Shortening line with PyQGIS

Question

I tried to shorten a given LineGeometry with some Python code.

from qgis.utils import iface

layer = iface.activeLayer()
fs = layer.selectedFeatures()

for f in fs:
    fg = f.geometry()
    geomSingleType = QgsWkbTypes.isSingleType(fg.wkbType())

    if fg.type() == QgsWkbTypes.LineGeometry:
        if geomSingleType:
            x = fg.asPolyline()
            print("length: ", fg.length())
        else:
            x = fg.asMultiPolyline()
            print("MultiLine: ", x, "length: ", fg.length())

    newLine = fg.extendLine(0,(-2.5))
    print("length: ", newLine .length())

That returns a line with an equal length:

length:  11.995677929842055
length:  11.995677929842055

When I use a value > 0 the length is updated:

length:  11.995677929842055
length:  14.495677929927327

So I'm able to make a line longer, but I want to make it shorter.

Does anyone have any advice for me?

The code is part of a bigger project - so solutions with QGIS UI will not help me.

Answer 1

If you want to modify existing geometries (actually shorten line features) can use or modify the script below. You can test this on selected features of a line layer by running in the Python console but please make a backup copy of the layer first.

from qgis.utils import iface

l = iface.activeLayer()

#retrieve feature geometry and check its length
f = l.getFeature(1)
print(f.geometry().length())

def shorten_line(feat, length):
    '''Returns a linestring geometry shortened by the given length.
    Takes two arguments- a QgsFeature object and a double value'''
    g = feat.geometry()
    v = [v for v in g.vertices()]
    v.remove(v[-1])
    v.append(QgsPoint(g.interpolate(g.length()-length).asPoint()))
    n = QgsLineString(v)
    return QgsGeometry(n)
    

# example usage ! This will permanently modify the selected features in
# your active layer!
l.selectByIds([1])
with edit(l):
    for f in l.selectedFeatures():
        l.changeGeometry(f.id(), shorten_line(f, 100.00))

#retrieve feature geometry and check its new length
f = l.getFeature(1)
print(f.geometry().length())