Tutorial for LWPolylineΒΆ

A lightweight polyline is defined as a single graphic entity. The LWPolyline differs from the old-style Polyline, which is defined as a group of subentities. LWPolyline display faster (in AutoCAD) and consume less disk space and RAM. LWPolylines are planar elements, therefore all coordinates have no value for the z axis.

Create a simple polyline:

import ezdxf

dwg = ezdxf.new('R2000')
msp = dwg.modelspace()

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

dwg.saveas("lwpolyline1.dxf")

Append points to a polyline:

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

line = msp.query('LWPOLYLINE')[0]  # take first LWPolyline
line.append_points([(8, 7), (10, 7)])

dwg.saveas("lwpolyline2.dxf")

Getting points always returns a 5-tuple (x, y, start_width, ent_width, bulge), start_width, end_width and bulge is 0 if not present (0 is the DXF default value if not present):

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

Use context manager to edit polyline:

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

line = msp.query('LWPOLYLINE')[0]  # take first LWPolyline

with line.points() 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).

    del points[-2:]  # delete last 2 points
    points.extend([(4, 7), (0, 7)])  # adding 2 other points
    # the same as one command
    # points[-2:] = [(4, 7), (0, 7)]
# implicit call of line.set_points(points) at context manager exit

dwg.saveas("lwpolyline3.dxf")

Each line segment can have a different start/end width, if omitted start/end width = 0:

dwg = ezdxf.new('R2000')
msp = dwg.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)

dwg.saveas("lwpolyline4.dxf")

The first vertex (point) carries the start/end width of the first segment, the second vertex of the second segment and so on, the start/end width value of the last vertex is ignored. Start/end width only works if the DXF attribute const_width is unset, to be sure delete it:

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

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

dwg = ezdxf.new('R2000')
msp = dwg.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)

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

The curved segment is drawn from the vertex with the defined bulge value to the following vertex, the curved segment is always a circle, The bulge defines the ratio of the arc sagitta (segment height h) to half line segment length (vertex distance), a bulge value of 1 defines a semicircle. bulge > 0 the curve is on the right side of the vertex connection line, bulge < 0 the curve is on the left side.

ezdxf v0.8.9 supports a user defined points 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/bulge1.png