Tutorial for LWPolylineΒΆ

The LWPolyline is defined as a single graphic entity, which differs from the old-style Polyline entity, which is defined as a group of sub-entities. LWPolyline display faster (in AutoCAD) and consume less disk space, it is a planar element, therefore all points in OCS as (x, y) tuples (LWPolyline.dxf.elevation is the z-axis value).

Create a simple polyline:

import ezdxf

doc = ezdxf.new("R2000")
msp = doc.modelspace()

points = [(0, 0), (3, 0), (6, 3), (6, 6)]
msp.add_lwpolyline(points)

doc.saveas("lwpolyline1.dxf")

Append multiple points to a polyline:

doc = ezdxf.readfile("lwpolyline1.dxf")
msp = doc.modelspace()

line = msp.query("LWPOLYLINE").first
if line is not None:
    line.append_points([(8, 7), (10, 7)])

doc.saveas("lwpolyline2.dxf")

The LWPOLYLINE entity always returns polyline points as 5-tuple (x, y, start_width, end_width, bulge), the start_width, end_width and bulge values are 0 if not present:

first_point = line[0]
x, y, start_width, end_width, bulge = first_point

Use the method points() as context manager to edit polyline points, this method was introduced because accessing single points was very slow in early versions of ezdxf, but now direct access by the index operator [] is very fast and using the context manager is not required anymore. The advantage of the context manager is the ability to use a user defined point format:

doc = ezdxf.readfile("lwpolyline2.dxf")
msp = doc.modelspace()

line = msp.query("LWPOLYLINE").first

with line.points("xyseb") as points:
    # points is a standard Python list
    # existing points are 5-tuples, but new points can be
    # set as (x, y, [start_width, [end_width, [bulge]]]) tuple
    # set start_width, end_width to 0 to be ignored (x, y, 0, 0, bulge).

    # delete last 2 points
    del points[-2:]
    # adding two points
    points.extend([(4, 7), (0, 7)])

doc.saveas("lwpolyline3.dxf")

Each line segment can have a different start- and end-width, if omitted start- and end-width is 0:

doc = ezdxf.new("R2000")
msp = doc.modelspace()

# point format = (x, y, [start_width, [end_width, [bulge]]])
# set start_width, end_width to 0 to be ignored (x, y, 0, 0, bulge).

points = [(0, 0, .1, .15), (3, 0, .2, .25), (6, 3, .3, .35), (6, 6)]
msp.add_lwpolyline(points)

doc.saveas("lwpolyline4.dxf")

The first point carries the start- and end-width of the first segment, the second point of the second segment and so on, the start- and end-width value of the last point is used for the closing segment if the polyline is closed else these values are ignored. Start- and end-width only works if the DXF attribute dxf.const_width is unset, to be sure delete it:

# no exception will be raised if const_width is already unset:
del line.dxf.const_width

LWPolyline can also have curved elements, they are defined by the Bulge value:

doc = ezdxf.new("R2000")
msp = doc.modelspace()

# point format = (x, y, [start_width, [end_width, [bulge]]])
# set start_width, end_width to 0 to be ignored (x, y, 0, 0, bulge).

points = [(0, 0, 0, .05), (3, 0, .1, .2, -.5), (6, 0, .1, .05), (9, 0)]
msp.add_lwpolyline(points)

doc.saveas("lwpolyline5.dxf")
../_images/LWPolyline5.PNG

The curved segment is drawn from the point which defines the bulge value to the following point, the curved segment is always an arc. The bulge value defines the ratio of the arc sagitta (segment height h) to half line segment length (point distance), a bulge value of 1 defines a semicircle. The curve is on the right side of the line for a bulge value > 0, and on the left side of the line for a bulge value < 0.

The user defined point format, default is xyseb:

  • x = x coordinate

  • y = y coordinate

  • s = start width

  • e = end width

  • b = bulge value

  • v = (x, y) as tuple

msp.add_lwpolyline([(0, 0, 0), (10, 0, 1), (20, 0, 0)], format="xyb")
msp.add_lwpolyline([(0, 10, 0), (10, 10, .5), (20, 10, 0)], format="xyb")
../_images/bulge.png