about density map

[Yellow-Champagne]

Hello author, I want to make a data set myself. I now take 200 images from a video, use the labelimg to get the .xml file, and then use the matlab program to extract the human coordinates to get the .mat file. When I use your program to convert the density map, the partial density map I get is empty, what is the problem. And I want to ask if the spread parameter σ mentioned in the paper are not obtained by k nearest neighbors, why is it set to 4 in your program?