WDR or HLC, any good at night for LPR?

GeoffC

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I'd like to be able to read the license plate of anyone entering/leaving my driveway. The driveway is U-shaped, so vehicles typically drive around the 'U at very slow speed', giving me a good view of both the front and rear plates.
I don't have a dedicated LPR camera, and I'm not looking for automated LPR (no character recognition), I'd just like to be able to review the video and make out a plate number.
During the day my Hikvision 4Mp camera gets a good enough image to read plates at about 30 feet (it's a 2.8mm lens). I was hoping that since my camera has the 1/1.8" sensor that it would do reasonably well at night. It does do well as we have lots of ambient lighting, and I'm still able to make out plates at about 30 feet if the plate isn't lit by bulbs at the sides of the plate. If the license plate is lit up, then all I get is a blob of light with no distinguishable letters.

I suspect that what I'm looking for isn't possible without setting shutter speeds so fast that the camera would be useless for anything other than viewing lighted license plates, but I was just wondering what settings there my be that may improve my chances (WDR?, HLC?) of reading lighted license plates while still leaving the camera able to do general surveillance? So far I'm finding that WDR and HLC seem to help a touch, but at the expense of considerable noise.
 

wittaj

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You are correct - unless you have tons of ambient light, you will need to switch to B/W and infrared with a faster shutter. 30 feet with a 2.8mm lens will be tough.

With that sensor, I would try first going to B/W at night and slowly increasing the shutter speed and see if you can find the mix that let's you read plates and still make out the surrounding - maybe but probably not.

HLC may help but as you found out, it adds noise, which usually means not enough ambient light to counter the effect.
 

GeoffC

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Thanks for the reply. Like you've suggested, it sounds like I need to park my car on the driveway with the lights on and play with the settings until I (hopefully) find a reasonable compromise in the settings.
 

wittaj

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Good luck - I hope you can find that compromise! I know many of us that run LPR have done just that - parked the car or have someone stop in the roadway so we can dial it in.

Since you are looking at just capturing your driveway, that does open up a lot more opportunities than a vehicle traveling down a road. I am thinking maybe 1/30 or 1/60 might be able to capture it.

We would love to see some day and night pics! Especially because my 2.8mm has trouble picking up plates of a parked car at 30 feet.
 

GeoffC

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I've tried even 1/120 shutter speed with no luck, that's why I started playing with WDR and HLC, but I've only done it for the vehicles of other people who stop by. I think that parking my car will allow me to fine tune everything. I'll pose some pics when I next get around to a tweaking session.

One thing I noticed on one of my sessions was that a night when a license plate was not lit up I initially couldn't read the plate and the surroundings was quite noisy/pixellated (the vehicle had been stationary for about 10 seconds). The driver then moved in the vehicle causing it to rock a tiny bit. Then it's as if something changed and the license plate snapped into focus with most of the noise/pixellation in the surroundings greatly reduced. Would this have something to do with using h.264+ encoding (bitrate perhaps automatically changing?).
 

wittaj

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Are you running CBR or VBR? Try CBR and see if it improves - and probably bump up the rate as well.
 

GeoffC

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I ran some tests and found that a combination of backlight compensation (BLC) (for street lights in the background) and about 40% HLC really helped a lot. I also added a tiny bit of brightness to compensate for a darkish image. A shutter speed of 1/60th of a second worked fine for the slow speeds going around our U-shaped driveway. Note that I have two 60-watt equivalent LED bulbs lighting the driveway from our coach lamps, so that helps a bit with the lighting.
It seems that the angle of the license plate makes a huge difference, where I had some perfectly clear license plates at about 35 feet (didn't even have to zoom in to read them) when the plate was perfectly perpendicular to the camera, and then they would be almost illegible with only a small change in angle.
I forgot to test with CBR, will try that later, perhaps that will help with the license plate angle issue as that may have been more a result of the higher bitrate not kicking in.
 

samplenhold

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It seems that the angle of the license plate makes a huge difference, where I had some perfectly clear license plates at about 35 feet (didn't even have to zoom in to read them) when the plate was perfectly perpendicular to the camera, and then they would be almost illegible with only a small change in angle.
Plate foreshortening due to high angle is, to me, the biggest impact on getting a good cap. See my discussion of LPR for newbies

 
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