vp9/encoder/vp9_skin_detection.c

   1 /*
   2  *  Copyright (c) 2015 The WebM project authors. All Rights Reserved.
   3  *
   4  *  Use of this source code is governed by a BSD-style license
   5  *  that can be found in the LICENSE file in the root of the source
   6  *  tree. An additional intellectual property rights grant can be found
   7  *  in the file PATENTS.  All contributing project authors may
   8  *  be found in the AUTHORS file in the root of the source tree.
   9  */
  10
  11 #include <limits.h>
  12 #include <math.h>
  13
  14 #include "vp9/common/vp9_blockd.h"
  15 #include "vp9/encoder/vp9_encoder.h"
  16 #include "vp9/encoder/vp9_skin_detection.h"
  17
  18 // Fixed-point skin color model parameters.
  19 static const int skin_mean[2] = {7463, 9614};                 // q6
  20 static const int skin_inv_cov[4] = {4107, 1663, 1663, 2157};  // q16
  21 static const int skin_threshold = 1570636;                    // q18
  22
  23 // Thresholds on luminance.
  24 static const int y_low = 20;
  25 static const int y_high = 220;
  26
  27 // Evaluates the Mahalanobis distance measure for the input CbCr values.
  28 static int evaluate_skin_color_difference(int cb, int cr) {
  29   const int cb_q6 = cb << 6;
  30   const int cr_q6 = cr << 6;
  31   const int cb_diff_q12 = (cb_q6 - skin_mean[0]) * (cb_q6 - skin_mean[0]);
  32   const int cbcr_diff_q12 = (cb_q6 - skin_mean[0]) * (cr_q6 - skin_mean[1]);
  33   const int cr_diff_q12 = (cr_q6 - skin_mean[1]) * (cr_q6 - skin_mean[1]);
  34   const int cb_diff_q2 = (cb_diff_q12 + (1 << 9)) >> 10;
  35   const int cbcr_diff_q2 = (cbcr_diff_q12 + (1 << 9)) >> 10;
  36   const int cr_diff_q2 = (cr_diff_q12 + (1 << 9)) >> 10;
  37   const int skin_diff = skin_inv_cov[0] * cb_diff_q2 +
  38       skin_inv_cov[1] * cbcr_diff_q2 +
  39       skin_inv_cov[2] * cbcr_diff_q2 +
  40       skin_inv_cov[3] * cr_diff_q2;
  41   return skin_diff;
  42 }
  43
  44 int vp9_skin_pixel(const uint8_t y, const uint8_t cb, const uint8_t cr) {
  45   if (y < y_low || y > y_high)
  46     return 0;
  47   else
  48     return (evaluate_skin_color_difference(cb, cr) < skin_threshold);
  49 }
  50
  51 #ifdef OUTPUT_YUV_SKINMAP
  52 // For viewing skin map on input source.
  53 void vp9_compute_skin_map(VP9_COMP *const cpi, FILE *yuv_skinmap_file) {
  54   int i, j, mi_row, mi_col;
  55   VP9_COMMON *const cm = &cpi->common;
  56   uint8_t *y;
  57   const uint8_t *src_y = cpi->Source->y_buffer;
  58   const uint8_t *src_u = cpi->Source->u_buffer;
  59   const uint8_t *src_v = cpi->Source->v_buffer;
  60   const int src_ystride = cpi->Source->y_stride;
  61   const int src_uvstride = cpi->Source->uv_stride;
  62   YV12_BUFFER_CONFIG skinmap;
  63   memset(&skinmap, 0, sizeof(YV12_BUFFER_CONFIG));
  64   if (vp9_alloc_frame_buffer(&skinmap, cm->width, cm->height,
  65                                cm->subsampling_x, cm->subsampling_y,
  66                                VP9_ENC_BORDER_IN_PIXELS, cm->byte_alignment)) {
  67       vp9_free_frame_buffer(&skinmap);
  68       return;
  69   }
  70   memset(skinmap.buffer_alloc, 128, skinmap.frame_size);
  71   y = skinmap.y_buffer;
  72   // Loop through 8x8 blocks and set skin map based on center pixel of block.
  73   // Set y to white for skin block, otherwise set to source with gray scale.
  74   // Ignore rightmost/bottom boundary blocks.
  75   for (mi_row = 0; mi_row < cm->mi_rows - 1; ++mi_row) {
  76     for (mi_col = 0; mi_col < cm->mi_cols - 1; ++mi_col) {
  77       // Use middle pixel for each 8x8 block for skin detection.
  78       // If middle pixel is skin, assign whole 8x8 block to skin.
  79       const uint8_t ysource = src_y[4 * src_ystride + 4];
  80       const uint8_t usource = src_u[2 * src_uvstride + 2];
  81       const uint8_t vsource = src_v[2 * src_uvstride + 2];
  82       const int is_skin = vp9_skin_pixel(ysource, usource, vsource);
  83       for (i = 0; i < 8; i++) {
  84         for (j = 0; j < 8; j++) {
  85           if (is_skin)
  86             y[i * src_ystride + j] = 255;
  87           else
  88             y[i * src_ystride + j] = src_y[i * src_ystride + j];
  89         }
  90       }
  91       y += 8;
  92       src_y += 8;
  93       src_u += 4;
  94       src_v += 4;
  95     }
  96     y += (src_ystride << 3) - ((cm->mi_cols - 1) << 3);
  97     src_y += (src_ystride << 3) - ((cm->mi_cols - 1) << 3);
  98     src_u += (src_uvstride << 2) - ((cm->mi_cols - 1) << 2);
  99     src_v += (src_uvstride << 2) - ((cm->mi_cols - 1) << 2);
 100   }
 101   vp9_write_yuv_frame_420(&skinmap, yuv_skinmap_file);
 102   vp9_free_frame_buffer(&skinmap);
 103 }
 104 #endif