This function allows you to switch between perspective view mode and orthographic view mode. The option is only available via the toolbar and is represented by the following icons 
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The main view and the secondary views are then displayed in 2D with no perspective angle, simulating a view perpendicular to a plane where the focal length would be towards infinity where the perspective angle is equal to zero. A sign in the upper right corner mentions that we are in ortho mode. Disengaging the function will return to perspective view mode with the view angle specified in the preferences.
Switching on the top view in orthographic view mode will allow you to see all the points vertically in 2D while switching on the front or right view will allow you to see all the points horizontally in 2D.
Perspective mode:
Orthographic mode:
