If you need to choose symbols based on more complicated criteria than the built-in renderers will provide, you can write your own custom QgsFeatureRendererV2 subclass using Python. For example, the following Python code implements a simple renderer that alternates between odd and even symbols as point features are displayed:
class OddEvenRenderer(QgsFeatureRendererV2):
def __init__(self):
QgsFeatureRendererV2.__init__(self, "OddEvenRenderer")
self.evenSymbol = QgsMarkerSymbolV2.createSimple({})
self.evenSymbol.setColor(QColor("light gray"))
self.oddSymbol = QgsMarkerSymbolV2.createSimple({})
self.oddSymbol.setColor(QColor("black"))
self.n = 0
def clone(self):
return OddEvenRenderer()
def symbolForFeature(self, feature):
self.n = self.n + 1
if self.n % 2 == 0:
return self.evenSymbol
else:
return self.oddSymbol
def startRender(self, context...