Pattern Definition

Defining a pattern is a relatively simple matter once you understand the method. Each pattern is made up a a number of line segments which are repeated over the area to be hatched. To define a pattern you must first decompose the pattern into a set of line segments which, when repeated at certain intervals will produce a recognisable pattern.

Lines beginning with "*" define the start of a pattern definition. The start of a pattern definition line contains 2 fields delimited by a comma `,' with the first field being the pattern name which is limited to 16 characters in length and the second field being a more descriptive name for the pattern.

Each pattern definition may have at most 500 pattern lines with each pattern line having at most 6 dash specifiers. Each pattern definition line will have at least 5 fields delimited by commas `,' and at most 11 fields.

The fields in each pattern definition line contain the following data.

angle, xorigin, yorigin, xoffset, 
yoffset, dash1, dash2, ..., dash6

All dimensions are typically expressed in centimetres but are really unitless as a scale factor must be defined when the entity fill attributes are defined to result in the correct size of the pattern.

Field	Contains

angle	The orientation of the line. Valid values are 0 to 360. 0 is a horizontal line 90 is a vertical line going up the page 270 is a vertical line going down the page
xorigin, yorigin	Together these define the start of the position of this line in the pattern. Start positions are generally around the 0,0 coordinates. The pattern is displaced to the required position when used by the plotting module.
xoffset yoffset	Together these define the displacement, in directions relative to the hatch line itself, to the start position of the next parallel line segment in the hatch pattern. The xoffset is the displacement along the hatch line. The yoffset is the displacement perpendicular to the hatch line, i.e. the spacing between the hatch lines.
dash1, dash2, etc.	These dash values define the pattern to be used for the hatch line. Positive values indicate the length of the line segment which is to be drawn. Negative values indicate the length of the line segment which is to be left blank. The sum of all the dash lengths, disregarding the sign, defines the pattern repeat length. No dashes is valid and indicates that a solid line will be drawn. Xoffset values are meaningless if there is no dash pattern.

A simple example patterns.ssi file is shown below with the actual pattern which it defines shown below.

*angle,Angle steel 
0, 0,0, 0,.275, .2,-.075 
90, 0,0, 0,.275, .2,-.075