I thought I'd share something that I discovered that doesn't seemed to be documented anywhere else regarding Line Crossing Detection on the Hikvision cameras. If you clear the detection area from your smart event, it will clear not only your line but your max. size rectangle also, which by default is the outer border of the image. Without that in place your line crossing no longer detects anything, as anything larger than 'nothing' won't be detected. It stumped me for quite a bit before I realised what was happening.
And something else I noticed. If you set a max. area box and it touches your line then it seems to disable the detection. I moved the box well clear of my line and everything worked as expected again.
And something else I noticed. If you set a max. area box and it touches your line then it seems to disable the detection. I moved the box well clear of my line and everything worked as expected again.