In other words, we can distinguish between the following three types of AR markers: digital markers (for example, an image on a computer screen), printed markers (for example, a photograph in a magazine), and natural markers (for example, a human face). In addition, AR markers can be classified as technical (for example, a QR code) and natural (for example, a photograph, a human face or a view of a real-world environment Technical markers are nothing else but clearly visible geometrical patterns and therefore they do not look userfriendly (see Figure 9). Natural markers, such as the cover of a magazine or an advertising poster, can be used in the same way, but they are much more attractive and common for the user. Though the difference between a printed AR marker or QR code and a human face might look massive, in terms of recognition technology, both of them contain some visual patterns (the pattern of a printed AR marker or QR code is quite obvious and the “pattern” of a human face is deeply concealed) that have to be identified, extracted and compared with a reference pattern on the server. Hence, we are dealing with different levels of the pattern recognition technology, rather than with different types of AR. All of the above examples are examples of Recognition-based AR applications.