VIEW HIDDEN Mode

                           

This command produces a view of the model with hidden lines (or hidden surfaces) removed. Several hidden view algorithms are provided, and their advantages are given in the table below.

Hidden views may be requested of the current sub-set of finite elements and in FEMVIEW of cut-away views and of multiple cross sections. Note that in FEMGEN , VIEW HIDDEN commands are only available if a mesh has been generated.

A hidden view may be turned off via the command VIEW HIDDEN OFF and in FEMVIEW it will be automatically turned off if certain other VIEW commands are issued.

Mode Comments

FILL [BACKGRND] This provides a hidden surface representation

[COLOUR] of the model using the overdrawing

[NORMAL] characteristics of a raster display. Polygons at the rear of the model are drawn first, and these are overwritten with polygons nearer to the viewer. The default is BACKGRND in which the model is colour filled in the current background colour. The option COLOUR will cause each part of the model to be filled with the colour currently assigned to it via the VIEW MESH part colour or VIEW GEOMETRY part colour command. The option NORMAL will cause a surface to be colour filled with red (white on monochrome display, light grey on greyscale display) if its surface normal is pointing away from the viewer and colour filled with blue (black on monochrome display, dark grey on greyscale display) if its surface normal is pointing towards the viewer. Currently the controls BACKGRND, COLOUR and NORMAL are available in FEMGEN only.

SHADE [Colour] This provides a shaded view of the model

[FACEWISE] using parallel light for illumination. The

[NODEWISE] quality of shading can be controlled (see

[SMOOTH] [`real'] below). The algorithm, and drawing technique

[HIGHLIGHT] [`real'] is the same as for `VIEW HIDDEN FILL', except when VIEW OPTIONS HIDDEN MESHFILL OFF is used in FEMGEN . (See note 5 for colours).

LINE This provides a hidden line view of the model in which each edge is clipped to display only those line segments which would be visible to the user at the current model rotation.

OFF Turns off hidden views (FEMGEN only).

 

Figure 2.132: Hidden line with shading

Notes:

1.

Control of the rendition with VIEW HIDDEN SHADE
With the VIEW HIDDEN SHADE FACEWISE command each element face will be displayed with a single shade dependent upon its average orientation to the current light sources. With the VIEW HIDDEN SHADE NODEWISE command each element face will be displayed with a range of shades dependent on the orientation of that face, in the region of each of its nodes, to the current light sources. Note that the picture will be as FACEWISE for planar faces.

With the VIEW HIDDEN SHADE SMOOTH [`real'] command each element face will be displayed with a range of shades dependent on the orientation of the model, averaged at each of its nodes, to the current light sources. An optional angle (default 30 degrees) can be specified to stop the averaging over adjacent faces.

The VIEW HIDDEN SHADE HIGHLIGHT [`real'] command is as for SMOOTH with the addition of reflection highlighting.

2.

Assignment of colour for hidden views
Colour can be assigned to the model in a variety of ways; for example, with the VIEW HIDDEN SHADE command or with the VIEW GEOMETRY part colour command. Colours can also be selected with the VIEW OPTIONS COLOUR command.

The FEMGEN command VIEW OPTIONS HIDDEN MESHFILL OFF will cause the program to ignore any colour assignment to mesh parts that are currently being displayed. The mesh elements will be coloured in the background colour as though the command VIEW HIDDEN FILL BACKGROUND had been issued. As colour assignments to geometry parts will be retained, this allows simultaneous display of VIEW HIDDEN FILL BACKGROUND and VIEW HIDDEN SHADE on different parts of the model.

3.

Producing pictures faster
With a graphics terminal that is capable of fast polygon fill, the VIEW HIDDEN FILL command (rather than VIEW HIDDEN LINE) is the fastest way of achieving a hidden view. There is very little overhead in requesting a shaded picture with VIEW HIDDEN SHADE.

Hidden views take longer to produce if a shrink factor has been applied. Normally, internal surfaces and bodies can be ignored when producing a hidden view, but if a shrink factor is applied, more of the model becomes visible and so more data must be processed. Shrink applied to geometric parts will incur a small penalty and shrink applied to mesh will incur a large time penalty. For this reason, users with large models are advised to experiment with the content, lighting and eye position of a picture with the shrink factor off, and then apply the shrink factor only when everything else in the picture is ready.

4.

Choice of algorithm for hidden views
A choice of a quick or more rigorous algorithm for use with the above hidden views can be selected via the VIEW OPTIONS HIDDEN command.

5.

Shaded views
The default condition is that a shaded view using in general six shades of blue will be drawn. On graphics displays with sufficient colours the user may specify a `Colour', either WHITE, RED, ORANGE, YELLOW, GREEN, BLUE or VIOLET; in which case a shaded view using in general fourteen shades of the specified colour will be drawn.

Multiple light sources are available, see VIEW OPTION LIGHT.

6.

Hard copy on pen plotters
For hard copy on a pen plotter clipped lines will be required and VIEW HIDDEN LINE would have to be used. (Even if VIEW HIDDEN FILL or VIEW HIDDEN SHADE is selected for screen display, a clipped line picture will none-the-less be sent to the plot file).

Examples:

1.

VIEW HIDDEN FILL NORMAL

Produces a hidden view of the model where each element face is colour filled in red or blue depending on whether its normal is pointing towards or away from the viewer.

2.

VIEW HIDDEN SHADE RED

Requests a red, shaded, hidden view of the model.

See also the following commands

'VIEW OPTIONS COLOUR'
'VIEW OPTIONS EDGES'
'VIEW OPTIONS HIDDEN'
'VIEW OPTIONS LIGHT'
'VIEW OPTIONS SHRINK'