The old method used to be template matching, and it probably still works well enough for problems like this. There are almost certainly now better approaches possible using Machine Learning.
In my experience, the main challenges with problems like this are dealing with varying lighting, scales, orientation and perspective. These can quickly become of diminishing returns – especially if the solution is provided as an app that provides near-immediate feedback.
In my experience, the main challenges with problems like this are dealing with varying lighting, scales, orientation and perspective. These can quickly become of diminishing returns – especially if the solution is provided as an app that provides near-immediate feedback.