Understanding hatches algorithm
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Instead of reinventing the wheel, have you thought about using the platform services APIs?
CADnoob
There are two stages in the algorithm for generating a cross hatch pattern for a shape. The first stage is to determine closed geometry with no dangling edges. The geometry may have internal islands. The second stage is to consider a ray passing through the geometry in the direction of a hatch line and determining its intersections. For geometry without dangling edges, a hatch line is drawn from the first to the second intersection, and if present, from the 3rd to the 4th intersection, and the 5th to the 6th, etc. If there are an odd number of intersections for a ray, the hatch line is drawn from the first intersection to the last.
Creating a hatch when using the select object option bypasses the first stage and can handle open geometry as well as geometry with dangling edges but often with undesirable results as seen below.
The internal point option for the hatch command calculates closed boundaries for the overall outline of the shape as well as any islands. The result is the same you would get from using the boundary command. Invalid hatching does not occur for trimmed boundaries. The boundary command projects the specified internal point onto a line that can be reached without crossing another line. From there, the routine walks around the lines from intersection to intersection. Next step is to determine if islands are present and then calculate the boundaries for them. The hatch pattern offset is determined by the location of the active UCS.
