Offset Line with Python in QGIS

Question

I wanted to know how could I do a offset line of one layer in a new layer.

I take my layers, even if I only have one active (streets)

canvas = iface.mapCanvas()
layers = canvas.layers()

In my case, layers is just one layer, and this layer have 600 lines. After that I don't know first create a new layer where put the result (the offsets) and the other problem create the offset ( -2 and 2). I saw other post trying to resolv my problem, but I tried to do that but didn't work.

I did that manually, but I want that in python.

Manually, I had this:

And after offset I got this:

But I tried to write several codes and all codes didn't work.

Answer 1

By using 'qgis:offsetline' of Processing Tool Box, you can do that with a few lines of code; as in following script.

import processing

layer = iface.activeLayer()

parameters = { 'DISTANCE' : 5, #offset of 5 meters
               'INPUT' : layer, 
               'JOIN_STYLE' : 1, 
               'MITER_LIMIT' : 1, 
               'OUTPUT' : 'TEMPORARY_OUTPUT', 
               'SEGMENTS' : 8 }

processing.runAndLoadResults('qgis:offsetline',
               parameters)

I ran above script with line layer (in red) of following image; where offset line was drawn in blue.

Answer 2

The Offset lines algorithm will do this for you. Have you considered using it instead of doing it in Python?

With Python, you can use the offsetCurve (double distance, int segments, JoinStyle joinStyle, double miterLimit) function for each feature geometry (https://qgis.org/api/classQgsGeometry.html#ad9508e6f1c6247ce6347f46740d61818).

To use it, you have to iterate over the features in your line layer and for each feature get its geometry and call the function:

afeature.geometry().offsetCurve(...)

How to use the offset line algorithm (GUI)

Offset lines - offset to one side (positive Distance):

Offset lines - offset to the other side (negative Distance):

Offset lines - result (input layer: black lines):