vpx/vp9/encoder/vp9_skin_detection.c

/*
 *  Copyright (c) 2015 The WebM project authors. All Rights Reserved.
 *
 *  Use of this source code is governed by a BSD-style license
 *  that can be found in the LICENSE file in the root of the source
 *  tree. An additional intellectual property rights grant can be found
 *  in the file PATENTS.  All contributing project authors may
 *  be found in the AUTHORS file in the root of the source tree.
 */

#include <limits.h>
#include <math.h>

#include "vp9/common/vp9_blockd.h"
#include "vp9/encoder/vp9_encoder.h"
#include "vp9/encoder/vp9_skin_detection.h"

// Fixed-point skin color model parameters.
static const int skin_mean[2] = {7463, 9614};                 // q6
static const int skin_inv_cov[4] = {4107, 1663, 1663, 2157};  // q16
static const int skin_threshold = 1570636;                    // q18

// Thresholds on luminance.
static const int y_low = 20;
static const int y_high = 220;

// Evaluates the Mahalanobis distance measure for the input CbCr values.
static int evaluate_skin_color_difference(int cb, int cr) {
  const int cb_q6 = cb << 6;
  const int cr_q6 = cr << 6;
  const int cb_diff_q12 = (cb_q6 - skin_mean[0]) * (cb_q6 - skin_mean[0]);
  const int cbcr_diff_q12 = (cb_q6 - skin_mean[0]) * (cr_q6 - skin_mean[1]);
  const int cr_diff_q12 = (cr_q6 - skin_mean[1]) * (cr_q6 - skin_mean[1]);
  const int cb_diff_q2 = (cb_diff_q12 + (1 << 9)) >> 10;
  const int cbcr_diff_q2 = (cbcr_diff_q12 + (1 << 9)) >> 10;
  const int cr_diff_q2 = (cr_diff_q12 + (1 << 9)) >> 10;
  const int skin_diff = skin_inv_cov[0] * cb_diff_q2 +
      skin_inv_cov[1] * cbcr_diff_q2 +
      skin_inv_cov[2] * cbcr_diff_q2 +
      skin_inv_cov[3] * cr_diff_q2;
  return skin_diff;
}

int vp9_skin_pixel(const uint8_t y, const uint8_t cb, const uint8_t cr) {
  if (y < y_low || y > y_high)
    return 0;
  else
    return (evaluate_skin_color_difference(cb, cr) < skin_threshold);
}

#ifdef OUTPUT_YUV_SKINMAP
// For viewing skin map on input source.
void vp9_compute_skin_map(VP9_COMP *const cpi, FILE *yuv_skinmap_file) {
  int i, j, mi_row, mi_col, num_bl;
  VP9_COMMON *const cm = &cpi->common;
  uint8_t *y;
  const uint8_t *src_y = cpi->Source->y_buffer;
  const uint8_t *src_u = cpi->Source->u_buffer;
  const uint8_t *src_v = cpi->Source->v_buffer;
  const int src_ystride = cpi->Source->y_stride;
  const int src_uvstride = cpi->Source->uv_stride;
  int y_bsize = 16;  // Use 8x8 or 16x16.
  int uv_bsize = y_bsize >> 1;
  int ypos = y_bsize >> 1;
  int uvpos = uv_bsize >> 1;
  int shy = (y_bsize == 8) ? 3 : 4;
  int shuv = shy - 1;
  int fac = y_bsize / 8;
  // Use center pixel or average of center 2x2 pixels.
  int mode_filter = 1;
  YV12_BUFFER_CONFIG skinmap;
  memset(&skinmap, 0, sizeof(YV12_BUFFER_CONFIG));
  if (vpx_alloc_frame_buffer(&skinmap, cm->width, cm->height,
                               cm->subsampling_x, cm->subsampling_y,
                               VP9_ENC_BORDER_IN_PIXELS, cm->byte_alignment)) {
      vpx_free_frame_buffer(&skinmap);
      return;
  }
  memset(skinmap.buffer_alloc, 128, skinmap.frame_size);
  y = skinmap.y_buffer;
  // Loop through blocks and set skin map based on center pixel of block.
  // Set y to white for skin block, otherwise set to source with gray scale.
  // Ignore rightmost/bottom boundary blocks.
  for (mi_row = 0; mi_row < cm->mi_rows - 1; mi_row += fac) {
    num_bl = 0;
    for (mi_col = 0; mi_col < cm->mi_cols - 1; mi_col += fac) {
      // Select pixel for each block for skin detection.
      // Use center pixel, or 2x2 average at center.
      uint8_t ysource = src_y[ypos * src_ystride + ypos];
      uint8_t usource = src_u[uvpos * src_uvstride + uvpos];
      uint8_t vsource = src_v[uvpos * src_uvstride + uvpos];
      uint8_t ysource2 = src_y[(ypos + 1) * src_ystride + ypos];
      uint8_t usource2 = src_u[(uvpos + 1) * src_uvstride + uvpos];
      uint8_t vsource2 = src_v[(uvpos + 1) * src_uvstride + uvpos];
      uint8_t ysource3 = src_y[ypos * src_ystride + (ypos + 1)];
      uint8_t usource3 = src_u[uvpos * src_uvstride + (uvpos  + 1)];
      uint8_t vsource3 = src_v[uvpos * src_uvstride + (uvpos +  1)];
      uint8_t ysource4 = src_y[(ypos + 1) * src_ystride + (ypos + 1)];
      uint8_t usource4 = src_u[(uvpos + 1) * src_uvstride + (uvpos  + 1)];
      uint8_t vsource4 = src_v[(uvpos + 1) * src_uvstride + (uvpos +  1)];
      if (mode_filter == 1) {
        ysource = (ysource + ysource2 + ysource3 + ysource4) >> 2;
        usource = (usource + usource2 + usource3 + usource4) >> 2;
        vsource = (vsource + vsource2 + vsource3 + vsource4) >> 2;
      }
      const int is_skin = vp9_skin_pixel(ysource, usource, vsource);
      for (i = 0; i < y_bsize; i++) {
        for (j = 0; j < y_bsize; j++) {
          if (is_skin)
            y[i * src_ystride + j] = 255;
          else
            y[i * src_ystride + j] = src_y[i * src_ystride + j];
        }
      }
      num_bl++;
      y += y_bsize;
      src_y += y_bsize;
      src_u += uv_bsize;
      src_v += uv_bsize;
    }
    y += (src_ystride << shy) - (num_bl << shy);
    src_y += (src_ystride << shy) - (num_bl << shy);
    src_u += (src_uvstride << shuv) - (num_bl << shuv);
    src_v += (src_uvstride << shuv) - (num_bl << shuv);
  }
  vp9_write_yuv_frame_420(&skinmap, yuv_skinmap_file);
  vpx_free_frame_buffer(&skinmap);
}
#endif