But there are also problems. Researchers who have tried to come up with 3-D visualizations have faced this three main problems: first in 3-D space it can become very difficult for the user to compare the different sizes of objects, because if one object is far away it becomes smaller and if it is closer it becomes larger. So, when objects are at different distances from the user, it becomes difficult for the user to make comparisons. Then another problem is the occlusion effect: since objects are at different distances one object can be behind another object. So, the object which is closer to the user hides, occludes the object that is behind, and this can be very dangerous for visualization, because important aspects of the visualization can become occluded. And a third problem seems a paradox, because one argument of 3-D supporters is that 3-D makes navigation easier but the researchers who do not agree with 3-D approaches also make the point that if you allow free navigation in a 3-D space, I mean 6 degrees of freedom navigation, the 3-D space can become very very difficult to navigate. In user studies, unconstrained navigation in 3-D space shows that with standard users it happens very frequently that users completely lose themselves in the visualization, becomes disoriented and do not find anymore their destination and they do not understand any more where they are inside the visualized dataset.