**3.4 Concluding remarks**

As can be deduced from the brief and non-exhaustive analysis made in the previous paragraphs, the goal of obtaining a general-purpose objective metric is far from being achieved.

There are many difficulties, such as the availability of well-designed test datasets or the need for extensive subjective tests for collecting subjective opinion. In the frameworks of virtual, augmented, and mixed reality, quality evaluation is even more complex. In fact, up to now no standardized guidelines for running subjective tests have been defined. Even the use of ACR (Absolute Category Rating), ACR-HR (Absolute Category Rating with Hidden Reference), DSIS (Double-Stimulus Impairment Scale), DSCQS (Double Stimulus Continuous Quality-Scale) for quality assessment is not well defined, since it mainly depends on the target application and the rendering device. The situation is not different for the virtual reality case.

In the next paragraphs, we will make a quick introduction of one of the systems currently among the most used to acquire a scene from multiple points of view.
