D3PLOT 22.1

FREE_EDGES... Menu: Controlling Free Edge Display of Element Borders

FREE_EDGES... menu: Controlling free edge display of element borders

LI line and HI hidden-line plots, and the hidden-line overlay of element borders normally draw all element edges. In "free-edge" mode you can choose to draw only those borders which are "free" edges.

Two separate options are provided for controlling the display of free edged in Line/Hidden plotting and in all other plotting modes. The defaults for these 2 options are:

Line/Hidden

Defaults to off

Other overlays

Defaults to on

"Free" edges can be displayed either as topological free edges or as "feature" lines: this is explained in more detail below.

Overlays apply to all plotting modes that are not wireframe of hidden, and overlay display itself is controlled in the OVERLAY panel. (See OVERLAY... Controlling the hidden-line overlay of element borders on data plots )

Individual element overlay styles can also be set in the PROPS panel ( Properties: Controlling Colour, Drawing Style, Transparency, Lighting Attributes and Overlay of Entities ).

What is a "free" edge?

A purely topological "free" edge is defined as an element (or face) border that is attached to only one element. This is illustrated in the two figures below.

What is a "feature line"?

The purely topological definition of a free edge can sometimes give unsatisfactory images since it is dictated purely by element connectivity, and not by the actual shape of the mesh.

A "feature line" occurs when adjacent elements, or faces of adjacent 3D elements, have outward normal vectors that are more than the "edge angle" apart. This has the effect of inserting extra lines where the mesh changes shape, giving a better idea of the underlying surface.

The "edge angle" is the same as that used to denote sharp edges during smooth shading: see the Edge Angle notes in the section on lighting.

Free edge overlay

Feature line overlay

In this image purely topological free edges have been used. The image is understandable but a lot of detail is absent from areas of mesh of the same part id. In this image feature lines with an "edge angle" of 20 degrees have been used. This has resulted in lines appearing in previously empty areas: in particular the mouth and eyes have acquired some detail, and the arms are better defined.

Feature lines are logically ORed with free edges when selected, and they can be used both as overlay on shaded/contoured plots, and as the edging mode for wire/hidden plots. They normally involve more vectors than pure free edges, so they take longer to draw and may have an adverse effect on animation speed.

Modifying the topological definition of a "free" edge

The definition above is open to modification: blanking elements can create free edges, as can volume-clipping and the borders between element materials (or contact surfaces). You can control whether or not each of these categories apply by setting the following switches:

BLANKING -> EDGES Influence of Blanking on free edge display.

Normally blanking elements does not create "free" edges, but if you turn the BLANKING -> EDGES switch on then the edges created by blanking are shown. This effect is shown in the figure below

CLIPPING -> EDGES Influence of Volume-Clipping on free edge display

In the same way that element borders created by blanking do not, by default, create free edges; those exposed by volume clipping do not either. This is illustrated in the figure below where a clipping volume is used to remove solid elements from the same wall as in the previous example.

MATERIALS -> EDGES Influence of material boundaries on free edge display

The border between materials of the same element type does not, by default, qualify as a free edge. (Borders between different element types always qualify as edges.) If MATERIALS -> EDGES is turned on then such borders are treated as free edges. This is illustrated in the figure below.

SURFACES -> EDGES Influence of contact surface boundaries on free edge display

You can think of contact surface boundaries in the same way as material boundaries: the different surface ids equate to the different material ids, and the same edge definition logic applies.

However:

The default setting of the SURFACES -> EDGES switch is on. The reason for this is that it would be very unusual to have two adjacent contact surfaces forming a single (geometric) surface, but it is quite common to have two or more coincident surfaces. In the coincident case it is useful to see the edges.


Notes on FREE_FACE options:

  • Free edges are only computed for 2 and 3-D entities, that is: solids, thin shells, thick shells and contact surface facets.
  • Free edge computation is worked out separately for each element type. For example an edge common to a shell and a solid will still be treated as a free edge.
  • Where elements are deleted, due to material failure, free edges may be created at that and subsequent complete states.
  • A "line" mode plot with free edges only is the fastest way of drawing something in D3PLOT since it combines minimal computation with only a small amount of screen vectors. It is a good way of assembling a quick animation
  • Precomputed free-edge dynamic viewing is available: see Graphics modes during dynamic viewing .


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