问为什么正向图像扭曲会在图像中留下空洞？

EN

/* Assuming the input image is not empty */
cv::Mat originalImage; // Not empty!

/* Our outputs */
cv::Mat homographyOutput, remapOutput;

cv::Mat H;             // This is our homography
cv::Mat map_x, map_y;  // These matrices are needed for cv::remap()

/* Compute mappings from homography */
map_x.create(originalImage.size(), CV_32FC1);
map_y.create(originalImage.size(), CV_32FC1);
for (int y = 0; y < originalImage.size().height; ++y)
  for (int x = 0; x < originalImage.size().width; ++x)
  {
    double x_mapped = H.at<double>(0,0)*x + H.at<double>(0,1)*y + 
                        H.at<double>(0,2);
    double y_mapped = H.at<double>(1,0)*x + H.at<double>(1,1)*y + 
                        H.at<double>(1,2);
    double w = H.at<double>(2,0)*x + H.at<double>(2,1)*y + 
                        H.at<double>(2,2);
    x_mapped /= w;
    y_mapped /= w;
    map_x.at<float>(y,x) = (float)x_mapped;
    map_y.at<float>(y,x) = (float)y_mapped;
  }

/* Now call remap */
cv::remap(originalImage, remapOutput, map_x, map_y);