Get Line Elements

Q How do I get line elements using MicroStation Python?

Q How do I get a zero-length line element using MicroStation Python?

A Here is a Python example that gets line elements from a DGN model. It tests for zero-length lines while building a list of PersistentElementRef.

Introduction

Line elements are often used in MicroStation. Lines have two flavours: simple lines and line-strings. A simple line has two vertices, the start and end points. A line-string has many vertices, from two to five thousand. If you want to get a list of vertices from a line-string, read Get Line Vertices.

Line elements are sometimes used to represent a point. In that case the start and end points are coincident, and the line has zero-length.

Although a point element is useful, it's often the case that after use they should be deleted from the DGN model. As a point is sometimes hard to see, especially in a busy part of a design, it's useful to have a utility that seeks points. This example shows how to harvest a list of zero-length lines.

Python Implementation

Get List of Elements

First, get a list of any element from a DGN model …

elements = dgnModel.GetGraphicElements()

Get List of Line Elements

Next, use a Python list comprehension to get a list of lines. That calls the following function IsLine to test if an element is a line element …

def IsLine (elemRef: PersistentElementRef, dgnModel: DgnModel):
    """ Test whether a DGN element is a line. """
    # Create an element handle and get its Handler
    eh = ElementHandle (elemRef, dgnModel)
    handler = eh.GetHandler()
    # Compare this handler to LineHandler class
    if isinstance (handler, LineHandler):
        # Found a line
        return True
    #else
    return False

lines = [elemRef for elemRef in dgnModel.GetGraphicElements() if IsLine(elemRef, dgnModel)]

Get List of Zero-Length Line Elements

Finally, use a Python list comprehension to get a list of zero-length lines. That calls the following function IsZeroLengthLine to test if an element is a line element that has zero length …

def IsZeroLengthLine (elemRef: PersistentElementRef, dgnModel: DgnModel, epsilon: float = 1e-8):
    """ Test whether a DGN element is a zero-length line.
    In this example we want elements of a specified type: a DGN line element of zero-length."""

    eh = ElementHandle (elemRef, dgnModel)
    handler = eh.GetHandler()
    if isinstance (handler, LineHandler):
        # Found a line: test for zero-length
        curve = ICurvePathQuery.ElementToCurveVector (eh)
        lineLength = curve.Length()
        msg = f"Line [{elemRef.GetElementId()}] length {lineLength}"
        MessageCenter.ShowDebugMessage (msg, msg, False)
        # Test the line length against some small quantity epsilon
        return lineLength < epsilon
    #else
    return False

zeroLengthlines = [elemRef for elemRef in dgnModel.GetGraphicElements() if IsZeroLengthLine(elemRef, dgnModel)]

Why perform test lineLength < epsilon and not lineLength == 0.0? When analysing floating-point numbers, we need to beware rounding and other issues.

Once you have a list of elements, it's straightforward to remove them.

Usage

Now you can check to see if your DGN model contains zero-length lines. Watch the messages in the MicroStation Messager Center.

Unpack the ZIP file and copy the Python file into a folder that MicroStation knows about.

Use MicroStation's Python Manager to find and execute the script.

Questions

Post questions about MicroStation programming to the MicroStation Programming Forum.