Vertex Expression
In order to specify a group of vertices in a polygon, the following three expressions are available.
The standard boolean connectives 'and', 'or' and 'not', and brackets can be used to construct a complex expression. The boolean connectives may be abbreviated as '&', '|', and '!', respectively.
comparison operators
Parts of a polygon can be selected using equality and inequality operators on the ID numbers assigned to each vertex; for example, id >= 100. Possible operators are =, ==, <>, !=, /=, <, <=, > and >=. When each vertex element has a user_data element, every attributes in the user_data element can be used to select vertices with equality and inequality operators.
within expression
A within expression selects vertices that exist within a specified distance from a set of atoms specified by the atom expression.
- within( <distance>, <atom_expression> )
Instead of a set of atoms, a single point coordinates, from which the distance is measured, can be specified. - within( <distance>, [x, y, z] )
- within( <distance>, {x, y, z} )
In case 2), vertices are selected according to their current coordinates displayed in the screen. On the other hand, coordinates written in the original polygon file are used in case 3).
box expression
A box expression selects vertices that exist in some cubes, where each cube is centered to each atom in a set of atoms specified by the atom expression. The syntax is similar to that of the within expression.
- box( <size>, <atom_expression> )
When a single point coordinates is given in the second argument, vertices that exist in a single cube, whose center is set to the passed coordinates, are selected. - box( <size>, [x, y, z] )
- box( <size>, {x, y, z} )
In every cases, the length of an edge of the cubes is twice as the specified value.