Context:
I am trying to create a Unity app for a mobile device that can recognize big objects like cars through the device's physical camera.
I was looking at Wikitude and Vuforia but they both only seem to work with small objects that can fit on a table top. Unless I am missing something?
I was also looking at Open CV and ARCore, but they only seem to work with rectangular "markers". Cars for example, is not rectangular and although the shape of the car will always be the same from the same angle, the background and the objects seen behind the car through the windows always differ causing the AR library to try to match pixels that are not part of the car.
Question:
How can one create a Unity app that can recognize a big object or a non-rectangular image through a mobile's physical camera?
Image coming from the camera does not contain scale of the object in any way - its just an array from pixels, as captured from the light detecting device (typicaly a Cmos sensor). The appliaction has no way of knowing if the object is big or small (at least until we get into depth cameras), so the methodology in recognising an image is exactly the same for big and for small objects.
As far as root of your question goes : recognizing an object based on the image is a very tricky subject, that only gets managable if we only teach the algorithm to recognise a marker or a QRCode - those are designed specifically to be easily recognizable, that's why ther's more libraries doing that sort of thing.
Vuforia tries to recognize all objects, which is amazing, and will not care about the scale of the object, so you can try to use it, this is pretty much it as far as easy solutions go.
However, this is likely to only work in the same lighting condititions, and will fail if the weather changes etc, if the car is dirty, wet and what not.
The general case, as in aproaching a human-like ability to recognize objects - not yet possible, humans train their neural networks for many years before they can recognize cars.
For more information please read this really informative thread on image recognition (in this case a Coca-Cola can which is far simpler than a general 'car')
Image Processing: Algorithm Improvement for 'Coca-Cola Can' Recognition
Here's an image showing that larger stuff looks the same for the camera