[FAQ]: 作者回答👉关于如何根据抠图模型的alpha合成新背景？

[DefTruth]

由于问这个问题的人比较多，我在lite里面增加了一些辅助函数来实现背景合成，但还没合并进主分支，具体细节可以参考以下这段逻辑，有需要的可以参考一下（目前不考虑性能的优化问题，有需要的同学可以自己根据这段逻辑做特定的性能优化）：

void lite::utils::swap_background(const cv::Mat &fgr_mat, const cv::Mat &pha_mat,
                                  const cv::Mat &bgr_mat, cv::Mat &out_mat,
                                  bool fgr_is_already_mul_pha)
{
  // user-friendly method for background swap.
  if (fgr_mat.empty() || pha_mat.empty() || bgr_mat.empty()) return;
  const unsigned int fg_h = fgr_mat.rows;
  const unsigned int fg_w = fgr_mat.cols;
  const unsigned int bg_h = bgr_mat.rows;
  const unsigned int bg_w = bgr_mat.cols;
  const unsigned int ph_h = pha_mat.rows;
  const unsigned int ph_w = pha_mat.cols;
  const unsigned int channels = fgr_mat.channels();
  if (channels != 3) return; // only support 3 channels.
  const unsigned int num_elements = fg_h * fg_w * channels;

  cv::Mat bg_mat_copy, ph_mat_copy, fg_mat_copy;
  if (bg_h != fg_h || bg_w != fg_w)
    cv::resize(bgr_mat, bg_mat_copy, cv::Size(fg_w, fg_h));
  else bg_mat_copy = bgr_mat; // ref only.
  if (ph_h != fg_h || ph_w != fg_w)
    cv::resize(pha_mat, ph_mat_copy, cv::Size(fg_w, fg_h));
  else ph_mat_copy = pha_mat; // ref only.
  if (ph_mat_copy.channels() == 1)
    cv::cvtColor(ph_mat_copy, ph_mat_copy, cv::COLOR_GRAY2BGR); // 0.~1.
  // convert mats to float32 points.
  if (bg_mat_copy.type() != CV_32FC3) bg_mat_copy.convertTo(bg_mat_copy, CV_32FC3); // 0.~255.
  if (ph_mat_copy.type() != CV_32FC3) ph_mat_copy.convertTo(ph_mat_copy, CV_32FC3); // 0.~1.
  if (fgr_mat.type() != CV_32FC3) fgr_mat.convertTo(fg_mat_copy, CV_32FC3); // 0.~255.
  else fg_mat_copy = fgr_mat; // ref only

  // element wise operations.
  out_mat = fg_mat_copy.clone();
  const float *fg_ptr = (float *) fg_mat_copy.data;
  const float *bg_ptr = (float *) bg_mat_copy.data;
  const float *ph_ptr = (float *) ph_mat_copy.data;
  float *mutable_out_ptr = (float *) out_mat.data;

  // TODO: add omp support instead of native loop.
  if (!fgr_is_already_mul_pha)
    for (unsigned int i = 0; i < num_elements; ++i)
      mutable_out_ptr[i] = fg_ptr[i] * ph_ptr[i] + (1.f - ph_ptr[i]) * bg_ptr[i];
  else
    for (unsigned int i = 0; i < num_elements; ++i)
      mutable_out_ptr[i] = fg_ptr[i] + (1.f - ph_ptr[i]) * bg_ptr[i];

  if (!out_mat.empty() && out_mat.type() != CV_8UC3)
    out_mat.convertTo(out_mat, CV_8UC3);
}

使用案例（MODNet还在开发中，此处仅用作参考示例）

static void test_default()
{
  std::string onnx_path = "../../../hub/onnx/cv/modnet_photographic_portrait_matting-512x512.onnx";
  std::string test_img_path = "../../../examples/lite/resources/test_lite_matting_input.jpg";
  std::string test_bgr_path = "../../../examples/lite/resources/test_lite_matting_bgr.jpg";
  std::string save_fgr_path = "../../../logs/test_lite_modnet_fgr.jpg";
  std::string save_pha_path = "../../../logs/test_lite_modnet_pha.jpg";
  std::string save_merge_path = "../../../logs/test_lite_modnet_merge.jpg";
  std::string save_swap_path = "../../../logs/test_lite_modnet_swap.jpg";

  lite::cv::matting::MODNet *modnet =
      new lite::cv::matting::MODNet(onnx_path, 16); // 16 threads

  lite::types::MattingContent content;
  cv::Mat img_bgr = cv::imread(test_img_path);
  cv::Mat bgr_mat = cv::imread(test_bgr_path);

  // 1. image matting.
  modnet->detect(img_bgr, content, true);

  if (content.flag)
  {
    if (!content.fgr_mat.empty()) cv::imwrite(save_fgr_path, content.fgr_mat);
    if (!content.pha_mat.empty()) cv::imwrite(save_pha_path, content.pha_mat * 255.);
    if (!content.merge_mat.empty()) cv::imwrite(save_merge_path, content.merge_mat);
    // swap background
    cv::Mat out_mat;
    lite::utils::swap_background(content.fgr_mat, content.pha_mat, bgr_mat, out_mat, true);
    if (!out_mat.empty())
    {
      cv::imwrite(save_swap_path, out_mat);
      std::cout << "Saved Swap Image Done!" << std::endl;
    }

    std::cout << "Default Version MGMatting Done!" << std::endl;
  }

  delete modnet;
}

效果示例

• 合成图
• 原图
• 背景图