Car detection from still pictures

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Hi. I'm planning on having a camera in a fixed location where cars park. The camera would sit just in front of the car at the same level of the cars, and would take pictures at a given interval (e.g. 1 minute). The camera would then get the same picture (except perhaps for lighting condition changes) until the car leaves and another one comes.

I would like to know the complexity of this task (e.g. are there any scripts that already solve this, or any kind of filter?).

I'm new to the OpenCV world. Any orientation (high level is fine) would be appreciated.

Regards,