Precision, Tolerances and Errors in Corner, Edge and Face Identification.
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Many calculations in SHAPE depend on finite absolute tolerances to decide whether corners are outside of any face, whether a corner derived from one triplet of faces is coincident with another, etc. Sometimes there are errors, for example extra corners, missing edges, or missing faces ( in the Shading or Face Fill options, or in 3D mode). Errors may occur either in the calculation of corners and edges, or during actual plotting
Situations which can cause problems:

1) Multiple edge/face intersections . When more than three edges (or more than three faces) intersect or nearly intersect at a point, the identification of the number of corners involved may be arbitrary. In some cases more than three faces intersect at a point and form a single true corner by symmetry - for example 3, 4, 6 or more faces in a pyramid in trigonal, tetragonal or hexagonal crystals. Tolerances must be large enough to identify these intersections correctly. In other cases the intersection of edges is fortuitous.

2) Low-angle face intersections . When faces are nearly parallel, the precision of locating intersections decreases.

3) Junctions of twin or epitaxial intergrowths . Special situations may arise in multiple crystals which do not occur for single crystals. There may be problems either during the calculation of corners and edges or during drawing.

Possible remedies:

1) The Remove Hidden Lines option in the Display menu controls the plotting of edges on one crystal which are hidden by another crystal.

2) Rotation . Missing lines of type 3) above can sometimes be restored by rotating a small amount, on the order of a degree or less.

3) Change of individual central distances . Problems of type 1) may often be cleared up by slightly changing the central distance of one of the forms involved, rather than all forms.

4) Change of all central distances . Since most of the tolerances are absolute (not relative), increasing all central distances by a common factor has the effect of reducing all tolerances by the inverse of this factor. There is an option for this in the Forms List dialog.

5) Change of criteria for showing edges in twins and epitaxial intergrowths. Certain types of edge may or may not be desirable to draw in intergrowths. The Edges in faces and Always show composition-plane edges options in the Preferences dialog control some such cases. Trial-and-error change of these parameters may give a more correct or more pleasing appearance.

6) Change of perspective distance . Increasing the Perspective distance sometimes clears up problems.

Note on units . Most distances used in SHAPE are in arbitrary units called central distance units (abbreviated cdu). These may be considered to correspond to any desired units on a real crystal, such as millimeters, centimeters, inches, etc. When fixed scale factors are used (Scaling dialog), it is necessary to specify the ratio of these units to the units (inches, centimeters or pixels) which are used to describe the screen or paper. Growth rate equations must be given in the appropriate units per time unit (also an arbitrary unit). Otherwise the absolute values of central distances have no effect - it is the relative values which determine crystal shape.