder_control->cfg.bipred && hi_prec_out) {
      inter_recon_14bit_frac_luma(state, ref,
                                  pu_in_tile.x, pu_in_tile.y,
                                  width, height,
                                  mv_param, hi_prec_out);
    } else {
      inter_recon_frac_luma(state, ref,
                            pu_in_tile.x, pu_in_tile.y,
                            width, height,
                            mv_param, lcu);
    }
  } else {
    // With an integer MV, copy pixels directly from the reference.
    const int lcu_pu_index = pu_in_lcu.y * LCU_WIDTH + pu_in_lcu.x;
    if (mv_is_outside_frame) {
      inter_cp_with_ext_border(ref->y, ref->width,
                               ref->width, ref->height,
                               &lcu->rec.y[lcu_pu_index], LCU_WIDTH,
                               width, height,
                               &mv_in_frame);
    } else {
      const int frame_mv_index = mv_in_frame.y * ref->width + mv_in_frame.x;
      kvz_pixels_blit(&ref->y[frame_mv_index],
                      &lcu->rec.y[lcu_pu_index],
                      width, height,
                      ref->width, LCU_WIDTH);
    }
  }

  if (state->encoder_control->chroma_format == KVZ_CSP_400) {
    return;
  }

  // Generate prediction for chroma.
  if (fractional_luma || fractional_chroma) {
    // With a fractional MV, do interpolation.
    if (state->encoder_control->cfg.bipred && hi_prec_out) {
      inter_recon_14bit_frac_chroma(state, ref,
                                    pu_in_tile.x, pu_in_tile.y,
                                    width, height,
                                    mv_param, hi_prec_out);
    } else {
      inter_recon_frac_chroma(state, ref,
                              pu_in_tile.x, pu_in_tile.y,
                              width, height,
                              mv_param, lcu);
    }
  } else {
    // With an integer MV, copy pixels directly from the reference.
    const int lcu_pu_index_c = pu_in_lcu.y / 2 * LCU_WIDTH_C + pu_in_lcu.x / 2;
    const vector2d_t mv_in_frame_c = { mv_in_frame.x / 2, mv_in_frame.y / 2 };

    if (mv_is_outside_frame) {
      inter_cp_with_ext_border(ref->u, ref->width / 2,
                               ref->width / 2, ref->height / 2,
                               &lcu->rec.u[lcu_pu_index_c], LCU_WIDTH_C,
                               width / 2, height / 2,
                               &mv_in_frame_c);
      inter_cp_with_ext_border(ref->v, ref->width / 2,
                               ref->width / 2, ref->height / 2,
                               &lcu->rec.v[lcu_pu_index_c], LCU_WIDTH_C,
                               width / 2, height / 2,
                               &mv_in_frame_c);
    } else {
      const int frame_mv_index = mv_in_frame_c.y * ref->width / 2 + mv_in_frame_c.x;

      kvz_pixels_blit(&ref->u[frame_mv_index],
                      &lcu->rec.u[lcu_pu_index_c],
                      width / 2, height / 2,
                      ref->width / 2, LCU_WIDTH_C);
      kvz_pixels_blit(&ref->v[frame_mv_index],
                      &lcu->rec.v[lcu_pu_index_c],
                      width / 2, height / 2,
                      ref->width / 2, LCU_WIDTH_C);
    }
  }
}

/**
 * \brief Reconstruct bi-pred inter block
 *
 * \param state     encoder state
 * \param ref1      reference picture to copy the data from
 * \param ref2      other reference picture to copy the data from
 * \param xpos      block x position
 * \param ypos      block y position
 * \param width     block width
 * \param height    block height
 * \param mv_param  motion vectors
 * \param lcu       destination lcu
 */
void kvz_inter_recon_lcu_bipred(const encoder_state_t * const state,
                                const kvz_picture * ref1,
                                const kvz_picture * ref2,
                                int32_t xpos,
                                int32_t ypos,
                                int32_t width,
                                int32_t height,
                                int16_t mv_param[2][2],
                                lcu_t* lcu)
{
  kvz_pixel temp_lcu_y[LCU_WIDTH*LCU_WIDTH];
  kvz_pixel temp_lcu_u[LCU_WIDTH_C*LCU_WIDTH_C];
  kvz_pixel temp_lcu_v[LCU_WIDTH_C*LCU_WIDTH_C];
  int temp_x, temp_y;
  int shift = 15 - KVZ_BIT_DEPTH;
  int offset = 1 << (shift - 1);

  const int hi_prec_luma_rec0 = mv_param[0][0] & 3 || mv_param[0][1] & 3;
  const int hi_prec_luma_rec1 = mv_param[1][0] & 3 || mv_param[1][1] & 3;

  const int hi_prec_chroma_rec0 = mv_param[0][0] & 7 || mv_param[0][1] & 7;
  const int hi_prec_chroma_rec1 = mv_param[1][0] & 7 || mv_param[1][1] & 7;

  hi_prec_buf_t* high_precision_rec0 = 0;
  hi_prec_buf_t* high_precision_rec1 = 0;
  if (hi_prec_chroma_rec0) high_precision_rec0 = kvz_hi_prec_buf_t_alloc(LCU_WIDTH*LCU_WIDTH);
  if (hi_prec_chroma_rec1) high_precision_rec1 = kvz_hi_prec_buf_t_alloc(LCU_WIDTH*LCU_WIDTH);
  //Reconstruct both predictors
  kvz_inter_recon_lcu(state, ref1, xpos, ypos, width, height, mv_param[0], lcu, high_precision_rec0);
  if (!hi_prec_luma_rec0){
    memcpy(temp_lcu_y, lcu->rec.y, sizeof(kvz_pixel) * 64 * 64);
  }
  if (!hi_prec_chroma_rec0){
    memcpy(temp_lcu_u, lcu->rec.u, sizeof(kvz_pixel) * 32 * 32);
    memcpy(temp_lcu_v, lcu->rec.v, sizeof(kvz_pixel) * 32 * 32);
  }
  kvz_inter_recon_lcu(state, ref2, xpos, ypos, width, height, mv_param[1], lcu, high_precision_rec1);

  // After reconstruction, merge the predictors by taking an average of each pixel
  for (temp_y = 0; temp_y < height; ++temp_y) {
    int y_in_lcu = ((ypos + temp_y) & ((LCU_WIDTH)-1));
    for (temp_x = 0; temp_x < width; ++temp_x) {
      int x_in_lcu = ((xpos + temp_x) & ((LCU_WIDTH)-1));
      int16_t sample0_y = (hi_prec_luma_rec0 ? high_precision_rec0->y[y_in_lcu * LCU_WIDTH + x_in_lcu] : (temp_lcu_y[y_in_lcu * LCU_WIDTH + x_in_lcu] << (14 - KVZ_BIT_DEPTH)));
      int16_t sample1_y = (hi_prec_luma_rec1 ? high_precision_rec1->y[y_in_lcu * LCU_WIDTH + x_in_lcu] : (lcu->rec.y[y_in_lcu * LCU_WIDTH + x_in_lcu] << (14 - KVZ_BIT_DEPTH)));
      lcu->rec.y[y_in_lcu * LCU_WIDTH + x_in_lcu] = (kvz_pixel)kvz_fast_clip_32bit_to_pixel((sample0_y + sample1_y + offset) >> shift);
    }

  }
  for (temp_y = 0; temp_y < height >> 1; ++temp_y) {
    int y_in_lcu = (((ypos >> 1) + temp_y) & (LCU_WIDTH_C - 1));
    for (temp_x = 0; temp_x < width >> 1; ++temp_x) {
      int x_in_lcu = (((xpos >> 1) + temp_x) & (LCU_WIDTH_C - 1));
      int16_t sample0_u = (hi_prec_chroma_rec0 ? high_precision_rec0->u[y_in_lcu * LCU_WIDTH_C + x_in_lcu] : (temp_lcu_u[y_in_lcu * LCU_WIDTH_C + x_in_lcu] << (14 - KVZ_BIT_DEPTH)));
      int16_t sample1_u = (hi_prec_chroma_rec1 ? high_precision_rec1->u[y_in_lcu * LCU_WIDTH_C + x_in_lcu] : (lcu->rec.u[y_in_lcu * LCU_WIDTH_C + x_in_lcu] << (14 - KVZ_BIT_DEPTH)));
      lcu->rec.u[y_in_lcu * LCU_WIDTH_C + x_in_lcu] = (kvz_pixel)kvz_fast_clip_32bit_to_pixel((sample0_u + sample1_u + offset) >> shift);

      int16_t sample0_v = (hi_prec_chroma_rec0 ? high_precision_rec0->v[y_in_lcu * LCU_WIDTH_C + x_in_lcu] : (temp_lcu_v[y_in_lcu * LCU_WIDTH_C + x_in_lcu] << (14 - KVZ_BIT_DEPTH)));
      int16_t sample1_v = (hi_prec_chroma_rec1 ? high_precision_rec1->v[y_in_lcu * LCU_WIDTH_C + x_in_lcu] : (lcu->rec.v[y_in_lcu * LCU_WIDTH_C + x_in_lcu] << (14 - KVZ_BIT_DEPTH)));
      lcu->rec.v[y_in_lcu * LCU_WIDTH_C + x_in_lcu] = (kvz_pixel)kvz_fast_clip_32bit_to_pixel((sample0_v + sample1_v + offset) >> shift);
    }
  }
  if (high_precision_rec0 != 0) kvz_hi_prec_buf_t_free(high_precision_rec0);
  if (high_precision_rec1 != 0) kvz_hi_prec_buf_t_free(high_precision_rec1);
}

/**
 * \brief Clear unused L0/L1 motion vectors and reference
 * \param cu coding unit to clear
 */
static void inter_clear_cu_unused(cu_info_t* cu)
{
  for (unsigned i = 0; i < 2; ++i) {
    if (cu->inter.mv_dir & (1 << i)) continue;

    cu->inter.mv[i][0] = 0;
    cu->inter.mv[i][1] = 0;
    cu->inter.mv_ref[i] = 255;
  }
}

/**
 * \brief Check whether a0 mv cand block is coded before the current block.
 * \param x       x-coordinate of the current block (in pixels)
 * \param y       y-coordinate of the current block (in pixels)
 * \param width   width of the current block (in pixels)
 * \param height  height of the current block (in pixels)
 * \return        True, if the a0 mv candidate block is coded before the
 *                current block. Otherwise false.
 */
static bool is_a0_cand_coded(int x, int y, int width, int height)
{
  int size = MIN(width & ~(width - 1), height & ~(height - 1));

  if (height != size) {
    // For SMP and AMP blocks the situation is equivalent to a square block
    // at the lower left corner of the PU.
    y = y + height - size;
  }

  while (size < LCU_WIDTH) {
    const int parent_size = 2 * size;
    const int cu_index    = (x % parent_size != 0) + 2 * (y % parent_size != 0);
    switch (cu_index) {
      case 0:
        // A0 is in the CU directly left of the parent CU so it has been
        // coded already.
        //    +---+---+
        //    | X |   |
        //    |---+---+
        // A0 |   |   |
        //    +---+---+
        return true;

      case 1:
        // A0 is in the CU that will be coded after the current CU.
        //    +---+---+
        //    |   | X |
        //    |---+---+
        //    |A0 |   |
        //    +---+---+
        return false;

      case 2:
        //    +---+---+
        //    |   |   |
        //    |---+---+
        //    | X |   |
        //    +---+---+
        // A0

        // Move to the parent block.
        y -= size;
        size = parent_size;
        break;

      case 3:
        // A0 is in the CU directly down of the parent CU so is has not
        // been coded yet.
        //    +---+---+
        //    |   |   |
        //    |---+---+
        //    |   | X |
        //    +---+---+
        //     A0
        return false;
    }
  }

  // For 64x64 blocks A0 candidate is located outside the LCU.
  return false;
}

/**
 * \brief Check whether b0 mv cand block is coded before the current block.
 * \param x       x-coordinate of the current block (in pixels)
 * \param y       y-coordinate of the current block (in pixels)
 * \param width   width of the current block (in pixels)
 * \param height  height of the current block (in pixels)
 * \return        True, if the b0 mv candidate block is coded before the
 *                current block. Otherwise false.
 */
static bool is_b0_cand_coded(int x, int y, int width, int height)
{
  int size = MIN(width & ~(width - 1), height & ~(height - 1));

  if (width != size) {
    // For SMP and AMP blocks the situation is equivalent to a square block
    // at the upper right corner of the PU.
    x = x + width - size;
  }

  while (size < LCU_WIDTH) {
    const int parent_size = 2 * size;
    const int cu_index    = (x % parent_size != 0) + 2 * (y % parent_size != 0);
    switch (cu_index) {
      case 0:
        // B0 is in the CU directly above the parent CU so it has been
        // coded already.
        //         B0
        //    +---+---+
        //    | X |   |
        //    |---+---+
        //    |   |   |
        //    +---+---+
        return true;

      case 1:
        //             B0
        //    +---+---+
        //    |   | X |
        //    |---+---+
        //    |   |   |
        //    +---+---+

        // Move to the parent block.
        x -= size;
        size = parent_size;
        break;

      case 2:
        //    +---+---+
        //    |   |B0 |
        //    |---+---+
        //    | X |   |
        //    +---+---+
        return true;

      case 3:
        // B0 is in the CU directly right of the parent CU so is has not
        // been coded yet.
        //    +---+---+
        //    |   |   | B0
        //    |---+---+
        //    |   | X |
        //    +---+---+
        return false;
    }
  }

  // The LCU to the right and up of the current LCU has been coded already.
  return true;
}


/**
 * \brief Get merge candidates for current block
 *
 * \param state     encoder control state to use
 * \param x         block x position in SCU
 * \param y         block y position in SCU
 * \param width     current block width
 * \param height    current block height
 * \param ref_list  which reference list, L0 is 1 and L1 is 2
 * \param ref_idx   index in the reference list
 * \param cand_out  will be filled with C3 and H candidates
 */
static void get_temporal_merge_candidates(const encoder_state_t * const state,
                                          int32_t x,
                                          int32_t y,
                                          int32_t width,
                                          int32_t height,
                                          uint8_t ref_list,
                                          uint8_t ref_idx,
                                          merge_candidates_t *cand_out)
{
  /*
  Predictor block locations
  _________
  |CurrentPU|
  | |C0|__  |
  |    |C3| |
  |_________|_
            |H|
  */

  cand_out->c3 = cand_out->h = NULL;

  // Find temporal reference
  if (state->frame->ref->used_size) {
    uint32_t colocated_ref;

    // Select L0/L1 ref_idx reference
    if (state->frame->ref_LX_size[ref_list-1] > ref_idx) {
      colocated_ref = state->frame->ref_LX[ref_list - 1][ref_idx];
    } else {
      // not found
      return;
    }

    cu_array_t *ref_cu_array = state->frame->ref->cu_arrays[colocated_ref];
    int cu_per_width = ref_cu_array->width / SCU_WIDTH;

    uint32_t xColBr = x + width;
    uint32_t yColBr = y + height;

    // H must be available
    if (xColBr < state->encoder_control->in.width &&
        yColBr < state->encoder_control->in.height) {
      int32_t H_offset = -1;

      // Y inside the current CTU / LCU
      if (yColBr % LCU_WIDTH != 0) {
        H_offset = ((xColBr >> 4) << 4) / SCU_WIDTH +
                  (((yColBr >> 4) << 4) / SCU_WIDTH) * cu_per_width;
      }

      if (H_offset >= 0) {
        // Only use when it's inter block
        if (ref_cu_array->data[H_offset].type == CU_INTER) {
          cand_out->h = &ref_cu_array->data[H_offset];
        }
      }
    }
    uint32_t xColCtr = x + (width / 2);
    uint32_t yColCtr = y + (height / 2);

    // C3 must be inside the LCU, in the center position of current CU
    if (xColCtr < state->encoder_control->in.width && yColCtr < state->encoder_control->in.height) {
      uint32_t C3_offset = ((xColCtr >> 4) << 4) / SCU_WIDTH + ((((yColCtr >> 4) << 4) / SCU_WIDTH) * cu_per_width);
      if (ref_cu_array->data[C3_offset].type == CU_INTER) {
        cand_out->c3 = &ref_cu_array->data[C3_offset];
      }
    }
  }
}

/**
 * \brief Get merge candidates for current block.
 *
 * The output parameters b0, b1, b2, a0, a1 are pointed to the
 * corresponding cu_info_t struct in lcu->cu, or set to NULL, if the
 * candidate is not available.
 *
 * \param x               block x position in pixels
 * \param y               block y position in pixels
 * \param width           block width in pixels
 * \param height          block height in pixels
 * \param picture_width   tile width in pixels
 * \param picture_height  tile height in pixels
 * \param lcu             current LCU
 * \param cand_out        will be filled with A and B candidates
 */
static void get_spatial_merge_candidates(int32_t x,
                                         int32_t y,
                                         int32_t width,
                                         int32_t height,
                                         int32_t picture_width,
                                         int32_t picture_height,
                                         lcu_t *lcu,
                                         merge_candidates_t *cand_out)
{
  /*
  Predictor block locations
  ____      _______
  |B2|______|B1|B0|
     |         |
     |  Cur CU |
   __|         |
  |A1|_________|
  |A0|
  */
  int32_t x_local = SUB_SCU(x); //!< coordinates from top-left of this LCU
  int32_t y_local = SUB_SCU(y);
  // A0 and A1 availability testing
  if (x != 0) {
    cu_info_t *a1 = LCU_GET_CU_AT_PX(lcu, x_local - 1, y_local + height - 1);
    // Do not check a1->coded because the block above is always coded before
    // the current one and the flag is not set when searching an SMP block.
    if (a1->type == CU_INTER) {
      inter_clear_cu_unused(a1);
      cand_out->a[1] = a1;
    }

    if (y_local + height < LCU_WIDTH && y + height < picture_height) {
      cu_info_t *a0 = LCU_GET_CU_AT_PX(lcu, x_local - 1, y_local + height);
      if (a0->type == CU_INTER && is_a0_cand_coded(x, y, width, height)) {
        inter_clear_cu_unused(a0);
        cand_out->a[0] = a0;
      }
    }
  }

  // B0, B1 and B2 availability testing
  if (y != 0) {
    cu_info_t *b0 = NULL;
    if (x + width < picture_width) {
      if (x_local + width < LCU_WIDTH) {
        b0 = LCU_GET_CU_AT_PX(lcu, x_local + width, y_local - 1);
      } else if (y_local == 0) {
        // Special case, top-right CU
        b0 = LCU_GET_TOP_RIGHT_CU(lcu);
      }
    }
    if (b0 && b0->type == CU_INTER && is_b0_cand_coded(x, y, width, height)) {
      inter_clear_cu_unused(b0);
      cand_out->b[0] = b0;
    }

    cu_info_t *b1 = LCU_GET_CU_AT_PX(lcu, x_local + width - 1, y_local - 1);
    // Do not check b1->coded because the block to the left is always coded
    // before the current one and the flag is not set when searching an SMP
    // block.
    if (b1->type == CU_INTER) {
      inter_clear_cu_unused(b1);
      cand_out->b[1] = b1;
    }

    if (x != 0) {
      cu_info_t *b2 = LCU_GET_CU_AT_PX(lcu, x_local - 1, y_local - 1);
      // Do not check b2->coded because the block above and to the left is
      // always coded before the current one.
      if (b2->type == CU_INTER) {
        inter_clear_cu_unused(b2);
        cand_out->b[2] = b2;
      }
    }
  }
}

/**
 * \brief Get merge candidates for current block.
 *
 * The output parameters b0, b1, b2, a0, a1 are pointed to the
 * corresponding cu_info_t struct in lcu->cu, or set to NULL, if the
 * candidate is not available.
 *
 * \param cua             cu information
 * \param x               block x position in pixels
 * \param y               block y position in pixels
 * \param width           block width in pixels
 * \param height          block height in pixels
 * \param picture_width   tile width in pixels
 * \param picture_height  tile height in pixels
 * \param cand_out        will be filled with A and B candidates
 */
static void get_spatial_merge_candidates_cua(const cu_array_t *cua,
                                             int32_t x,
                                             int32_t y,
                                             int32_t width,
                                             int32_t height,
                                             int32_t picture_width,
                                             int32_t picture_height,
                                             merge_candidates_t *cand_out)
{
  /*
  Predictor block locations
  ____      _______
  |B2|______|B1|B0|
     |         |
     |  Cur CU |
   __|         |
  |A1|_________|
  |A0|
  */
  int32_t x_local = SUB_SCU(x); //!< coordinates from top-left of this LCU
  int32_t y_local = SUB_SCU(y);
  // A0 and A1 availability testing
  if (x != 0) {
    const cu_info_t *a1 = kvz_cu_array_at_const(cua, x - 1, y + height - 1);
    // The block above is always coded before the current one.
    if (a1->type == CU_INTER) {
      cand_out->a[1] = a1;
    }

    if (y_local + height < LCU_WIDTH && y + height < picture_height) {
      const cu_info_t *a0 = kvz_cu_array_at_const(cua, x - 1, y + height);
      if (a0->type == CU_INTER && is_a0_cand_coded(x, y, width, height)) {
        cand_out->a[0] = a0;
      }
    }
  }

  // B0, B1 and B2 availability testing
  if (y != 0) {
    if (x + width < picture_width && (x_local + width < LCU_WIDTH || y_local == 0)) {
      const cu_info_t *b0 = kvz_cu_array_at_const(cua, x + width, y - 1);
      if (b0->type == CU_INTER && is_b0_cand_coded(x, y, width, height)) {
        cand_out->b[0] = b0;
      }
    }

    const cu_info_t *b1 = kvz_cu_array_at_const(cua, x + width - 1, y - 1);
    // The block to the left is always coded before the current one.
    if (b1->type == CU_INTER) {
      cand_out->b[1] = b1;
    }

    if (x != 0) {
      const cu_info_t *b2 = kvz_cu_array_at_const(cua, x - 1, y - 1);
      // The block above and to the left is always coded before the current
      // one.
      if (b2->type == CU_INTER) {
        cand_out->b[2] = b2;
      }
    }
  }
}

static INLINE int16_t get_scaled_mv(int16_t mv, int scale)
{
  int32_t scaled = scale * mv;
  return CLIP(-32768, 32767, (scaled + 127 + (scaled < 0)) >> 8);
}

static void apply_mv_scaling_pocs(int32_t current_poc,
                                  int32_t current_ref_poc,
                                  int32_t neighbor_poc,
                                  int32_t neighbor_ref_poc,
                                  int16_t mv_cand[2])
{
  int32_t diff_current  = current_poc  - current_ref_poc;
  int32_t diff_neighbor = neighbor_poc - neighbor_ref_poc;

  if (diff_current == diff_neighbor) return;

  diff_current  = CLIP(-128, 127, diff_current);
  diff_neighbor = CLIP(-128, 127, diff_neighbor);

  int scale = CLIP(-4096, 4095,
    (diff_current * ((0x4000 + (abs(diff_neighbor) >> 1)) / diff_neighbor) + 32) >> 6);

  mv_cand[0] = get_scaled_mv(mv_cand[0], scale);
  mv_cand[1] = get_scaled_mv(mv_cand[1], scale);
}

static INLINE void apply_mv_scaling(const encoder_state_t *state,
                                    const cu_info_t *current_cu,
                                    const cu_info_t *neighbor_cu,
                                    int8_t current_reflist,
                                    int8_t neighbor_reflist,
                                    int16_t mv_cand[2])
{
  apply_mv_scaling_pocs(state->frame->poc,
                        state->frame->ref->pocs[
                          state->frame->ref_LX[current_reflist][
                            current_cu->inter.mv_ref[current_reflist]]],
                        state->frame->poc,
                        state->frame->ref->pocs[
                          state->frame->ref_LX[neighbor_reflist][
                            neighbor_cu->inter.mv_ref[neighbor_reflist]]],
                        mv_cand);
}

/**
 * \brief Try to add a temporal MVP or merge candidate.
 *
 * \param state         encoder state
 * \param current_ref   index of the picture referenced by the current CU
 * \param colocated     colocated CU
 * \param reflist       either 0 (for L0) or 1 (for L1)
 * \param[out] mv_out   Returns the motion vector
 *
 * \return Whether a temporal candidate was added or not.
 */
static bool add_temporal_candidate(const encoder_state_t *state,
                                   uint8_t current_ref,
                                   const cu_info_t *colocated,
                                   int32_t reflist,
                                   int16_t mv_out[2])
{
  if (!colocated) return false;

  int colocated_ref;
  if (state->frame->ref_LX_size[0] > 0) {
    // get the first reference from L0 if it exists
    colocated_ref = state->frame->ref_LX[0][0];
  } else {
    // otherwise no candidate added
    return false;
  }

  int cand_list = colocated->inter.mv_dir & (1 << reflist) ? reflist : !reflist;

  mv_out[0] = colocated->inter.mv[cand_list][0];
  mv_out[1] = colocated->inter.mv[cand_list][1];
  apply_mv_scaling_pocs(
    state->frame->poc,
    state->frame->ref->pocs[current_ref],
    state->frame->ref->pocs[colocated_ref],
    state->frame->ref->images[colocated_ref]->ref_pocs[
      state->frame->ref->ref_LXs[colocated_ref]
        [cand_list][colocated->inter.mv_ref[cand_list]]],
    mv_out
  );

  return true;
}

static INLINE bool add_mvp_candidate(const encoder_state_t *state,
                                     const cu_info_t *cur_cu,
                                     const cu_info_t *cand,
                                     int8_t reflist,
                                     bool scaling,
                                     int16_t mv_cand_out[2])
{
  if (!cand) return false;

  assert(cand->inter.mv_dir != 0);
  const int cand_list = cand->inter.mv_dir & (1 << reflist) ? reflist : !reflist;

  if (scaling) {
    mv_cand_out[0] = cand->inter.mv[cand_list][0];
    mv_cand_out[1] = cand->inter.mv[cand_list][1];
    apply_mv_scaling(state, cur_cu, cand, reflist, cand_list, mv_cand_out);
    return true;
  }

  if (cand->inter.mv_dir & (1 << cand_list) &&
      state->frame->ref_LX[cand_list][cand->inter.mv_ref[cand_list]] == 
      state->frame->ref_LX[reflist][cur_cu->inter.mv_ref[reflist]])
  {
    mv_cand_out[0] = cand->inter.mv[cand_list][0];
    mv_cand_out[1] = cand->inter.mv[cand_list][1];
    return true;
  }

  return false;
}

/**
 * \brief Pick two mv candidates from the spatial and temporal candidates.
 */
static void get_mv_cand_from_candidates(const encoder_state_t * const state,
                                        int32_t x,
                                        int32_t y,
                                        int32_t width,
                                        int32_t height,
                                        const merge_candidates_t *merge_cand,
                                        const cu_info_t *cur_cu,
                                        int8_t reflist,
                                        int16_t mv_cand[2][2])
{
  const cu_info_t *const *a = merge_cand->a;
  const cu_info_t *const *b = merge_cand->b;
  const cu_info_t *c3 = merge_cand->c3;
  const cu_info_t *h  = merge_cand->h;

  uint8_t candidates = 0;
  uint8_t b_candidates = 0;

  // Left predictors without scaling
  for (int i = 0; i < 2; i++) {
    if (add_mvp_candidate(state, cur_cu, a[i], reflist, false, mv_cand[candidates])) {
      candidates++;
      break;
    }
  }

  // Left predictors with scaling
  if (candidates == 0) {
    for (int i = 0; i < 2; i++) {
      if (add_mvp_candidate(state, cur_cu, a[i], reflist, true, mv_cand[candidates])) {
        candidates++;
        break;
      }
    }
  }

  // Top predictors without scaling
  for (int i = 0; i < 3; i++) {
    if (add_mvp_candidate(state, cur_cu, b[i], reflist, false, mv_cand[candidates])) {
      b_candidates++;
      break;
    }
  }

  candidates += b_candidates;

  // When a1 or a0 is available, we dont check for secondary B candidates.
  if (a[0] || a[1]) {
    b_candidates = 1;
  } else if (candidates != 2) {
    b_candidates = 0;
  }

  if (!b_candidates) {
    // Top predictors with scaling
    for (int i = 0; i < 3; i++) {
      if (add_mvp_candidate(state, cur_cu, b[i], reflist, true, mv_cand[candidates])) {
        candidates++;
        break;
      }
    }
  }

  // Remove identical candidate
  if (candidates == 2 && mv_cand[0][0] == mv_cand[1][0] && mv_cand[0][1] == mv_cand[1][1]) {
    candidates = 1;
  }

  // Use Temporal Motion Vector Prediction when enabled.
  // TMVP required at least two sequential P/B-frames.
  bool can_use_tmvp =
    state->encoder_control->cfg.tmvp_enable &&
    state->frame->poc > 1 &&
    state->frame->ref->used_size &&
    candidates < AMVP_MAX_NUM_CANDS &&
    (h != NULL || c3 != NULL);

  if (can_use_tmvp && add_temporal_candidate(state,
                                             state->frame->ref_LX[reflist][cur_cu->inter.mv_ref[reflist]],
                                             (h != NULL) ? h : c3,
                                             reflist,
                                             mv_cand[candidates])) {
    candidates++;
  }

  // Fill with (0,0)
  while (candidates < AMVP_MAX_NUM_CANDS) {
    mv_cand[candidates][0] = 0;
    mv_cand[candidates][1] = 0;
    candidates++;
  }
}

/**
 * \brief Get MV prediction for current block.
 *
 * \param state     encoder state
 * \param x         block x position in pixels
 * \param y         block y position in pixels
 * \param width     block width in pixels
 * \param height    block height in pixels
 * \param mv_cand   Return the motion vector candidates.
 * \param cur_cu    current CU
 * \param lcu       current LCU
 * \param reflist   reflist index (either 0 or 1)
 */
void kvz_inter_get_mv_cand(const encoder_state_t * const state,
                           int32_t x,
                           int32_t y,
                           int32_t width,
                           int32_t height,
                           int16_t mv_cand[2][2],
                           cu_info_t* cur_cu,
                           lcu_t *lcu,
                           int8_t reflist)
{
  merge_candidates_t merge_cand = { {0, 0}, {0, 0, 0}, 0, 0 };

  get_spatial_merge_candidates(x, y, width, height,
                               state->tile->frame->width,
                               state->tile->frame->height,
                               lcu,
                               &merge_cand);
  get_temporal_merge_candidates(state, x, y, width, height, 1, 0, &merge_cand);
  get_mv_cand_from_candidates(state, x, y, width, height, &merge_cand, cur_cu, reflist, mv_cand);
}

/**
 * \brief Get MV prediction for current block using state->tile->frame->cu_array.
 *
 * \param state     encoder state
 * \param x         block x position in pixels
 * \param y         block y position in pixels
 * \param width     block width in pixels
 * \param height    block height in pixels
 * \param mv_cand   Return the motion vector candidates.
 * \param cur_cu    current CU
 * \param reflist   reflist index (either 0 or 1)
 */
void kvz_inter_get_mv_cand_cua(const encoder_state_t * const state,
                               int32_t x,
                               int32_t y,
                               int32_t width,
                               int32_t height,
                               int16_t mv_cand[2][2],
                               const cu_info_t* cur_cu,
                               int8_t reflist)
{
  merge_candidates_t merge_cand = { {0, 0}, {0, 0, 0}, 0, 0 };

  const cu_array_t *cua = state->tile->frame->cu_array;
  get_spatial_merge_candidates_cua(cua,
                                   x, y, width, height,
                                   state->tile->frame->width, state->tile->frame->height,
                                   &merge_cand);
  get_temporal_merge_candidates(state, x, y, width, height, 1, 0, &merge_cand);
  get_mv_cand_from_candidates(state, x, y, width, height, &merge_cand, cur_cu, reflist, mv_cand);
}

static bool is_duplicate_candidate(const cu_info_t* cu1, const cu_info_t* cu2)
{
  if (!cu2) return false;
  if (cu1->inter.mv_dir != cu2->inter.mv_dir) return false;

  for (int reflist = 0; reflist < 2; reflist++) {
    if (cu1->inter.mv_dir & (1 << reflist)) {
      if (cu1->inter.mv[reflist][0]  != cu2->inter.mv[reflist][0]  ||
          cu1->inter.mv[reflist][1]  != cu2->inter.mv[reflist][1]  ||
          cu1->inter.mv_ref[reflist] != cu2->inter.mv_ref[reflist]) {
        return false;
      }
    }
  }

  return true;
}

static bool add_merge_candidate(const cu_info_t *cand,
                                const cu_info_t *possible_duplicate1,
                                const cu_info_t *possible_duplicate2,
                                inter_merge_cand_t *merge_cand_out)
{
  if (!cand ||
      is_duplicate_candidate(cand, possible_duplicate1) ||
      is_duplicate_candidate(cand, possible_duplicate2)) {
    return false;
  }

  merge_cand_out->mv[0][0] = cand->inter.mv[0][0];
  merge_cand_out->mv[0][1] = cand->inter.mv[0][1];
  merge_cand_out->mv[1][0] = cand->inter.mv[1][0];
  merge_cand_out->mv[1][1] = cand->inter.mv[1][1];
  merge_cand_out->ref[0]   = cand->inter.mv_ref[0]; // L0/L1 references
  merge_cand_out->ref[1]   = cand->inter.mv_ref[1];
  merge_cand_out->dir      = cand->inter.mv_dir;
  return true;
}

/**
 * \brief Get merge predictions for current block
 * \param state     the encoder state
 * \param x         block x position in SCU
 * \param y         block y position in SCU
 * \param width     block width
 * \param height    block height
 * \param use_a1    true, if candidate a1 can be used
 * \param use_b1    true, if candidate b1 can be used
 * \param mv_cand   Returns the merge candidates.
 * \param lcu       lcu containing the block
 * \param ref_idx   current reference index (used only by TMVP)
 * \return          number of merge candidates
 */
uint8_t kvz_inter_get_merge_cand(const encoder_state_t * const state,
                                 int32_t x, int32_t y,
                                 int32_t width, int32_t height,
                                 bool use_a1, bool use_b1,
                                 inter_merge_cand_t mv_cand[MRG_MAX_NUM_CANDS],
                                 lcu_t *lcu,
                                 uint8_t ref_idx)
{
  uint8_t candidates = 0;
  int8_t zero_idx = 0;

  merge_candidates_t merge_cand = { {0, 0}, {0, 0, 0}, 0, 0 };

  get_spatial_merge_candidates(x, y, width, height,
                               state->tile->frame->width,
                               state->tile->frame->height,
                               lcu,
                               &merge_cand);

  const cu_info_t **a = merge_cand.a;
  const cu_info_t **b = merge_cand.b;

  if (!use_a1) a[1] = NULL;
  if (!use_b1) b[1] = NULL;

  if (add_merge_candidate(a[1], NULL, NULL, &mv_cand[candidates])) candidates++;
  if (add_merge_candidate(b[1], a[1], NULL, &mv_cand[candidates])) candidates++;
  if (add_merge_candidate(b[0], b[1], NULL, &mv_cand[candidates])) candidates++;
  if (add_merge_candidate(a[0], a[1], NULL, &mv_cand[candidates])) candidates++;
  if (candidates < 4 &&
      add_merge_candidate(b[2], a[1], b[1], &mv_cand[candidates])) candidates++;

  bool can_use_tmvp =
    state->encoder_control->cfg.tmvp_enable &&
    candidates < MRG_MAX_NUM_CANDS &&
    state->frame->ref->used_size;

  if (can_use_tmvp) {
    mv_cand[candidates].dir = 0;

    const int max_reflist = (state->frame->slicetype == KVZ_SLICE_B ? 1 : 0);
    for (int reflist = 0; reflist <= max_reflist; reflist++) {
      // Fetch temporal candidates for the current CU
      get_temporal_merge_candidates(state, x, y, width, height, 1, 0, &merge_cand);
      // TODO: enable L1 TMVP candidate
      // get_temporal_merge_candidates(state, x, y, width, height, 2, 0, &merge_cand);

      const cu_info_t *temporal_cand =
        (merge_cand.h != NULL) ? merge_cand.h : merge_cand.c3;

      if (add_temporal_candidate(state,
                                 ref_idx,
                                 temporal_cand,
                                 reflist,
                                 mv_cand[candidates].mv[reflist])) {
        mv_cand[candidates].ref[reflist] = 0;
        mv_cand[candidates].dir |= (1 << reflist);
      }
    }

    if (mv_cand[candidates].dir != 0) candidates++;
  }

  if (candidates < MRG_MAX_NUM_CANDS && state->frame->slicetype == KVZ_SLICE_B) {
    #define NUM_PRIORITY_LIST 12;
    static const uint8_t priorityList0[] = { 0, 1, 0, 2, 1, 2, 0, 3, 1, 3, 2, 3 };
    static const uint8_t priorityList1[] = { 1, 0, 2, 0, 2, 1, 3, 0, 3, 1, 3, 2 };
    uint8_t cutoff = candidates;
    for (int32_t idx = 0; idx<cutoff*(cutoff - 1) && candidates != MRG_MAX_NUM_CANDS; idx++) {
      uint8_t i = priorityList0[idx];
      uint8_t j = priorityList1[idx];
      if (i >= candidates || j >= candidates) break;

      // Find one L0 and L1 candidate according to the priority list
      if ((mv_cand[i].dir & 0x1) && (mv_cand[j].dir & 0x2)) {
        mv_cand[candidates].dir = 3;

        // get Mv from cand[i] and cand[j]
        mv_cand[candidates].mv[0][0]  = mv_cand[i].mv[0][0];
        mv_cand[candidates].mv[0][1]  = mv_cand[i].mv[0][1];
        mv_cand[candidates].mv[1][0]  = mv_cand[j].mv[1][0];
        mv_cand[candidates].mv[1][1]  = mv_cand[j].mv[1][1];
        mv_cand[candidates].ref[0]    = mv_cand[i].ref[0];
        mv_cand[candidates].ref[1]    = mv_cand[j].ref[1];

        if (state->frame->ref_LX[0][mv_cand[i].ref[0]] ==
            state->frame->ref_LX[1][mv_cand[j].ref[1]]
            &&
            mv_cand[i].mv[0][0] == mv_cand[j].mv[1][0] && 
            mv_cand[i].mv[0][1] == mv_cand[j].mv[1][1]) {
          // Not a candidate
        } else {
          candidates++;
        }
      }
    }
  }

  int num_ref = state->frame->ref->used_size;

  if (candidates < MRG_MAX_NUM_CANDS && state->frame->slicetype == KVZ_SLICE_B) {
    int j;
    int ref_negative = 0;
    int ref_positive = 0;
    for (j = 0; j < state->frame->ref->used_size; j++) {
      if (state->frame->ref->pocs[j] < state->frame->poc) {
        ref_negative++;
      } else {
        ref_positive++;
      }
    }
    num_ref = MIN(ref_negative, ref_positive);
  }

  // Add (0,0) prediction
  while (candidates != MRG_MAX_NUM_CANDS) {
    mv_cand[candidates].mv[0][0] = 0;
    mv_cand[candidates].mv[0][1] = 0;
    mv_cand[candidates].ref[0] = (zero_idx >= num_ref - 1) ? 0 : zero_idx;
    mv_cand[candidates].ref[1] = mv_cand[candidates].ref[0];
    mv_cand[candidates].dir = 1;
    if (state->frame->slicetype == KVZ_SLICE_B) {
      mv_cand[candidates].mv[1][0] = 0;
      mv_cand[candidates].mv[1][1] = 0;
      mv_cand[candidates].dir = 3;
    }
    zero_idx++;
    candidates++;
  }

  return candidates;
}