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[FEATURE][processing] New algorithm for topological coloring of polygons

This ports to old (pre 2.0!!) topocolor plugin to processing. It's based
off my beta 2.x fork (never publicly released) which implemented
a bunch of improvements to the algorithm allowing for minimal number
of required colors and also balanced counts of features assigned
each individual color.

** Pretty sure this plugin was highlighted in Victor's presentation
about plugins-which-shouldn't-be-plugins-and-should-be-processing-algs
instead. It's a prime example of a plugin where the amount of code
required for gui+setup exceeded the actual "guts" of the plugin by
a huge factor, and which is much more useful when it can be
integrated into a larger processing model.
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nyalldawson committed Feb 22, 2017
1 parent bde4ff9 commit 1cf0a20267de67d225061a351276e90b5c886fc8
@@ -544,6 +544,13 @@ qgis:symmetricaldifference: >
qgis:texttofloat: >
This algorithm modifies the type of a given attribute in a vector layer, converting a text attribute containing numeric strings into a numeric attribute.

qgis:topologicalcoloring: >
This algorithm assigns a color index to polygon features in such a way that no adjacent polygons share the same color index.

The algorithm attempts to assign colors so that the total number of colors required is minimized, whilst keeping the count of features assigned to each individual color index balanced.

The color index is saved to a new attribute named color_idx.

qgis:translate: >
This algorithm moves the geometries within a layer, by offsetting them with a specified x and y displacement.

@@ -595,3 +602,4 @@ qgis:fixgeometries: >
This algorithm attempts to create a valid representation of a given invalid geometry without losing any of the input vertices. Already-valid geometries are returned without further intervention. Always outputs multi-geometry layer.

NOTE: M values will be dropped from the output.

@@ -186,6 +186,7 @@
from .FixGeometry import FixGeometry
from .ExecuteSQL import ExecuteSQL
from .FindProjection import FindProjection
from .TopoColors import TopoColor

pluginPath = os.path.normpath(os.path.join(
os.path.split(os.path.dirname(__file__))[0], os.pardir))
@@ -255,7 +256,8 @@ def __init__(self):
ShortestPathPointToPoint(), ShortestPathPointToLayer(),
ShortestPathLayerToPoint(), ServiceAreaFromPoint(),
ServiceAreaFromLayer(), TruncateTable(), Polygonize(),
FixGeometry(), ExecuteSQL(), FindProjection()
FixGeometry(), ExecuteSQL(), FindProjection(),

if hasPlotly:
@@ -0,0 +1,218 @@
# -*- coding: utf-8 -*-

Date : February 2017
Copyright : (C) 2017 by Nyall Dawson
Email : nyall dot dawson at gmail dot com
* *
* This program is free software; you can redistribute it and/or modify *
* it under the terms of the GNU General Public License as published by *
* the Free Software Foundation; either version 2 of the License, or *
* (at your option) any later version. *
* *

__author__ = 'Nyall Dawson'
__date__ = 'February 2017'
__copyright__ = '(C) 2017, Nyall Dawson'

# This will get replaced with a git SHA1 when you do a git archive323

__revision__ = '$Format:%H$'

import os
import operator
from collections import defaultdict, deque

from qgis.core import (QgsField,

from qgis.PyQt.QtCore import (QVariant)

from processing.core.GeoAlgorithm import GeoAlgorithm
from processing.core.parameters import ParameterVector
from processing.core.outputs import OutputVector
from import dataobjects, vector

pluginPath = os.path.split(os.path.split(os.path.dirname(__file__))[0])[0]

class TopoColor(GeoAlgorithm):

def defineCharacteristics(self):, self.i18n_name = self.trAlgorithm('Topological coloring'), self.i18n_group = self.trAlgorithm('Cartographic tools')
self.tags ='topocolor,colors,graph,adjacent,assign')

self.addParameter(ParameterVector(self.INPUT_LAYER,'Input layer'), [dataobjects.TYPE_VECTOR_POLYGON]))

self.addOutput(OutputVector(self.OUTPUT_LAYER,'Colored'), datatype=[dataobjects.TYPE_VECTOR_POLYGON]))

def processAlgorithm(self, feedback):
layer = dataobjects.getObjectFromUri(

fields = layer.fields()
fields.append(QgsField('color_id', QVariant.Int))

writer = self.getOutputFromName(

# use a deque so we can drop features as we write them
# it's a bit friendlier on memory usage
features = deque(f for f in vector.features(layer))

topology, id_graph = self.compute_graph(features, feedback)
feature_colors = ColoringAlgorithm.balanced(topology, feedback)

max_colors = max(feature_colors.values())
feedback.pushInfo('{} colors required').format(max_colors))

total = 20.0 / len(features)
current = 0
while features:
input_feature = features.popleft()
output_feature = input_feature
attributes = input_feature.attributes()
if in feature_colors:

current += 1
feedback.setProgress(80 + int(current * total))

del writer

def compute_graph(features, feedback, create_id_graph=False):
""" compute topology from a layer/field """
s = Graph(sort_graph=False)
id_graph = None
if create_id_graph:
id_graph = Graph(sort_graph=True)

# skip features without geometry
features_with_geometry = dict((, f) for f in features if f.hasGeometry())

total = 70.0 / len(features_with_geometry)
index = QgsSpatialIndex()

i = 0
for feature_id, f in features_with_geometry.items():
engine = QgsGeometry.createGeometryEngine(f.geometry().geometry())

feature_bounds = f.geometry().boundingBox()
# grow bounds a little so we get touching features
feature_bounds.grow(feature_bounds.width() * 0.01)
intersections = index.intersects(feature_bounds)
for l2 in intersections:
f2 = features_with_geometry[l2]
if engine.intersects(f2.geometry().geometry()):
if id_graph:

i += 1
feedback.setProgress(int(i * total))

for feature_id, f in features_with_geometry.items():
if not feature_id in s.node_edge:
s.add_edge(feature_id, None)

return s, id_graph

class ColoringAlgorithm:

def balanced(graph, feedback):
feature_colors = {}
# start with 4 colors in pool
color_pool = set(range(1, 5))

# calculate count of neighbours
neighbour_count = defaultdict(int)
for feature_id, neighbours in graph.node_edge.items():
neighbour_count[feature_id] += len(neighbours)

# sort features by neighbour count - we want to handle those with more neighbours first
sorted_by_count = [feature_id for feature_id in sorted(neighbour_count.items(),
# counts for each color already assigned
color_counts = defaultdict(int)
for c in color_pool:
color_counts[c] = 0

total = 10.0 / len(sorted_by_count)
i = 0

for (feature_id, n) in sorted_by_count:
# first work out which already assigned colors are adjacent to this feature
adjacent_colors = set()
for neighbour in graph.node_edge[feature_id]:
if neighbour in feature_colors:

# from the existing colors, work out which are available (ie non-adjacent)
available_colors = color_pool.difference(adjacent_colors)

if len(available_colors) == 0:
# no existing colors available for this feature, so add new color to pool
feature_color = len(color_pool) + 1
# choose least used available color
counts = [(c, v) for c, v in color_counts.items() if c in available_colors]
feature_color = sorted(counts, key=operator.itemgetter(1))[0][0]
feature_colors[feature_id] = feature_color
color_counts[feature_color] += 1

i += 1
feedback.setProgress(70 + int(i * total))

return feature_colors

class Graph:

def __init__(self, sort_graph=True):
self.sort_graph = sort_graph
self.node_edge = {}

def add_edge(self, i, j):
ij = [i, j]
if self.sort_graph:
(i, j) = ij
if i in self.node_edge:
self.node_edge[i] = {j}

def make_full(self):
g = Graph(sort_graph=False)
for k in self.node_edge.keys():
for v in self.node_edge[k]:
g.add_edge(v, k)
g.add_edge(k, v)
return g
@@ -0,0 +1,41 @@

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