How to draw perpendicular/Parallel line using pyQGIS?

Question

While digitizing a line with multiple segment, how to create a particular segment parallel to (or) Perpendicular to another feature using pyQgis?

Answer 1

It is a problem of analytical geometry and you can use vector algebra or the direction cosines, for example.

• for perpendicular lines, a solution is given in How to draw perpendicular lines in QGIS?

• for parallel lines, you can use the solution of Draw a parallel line (normalized offset)

  def pair(list):
      '''Iterate over pairs in a list -> iterate over pairs of segments of a line '''
      for i in range(1, len(list)):
          yield list[i-1], list[i]
  
  import math
  # original line
  layer = qgis.utils.iface.activeLayer()
  # iterate over segments of the line
  for elem in layer.getFeatures():  
       line = elem.geometry().asPolyline()
       for seg_start, seg_end in pair(line):
            x1,y1 = QgsPoint(seg_start)
            x2,y2 = QgsPoint(seg_end)
            length = math.sqrt(line_start.sqrDist(line_end))
            x1p = x1 + 1500 * ((y2-y1) / length)
            x2p = x2 + 1500 * ((y2-y1) / length)
            y1p = y1 + 1500 * ((x1-x2) / length)
            y2p = y2 + 1500 * ((x1-x2) / length)
            result= QgsGeometry.fromPolyline([ QgsPoint(x1p,y1p),QgsPoint(x2p,y2p)])
  

Result (original polyline in red, and if you preserve the original length of the segments, the resulting parallel segments, in green, intersects )

• you can also use the direction cosines of the segments, starting from an original point: if the lines are parallels, they have the same orientation/direction (azimuth in PyQGIS):

  def cosdir(azim):
     az = math.radians(azim)
     cosa = math.sin(az)
     cosb = math.cos(az)
     return cosa,cosb
  
  # original point
  point = QgsPoint(147352.43, 94305.21)
  for elem in layer.getFeatures():  
  line = elem.geometry().asPolyline()
  for seg_start, seg_end in pair(line):
      line_start = QgsPoint(seg_start)
      line_end = QgsPoint(seg_end) 
      length = math.sqrt(line_start.sqrDist(line_end))
      # direction cosines from the azimuth
      cosa, cosb = cosdir(line_start.azimuth(line_end))  
      # generate the points  in the same direction    
      resulting_point = QgsPoint(point.x()+(length*cosa), point.y()+(length*cosb))
      result= QgsGeometry.fromPolyline([point,resulting_point])
      point = resulting_point