The Trouble with Getting Into LPR

[samplenhold]

I have been into cams as a hobby now for a little over two years. I originally had hoped to do LPR, but as time went by I thought it would never happen, mostly due to where could I put cams and how to get cable to them.

But then COVID hit and I found I had a lot of time on my hands just sitting around on my ass on the front porch. Which got me to thinking about LPR again. So I made a plan and tested multiple ideas. Finally bought the cams, built the box and installed them. Got everything hooked up this past Tuesday. Thanks to all of the folks that came before me and did the hard lifting on getting night image settings worked out and all of the talk about how much zoom. I stole the settings mostly from @bigredfish and @wittaj but of course those were just starting values. It did not take me long to get really good night caps, if I may say so myself.

But now there are other issues.

Like no matter where I am, like this afternoon at House Of Pies looking out the window while eating a patty melt, I spot a pickup with a hitch ball partially blocking the plate and I think to myself, that would be a bitch to get a cap of. But then I realize that if the truck was going by my West LPR cam, it might have enough angle to grab it. So now all I see are potential plate caps! Crap, that car does not have a front plate! What an asshole!

And what is it with police cars at night? Do they coat the plates so they smurf up the view from IR? Why? Why would they do that? Check these out from last night.

This is from LPR-W, car is eastbound.
But here is a pickup a half hour later. It is fine.

Here it is as it travels past LPR-E at the intersection:
And here it is from my intersection overview cam. Kind of pisses me off.

Then there is the problem of cars sneaking past the cams without triggering them. How do they do that? I get every guy walking his dog, every bike rider, every mom running with a baby carriage, but a big ass FedEx truck zips right by and no trigger? Whiskey Tango Foxtrot! This is especially troublesome at night. I'll get a car going westbound and get a good front trigger on LPR-E but LPR-W does not notice it. Or the other way around. Or neither see it. I did realize that sometimes it is that he car is going too fast through the scene so I had to set the Make Time to 0.2 seconds. That fixed some of it. I have used the option 'Test run video through motion detector' and sometimes get strange results. Like the car did not trip it in real time, but running it through on playback it shows as tripped.

I have yet to decide how I am going to compile the plates. Have read all of the threads on openalpr and plate recognizer and the wonderful scripts that members have put together. But if I understand correctly how each of these work, it takes the alert JPG from BI as the shot to analyze. My problem is that half the time the shot does not have the plate in the cap. It is either before the plate enters the zone (like headlights did the trigger), or it is too late to grab the plate part of the vehicle. How do you solve that?

Well ranting about this made me feel better. No, wait, it did not. If you have not dove down the LPR rabbit hole, beware it will grab you by the …. and never let go. Like in 'The Hangover Part 2'..."Bangkok's got him now. Once that happens, it never lets go".

 

[wittaj]

I have found that IVS doesn't work for the zoom needed as the camera needs time to recognize the motion and did it cross a zone. If the vehicle is in the frame for less than a second, IVS probably will miss it.

It takes some trial and error in BI, but setting up a zone works well. You will have to run a separate night profile and make the target a lot smaller as most will only see head/tail lights and the plate on the image. My target at night is about the size of the plate.

Here is my setup and you can see from the alert images, I am getting them. In my case, it was draw a zone where I did so that the cars going right to left, I catch the front plate, and cars going left to right, I catch the back plate. It is some trial and error to get the FOV and zone line drawn to where you get them, but you see the idea in how to get it to trigger an alert with the vehicle in the frame. I do zoom in at night as this FOV for nightime was just a little too wide, but it is fine for daytime.

And then if you only wanted to catch vehicles going in one direction, simply add another zone to account for that.

What I found was that the tail lights were still bright enough to trigger even at night. But you have to make the object size A LOT smaller since the image is black and all you see are the head/tail lights and plate.

 

[cjowers]

I have yet to decide how I am going to compile the plates. Have read all of the threads on openalpr and plate recognizer and the wonderful scripts that members have put together. But if I understand correctly how each of these work, it takes the alert JPG from BI as the shot to analyze. My problem is that half the time the shot does not have the plate in the cap. It is either before the plate enters the zone (like headlights did the trigger), or it is too late to grab the plate part of the vehicle. How do you solve that?

I'm waiting for someone to create an object detection script (just for plates, like the latest YOLO or similar) so I can easily filter out the images I want to run through whatever plate recognition setup that I eventually have. Cuz even with an 'unlimited' subscription youre still wasting computation/data/space, and you are still going to miss plates - that is UNLESS you capture multiple images per trigger (and filter out the plates). This is in my setup at least, where I am purposely making some compromises, but many others will have challenging environments as well I think.

Is focus an issue in any of those photos? The last one looks like it's focused up near the stop sign. I assume it's fine, but obviously if any cams are out, reliable motion detection might struggle.

 

[samplenhold]

Is focus an issue in any of those photos?

No, focus is fine. This only happens on police cars. You can see the pickup truck is fine.

 

[sebastiantombs]

Maybe use a clone of an overview camera to trigger the LPR? That might solve the missed triggers but does add another complication.

 

[samplenhold]

Maybe use a clone of an overview camera to trigger the LPR?

Had not thought of that. That brings up interesting possibilities. The big problem is cars headed south and turning left at the stop sign. Since the cam is zoomed in enough to get the plates of car travelling e-w, the south-turning-east cars always fill the entire frame before the plate is visible.

 

[biggen]

Maybe use a clone of an overview camera to trigger the LPR? That might solve the missed triggers but does add another complication.

Bingo. This is exactly how I do it except I don't use a clone of the overview. I use the actual overview feed to trigger the alert on my LPR camera. So I don't have any zones or motion or anything on my LPR camera. Just it receiving a trigger from the overview.

When I go to look at the actual alert video on my LPR, it takes a second or two for the car to enter the frame since the overview camera has a much larger FOV than the LPR. I'm ok with this as I'm not storing LPR data. I just want it recorded for future use if I ever need to go look at it.