How to use an image without targets (negative image)as a train image to train a detection net?

When I use an image in which there are targets to train a detection network, I can use the xml annotation to get the idx and rec file. But when I want to use an image in which there isnot any targets, I do not know how to do. For the xml, it need the position(xmin,xmax,ymin and ymax)of a target. So there are no x or y for the negetive image to generate the xml annotation.
And I also tried to write an xml annotation like others except the object part, it is not work.
How can I use the negative image to train the detection network like SSD?
@zhreshold

I guess you are talking about the xml file defined at https://gluon-cv.mxnet.io/build/examples_datasets/detection_custom.html Can you try putting 0 in all xmin, xmax, ymin and ymax for negative images? Does it work?

Yes, the xml file is defined like the url you gave.
And I tried the way that setting 0 on all xmin, xmax, ymin and ymax. The network can run but in fact these negative images do nothing to the results. From the detection results, I see there is not any performance improvement.
So, I think this way is not OK.
Do you have some idea else for the problem?
Thanks for your reply and I am very looking forward to it.