GIS, Open-Source

Generative Pseudo-Random Polygon Fill Patterns in QGIS

QGIS doesn’t support pseudo-random fill patterns out-of-the-box. However, using the Geometry Generator we can achieve the same effect.

Random fill pattern? Here’s a polygon with such a fill. These are useful if, say, we want to represent an area of intermittent water.

Screen Shot 2017-05-05 at 19.53.11
Clipped Grid with 100% randomness

A typical way to achieve this effect would be to generate a random fill texture and use a pattern fill. This is less computationally intensive to render, however QGIS cannot render a texture fill as a vector even in vector output formats, which means all vector fills will be rasterised. If that means nothing to you, don’t worry.

The typical way to generate a randomised fill pattern is firstly to draw a bounding box around the feature of interest, create a grid of points that covers the feature, then only retain those points that also intersect the feature of interest. In effect, the feature geometry is used as a clipping mask for a uniform grid.

For the points that remain after clipping, we can optionally add an amount of randomness to the X,Y value of each grid intersection between zero and the size of a grid element. With no randomness, of course we see the grid pattern.

The QGIS geometry generator comes in useful here as it can be fed the geometry of a feature, in this case a Polygon, that is fed to a PyQGIS script that returns a multipoint geometry which is then symbolised.

The Python code is below:

from qgis.core import *
from qgis.gui import *
import math
import random

"""
Define a grid based on the interval and the bounding box of
the feature. Grid will minimally cover the feature and be centre aligned

Create a multi-point geometry at the grid intersections where
the grid is enclosed by the feature - i.e. apply a clipping mask

Random value determines amount of randomness in X/Y within its
grid square a particular feature is allowed to have
"""
@qgsfunction(args='auto', group='Custom')
def fillGrid(xInterval, yInterval, rand, feature, parent):
  box = feature.geometry().boundingBox()

  #Create a grid that minimally covers the boundary
  #using the supplied intervals and centre it
  countX = math.ceil(box.width() / xInterval)
  countY = math.ceil(box.height() / yInterval)

  #Align the grid
  gridX = countX * xInterval
  gridY = countY * yInterval
  dX= gridX - box.width()
  dY= gridY - box.height()
  xMin = box.xMinimum() - (dX/2)
  yMin = box.yMinimum() - (dY/2)

  points = []
  #+1 to draw a symbol on the n+1th grid element
  for xOff in range(countX+1):
    for yOff in range(countY+1):

      ptX = xMin + xOff*(xInterval) + rand * random.uniform(0,xInterval)
      ptY = yMin + yOff*(yInterval) + rand * random.uniform(0,xInterval)

      pt = QgsPoint(ptX,ptY)
      point = QgsGeometry.fromPoint(pt)
      if feature.geometry().contains(point):
        points.append(pt)

  return QgsGeometry.fromMultiPoint(points)

Finally, in the symbology options for the layer, select ‘Geometry Generator’ as the fill for the layer, and call the Python function with values for X/Y intervals and the proportion (0-1) of randomness to add.

Screen Shot 2017-05-05 at 19.40.31

Note that the multipoint geometry returned is independent of the zoom level and also uses the co-ordinate reference system of the source layer, not the display layer. The eagle eyed may have noticed that in the screenshots above that a transformation appears to have been applied to the grid.

Indeed it has. The source layer is in WGS-84, while the map has a UTM projection.

Tackling scale and CRS dependence are topics for a later post.

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