In FRED, elements are collections of surfaces and curves used to represent objects such as lenses, prisms, mirrors and basic geometrical objects (element primitives). Surfaces and curves in elements and element primitives can be viewed and edited and the surface type, materials, coatings, scatter, grating, glue, and visualization properties can be altered, but the Surface trimming volumes ( i.e. apertures ) cannot be altered on the individual surfaces. The trimming volume can only be changed in the corresponding element/element primitive edit/view dialog, made available by right mouse clicking on the element/element primitive node in the object tree and selecting "Edit / View ..." from the list menu.
Custom elements are arbitrary collections of surfaces and curves through which the user can define any custom geometry. In a custom element, each surface must be edited separately.
The advantage of an element is that lens, prism, mirror or element primitive properties can be edited via the "Edit/View ..." dialogs and all of the surfaces making up that element/element primitive will be updated together according to the specifications given. In contrast, if an element or element primitive is converted to a custom element, then each surface must be edited separately.
Surfaces and curves can be added to elements and element primitives, but when edited only the surfaces that are part of the original element are changed. The surfaces added to the element do not change unless they are edited separately.
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