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Usare i Vettori

Questa sezione riassume le varie azioni che si possono eseguire con i vettori.

Retrieving information about attributes

You can retrieve information about the fields associated with a vector layer by calling pendingFields() on a QgsVectorLayer instance:

# "layer" is a QgsVectorLayer instance
for field in layer.pendingFields():
    print field.name(), field.typeName()

Nota

Starting from QGIS 2.12 there is also a fields() in QgsVectorLayer which is an alias to pendingFields().

Selecting features

In QGIS desktop, features can be selected in different ways, the user can click on a feature, draw a rectangle on the map canvas or use an expression filter. Selected features are normally highlighted in a different color (default is yellow) to draw user’s attention on the selection. Sometimes can be useful to programmatically select features or to change the default color.

To change the selection color you can use setSelectionColor() method of QgsMapCanvas as shown in the following example:

iface.mapCanvas().setSelectionColor( QColor("red") )

To add add features to the selected features list for a given layer, you can call setSelectedFeatures() passing to it the list of features IDs:

# Get the active layer (must be a vector layer)
layer = iface.activeLayer()
# Get the first feature from the layer
feature = layer.getFeatures().next()
# Add this features to the selected list
layer.setSelectedFeatures([feature.id()])

To clear the selection, just pass an empty list:

layer.setSelectedFeatures([])

Iterare un Vettore.

Iterating over the features in a vector layer is one of the most common tasks. Below is an example of the simple basic code to perform this task and showing some information about each feature. the layer variable is assumed to have a QgsVectorLayer object

iter = layer.getFeatures()
for feature in iter:
    # retrieve every feature with its geometry and attributes
    # fetch geometry
    geom = feature.geometry()
    print "Feature ID %d: " % feature.id()

    # show some information about the feature
    if geom.type() == QGis.Point:
        x = geom.asPoint()
        print "Point: " + str(x)
    elif geom.type() == QGis.Line:
        x = geom.asPolyline()
        print "Line: %d points" % len(x)
    elif geom.type() == QGis.Polygon:
        x = geom.asPolygon()
        numPts = 0
        for ring in x:
            numPts += len(ring)
        print "Polygon: %d rings with %d points" % (len(x), numPts)
    else:
        print "Unknown"

    # fetch attributes
    attrs = feature.attributes()

    # attrs is a list. It contains all the attribute values of this feature
    print attrs

Accessing attributes

Attributes can be referred to by their name.

print feature['name']

Alternatively, attributes can be referred to by index. This is will be a bit faster than using the name. For example, to get the first attribute:

print feature[0]

Iterare le caratteristiche selezionate

if you only need selected features, you can use the selectedFeatures() method from vector layer:

selection = layer.selectedFeatures()
print len(selection)
for feature in selection:
    # do whatever you need with the feature

Another option is the Processing features() method:

import processing
features = processing.features(layer)
for feature in features:
    # do whatever you need with the feature

By default, this will iterate over all the features in the layer, in case there is no selection, or over the selected features otherwise. Note that this behavior can be changed in the Processing options to ignore selections.

Iterare un sottoinsieme di caratteristiche

Nel caso si voglia iterare su un sottoinsieme di geometrie in un vettore, ad esempio quelle di un’area specifica, si deve aggiungere l’oggetto QgsFeatureRequest alla chiamata getFeatures(). Di seguito un esempio

request = QgsFeatureRequest()
request.setFilterRect(areaOfInterest)
for feature in layer.getFeatures(request):
    # do whatever you need with the feature

If you need an attribute-based filter instead (or in addition) of a spatial one like shown in the example above, you can build an QgsExpression object and pass it to the QgsFeatureRequest constructor. Here’s an example

# The expression will filter the features where the field "location_name" contains
# the word "Lake" (case insensitive)
exp = QgsExpression('location_name ILIKE \'%Lake%\'')
request = QgsFeatureRequest(exp)

See Espressioni, Filtraggio e Calcolo di Valori for the details about the syntax supported by QgsExpression.

The request can be used to define the data retrieved for each feature, so the iterator returns all features, but returns partial data for each of them.

# Only return selected fields
request.setSubsetOfAttributes([0,2])
# More user friendly version
request.setSubsetOfAttributes(['name','id'],layer.pendingFields())
# Don't return geometry objects
request.setFlags(QgsFeatureRequest.NoGeometry)

Suggerimento

If you only need a subset of the attributes or you don’t need the geometry information, you can significantly increase the speed of the features request by using QgsFeatureRequest.NoGeometry flag or specifying a subset of attributes (possibly empty) like shown in the example above.

Modificare i Vettori

Most vector data providers support editing of layer data. Sometimes they support just a subset of possible editing actions. Use the capabilities() function to find out what set of functionality is supported

caps = layer.dataProvider().capabilities()
# Check if a particular capability is supported:
caps & QgsVectorDataProvider.DeleteFeatures
# Print 2 if DeleteFeatures is supported

For a list of all available capabilities, please refer to the API Documentation of QgsVectorDataProvider

To print layer’s capabilities textual description in a comma separated list you can use capabilitiesString() as in the following example:

caps_string = layer.dataProvider().capabilitiesString()
# Print:
# u'Add Features, Delete Features, Change Attribute Values,
# Add Attributes, Delete Attributes, Create Spatial Index,
# Fast Access to Features at ID, Change Geometries,
# Simplify Geometries with topological validation'

By using any of the following methods for vector layer editing, the changes are directly committed to the underlying data store (a file, database etc). In case you would like to do only temporary changes, skip to the next section that explains how to do modifications with editing buffer.

Nota

If you are working inside QGIS (either from the console or from a plugin), it might be necessary to force a redraw of the map canvas in order to see the changes you’ve done to the geometry, to the style or to the attributes:

# If caching is enabled, a simple canvas refresh might not be sufficient
# to trigger a redraw and you must clear the cached image for the layer
if iface.mapCanvas().isCachingEnabled():
    layer.setCacheImage(None)
else:
    iface.mapCanvas().refresh()

Aggiungi Geometrie

Create some QgsFeature instances and pass a list of them to provider’s addFeatures() method. It will return two values: result (true/false) and list of added features (their ID is set by the data store).

To set up the attributes you can either initialize the feature passing a QgsFields instance or call initAttributes() passing the number of fields you want to be added.

if caps & QgsVectorDataProvider.AddFeatures:
    feat = QgsFeature(layer.pendingFields())
    feat.setAttributes([0, 'hello'])
    # Or set a single attribute by key or by index:
    feat.setAttribute('name', 'hello')
    feat.setAttribute(0, 'hello')
    feat.setGeometry(QgsGeometry.fromPoint(QgsPoint(123, 456)))
    (res, outFeats) = layer.dataProvider().addFeatures([feat])

Elimina Geometrie

To delete some features, just provide a list of their feature IDs

if caps & QgsVectorDataProvider.DeleteFeatures:
    res = layer.dataProvider().deleteFeatures([5, 10])

Modifica Geometrie

It is possible to either change feature’s geometry or to change some attributes. The following example first changes values of attributes with index 0 and 1, then it changes the feature’s geometry

fid = 100   # ID of the feature we will modify

if caps & QgsVectorDataProvider.ChangeAttributeValues:
    attrs = { 0 : "hello", 1 : 123 }
    layer.dataProvider().changeAttributeValues({ fid : attrs })

if caps & QgsVectorDataProvider.ChangeGeometries:
    geom = QgsGeometry.fromPoint(QgsPoint(111,222))
    layer.dataProvider().changeGeometryValues({ fid : geom })

Suggerimento

If you only need to change geometries, you might consider using the QgsVectorLayerEditUtils which provides some of useful methods to edit geometries (translate, insert or move vertex etc.)

Aggiungere e Rimuovere Campi

To add fields (attributes), you need to specify a list of field definitions. For deletion of fields just provide a list of field indexes.

if caps & QgsVectorDataProvider.AddAttributes:
    res = layer.dataProvider().addAttributes([QgsField("mytext", QVariant.String), QgsField("myint", QVariant.Int)])

if caps & QgsVectorDataProvider.DeleteAttributes:
    res = layer.dataProvider().deleteAttributes([0])

After adding or removing fields in the data provider the layer’s fields need to be updated because the changes are not automatically propagated.

layer.updateFields()

Modificare i Vettori con un Buffer di Modifica

Quando modifichi i vettori contenti in QGIS, devi prima attivare la modalità di modifica per un particolare layer, successivamente eseguire alcune modifiche e alla fine applicare (o annullare) i cambiamenti. Tutte le modifiche che fai non vengono scritte fino a quando non le applichi — si trovano in un buffer di modifica in memoria del vettore. È possibile usare questa funzionalità anche programmaticamente — è solo un altro metodo per modificare il vettore che è complementare all’uso diretto delle sorgenti dati. Usa questa opzione quando si forniscono alcuni strumenti dell’interfaccia grafica per la modifica del vettore, siccome questo permetterà all’utente di applicare/annullare e permetter l’uso di annulla/ripristina. Quando si applicano le modifiche, tutte le modifiche dal buffer di modifica sono salvate nella sorgente dati.

To find out whether a layer is in editing mode, use isEditable() — the editing functions work only when the editing mode is turned on. Usage of editing functions

# add two features (QgsFeature instances)
layer.addFeatures([feat1,feat2])
# delete a feature with specified ID
layer.deleteFeature(fid)

# set new geometry (QgsGeometry instance) for a feature
layer.changeGeometry(fid, geometry)
# update an attribute with given field index (int) to given value (QVariant)
layer.changeAttributeValue(fid, fieldIndex, value)

# add new field
layer.addAttribute(QgsField("mytext", QVariant.String))
# remove a field
layer.deleteAttribute(fieldIndex)

In order to make undo/redo work properly, the above mentioned calls have to be wrapped into undo commands. (If you do not care about undo/redo and want to have the changes stored immediately, then you will have easier work by editing with data provider.) How to use the undo functionality

layer.beginEditCommand("Feature triangulation")

# ... call layer's editing methods ...

if problem_occurred:
  layer.destroyEditCommand()
  return

# ... more editing ...

layer.endEditCommand()

The beginEditCommand() will create an internal “active” command and will record subsequent changes in vector layer. With the call to endEditCommand() the command is pushed onto the undo stack and the user will be able to undo/redo it from GUI. In case something went wrong while doing the changes, the destroyEditCommand() method will remove the command and rollback all changes done while this command was active.

To start editing mode, there is startEditing() method, to stop editing there are commitChanges() and rollBack() — however normally you should not need these methods and leave this functionality to be triggered by the user.

You can also use the with edit(layer)-statement to wrap commit and rollback into a more semantic code block as shown in the example below:

with edit(layer):
  f = layer.getFeatures().next()
  f[0] = 5
  layer.updateFeature(f)

This will automatically call commitChanges() in the end. If any exception occurs, it will rollBack() all the changes. In case a problem is encountered within commitChanges() (when the method returns False) a QgsEditError exception will be raised.

Usare l’Indice Spaziale

Spatial indexes can dramatically improve the performance of your code if you need to do frequent queries to a vector layer. Imagine, for instance, that you are writing an interpolation algorithm, and that for a given location you need to know the 10 closest points from a points layer, in order to use those point for calculating the interpolated value. Without a spatial index, the only way for QGIS to find those 10 points is to compute the distance from each and every point to the specified location and then compare those distances. This can be a very time consuming task, especially if it needs to be repeated for several locations. If a spatial index exists for the layer, the operation is much more effective.

Think of a layer without a spatial index as a telephone book in which telephone numbers are not ordered or indexed. The only way to find the telephone number of a given person is to read from the beginning until you find it.

Spatial indexes are not created by default for a QGIS vector layer, but you can create them easily. This is what you have to do:

  • create spatial index — the following code creates an empty index
index = QgsSpatialIndex()
  • add features to index — index takes QgsFeature object and adds it

    to the internal data structure. You can create the object manually or use one from previous call to provider’s nextFeature()

index.insertFeature(feat)
  • once spatial index is filled with some values, you can do some queries
# returns array of feature IDs of five nearest features
nearest = index.nearestNeighbor(QgsPoint(25.4, 12.7), 5)

# returns array of IDs of features which intersect the rectangle
intersect = index.intersects(QgsRectangle(22.5, 15.3, 23.1, 17.2))

Scrivere Vettori

Puoi scrivere file vettoriali usando la classe QgsVectorFileWriter. Essa supporta qualsiasi altro tipo di file vettoriale che supporta OGR (shapefile, GeoJSON, KML e altri).

Ci sono due possibilità per esportare un vettore:

  • from an instance of QgsVectorLayer
  error = QgsVectorFileWriter.writeAsVectorFormat(layer, "my_shapes.shp", "CP1250", None, "ESRI Shapefile")

  if error == QgsVectorFileWriter.NoError:
      print "success!"

  error = QgsVectorFileWriter.writeAsVectorFormat(layer, "my_json.json", "utf-8", None, "GeoJSON")
  if error == QgsVectorFileWriter.NoError:
      print "success again!"

The third parameter specifies output text encoding. Only some drivers need this
for correct operation - shapefiles are one of those --- however in case you
are not using international characters you do not have to care much about
the encoding. The fourth parameter that we left as ``None`` may specify
destination CRS --- if a valid instance of :class:`QgsCoordinateReferenceSystem`
is passed, the layer is transformed to that CRS.

For valid driver names please consult the `supported formats by OGR`_ --- you
should pass the value in the "Code" column as the driver name. Optionally
you can set whether to export only selected features, pass further
driver-specific options for creation or tell the writer not to create
attributes --- look into the documentation for full syntax.
  • directly from features
# define fields for feature attributes. A QgsFields object is needed
fields = QgsFields()
fields.append(QgsField("first", QVariant.Int))
fields.append(QgsField("second", QVariant.String))

# create an instance of vector file writer, which will create the vector file.
# Arguments:
# 1. path to new file (will fail if exists already)
# 2. encoding of the attributes
# 3. field map
# 4. geometry type - from WKBTYPE enum
# 5. layer's spatial reference (instance of
#    QgsCoordinateReferenceSystem) - optional
# 6. driver name for the output file
writer = QgsVectorFileWriter("my_shapes.shp", "CP1250", fields, QGis.WKBPoint, None, "ESRI Shapefile")

if writer.hasError() != QgsVectorFileWriter.NoError:
    print "Error when creating shapefile: ",  w.errorMessage()

# add a feature
fet = QgsFeature()
fet.setGeometry(QgsGeometry.fromPoint(QgsPoint(10,10)))
fet.setAttributes([1, "text"])
writer.addFeature(fet)

# delete the writer to flush features to disk
del writer

Provider in Memoria

Il provider in memoria è previsto per essere usato principalmente da sviluppatori di plugin o applicazioni di terze parti. Esso non memorizza dati sul disco, permettendo agli sviluppatori di usarlo come backend veloce per alcuni layer temporanei.

Il provider supporta campi di tipo string, int e double.

Il provider in memoria supporta anche l’indicizzazione spaziale, che si abilita richiamando la funzione createSpatialIndex() del provider. Una volta creato l’indice spaziale sarai in grado di iterare più velocemente sugli elementi contenuti in regioni più piccole (poiché non è necessario scorrere tutti gli elementi, ma solo quelli nel rettangolo specificato).

Un provider in memoria è creato passando "memory" come stringa sorgente al costruttore QgsVectorLayer.

Il costruttore inoltre accetta un URI che definisce il tipo di geometria del vettore, una tra: "Point", "LineString", "Polygon", "MultiPoint", "MultiLineString", o "MultiPolygon".

L’URI può anche specificare il sistema di riferimento delle coordinate, i campi e l’indicizzazione del provider in memoria nell’URI. La sintassi è:

crs=definizione

Specifica il sistema di riferimento delle coordinate dove la definizione può essere una qualsiasi delle forme accettate da QgsCoordinateReferenceSystem.createFromString()

index=yes

Specifica che il provider userà un indice spaziale

field=nome:tipo(lunghezza,precisione)

Specifica un attributo del vettore. L’attributo ha un nome, e facoltativamente un tipo (intero, double, o string), lunghezza, e precisione. Ci sono possono essere definizioni di campo multiple

The following example of a URI incorporates all these options

"Point?crs=epsg:4326&field=id:integer&field=name:string(20)&index=yes"

The following example code illustrates creating and populating a memory provider

# create layer
vl = QgsVectorLayer("Point", "temporary_points", "memory")
pr = vl.dataProvider()

# add fields
pr.addAttributes([QgsField("name", QVariant.String),
                    QgsField("age",  QVariant.Int),
                    QgsField("size", QVariant.Double)])
vl.updateFields() # tell the vector layer to fetch changes from the provider

# add a feature
fet = QgsFeature()
fet.setGeometry(QgsGeometry.fromPoint(QgsPoint(10,10)))
fet.setAttributes(["Johny", 2, 0.3])
pr.addFeatures([fet])

# update layer's extent when new features have been added
# because change of extent in provider is not propagated to the layer
vl.updateExtents()

Finally, let’s check whether everything went well

# show some stats
print "fields:", len(pr.fields())
print "features:", pr.featureCount()
e = layer.extent()
print "extent:", e.xMiniminum(), e.yMinimum(), e.xMaximum(), e.yMaximum()

# iterate over features
f = QgsFeature()
features = vl.getFeatures()
for f in features:
    print "F:", f.id(), f.attributes(), f.geometry().asPoint()

Apparenza (Simbologia) dei Vettori

Quando un vettore deve essere visualizzato, l’aspetto dei dati è dato dal visualizzatore e dai simboli associati al vettore. I simboli sono classi che si occupano del disegno di rappresentazione visiva delle geometrie, mentre i visualizzatori determinano quale simbolo sarà usato per una particolare geometria.

Il visualizzatore per un dato vettore può essere ottenuto come mostrato sotto:

renderer = layer.rendererV2()

And with that reference, let us explore it a bit

print "Type:", rendererV2.type()

Ci sono vari tipi di visualizzatori noti disponibili nella libreria di base di QGIS:

Tipo

Classe

Descrizione

singleSymbol QgsSingleSymbolRendererV2

Visualizza tutte le geometria con lo stesso simbolo

categorizedSymbol QgsCategorizedSymbolRendererV2

Visualizza le geometria usando un simbolo diverso per ogni categoria

graduatedSymbol QgsGraduatedSymbolRendererV2

Visualizza le geometrie usando un simbolo diverso per ogni intervallo di valori

There might be also some custom renderer types, so never make an assumption there are just these types. You can query QgsRendererV2Registry singleton to find out currently available renderers:

print QgsRendererV2Registry.instance().renderersList()
# Print:
[u'singleSymbol',
u'categorizedSymbol',
u'graduatedSymbol',
u'RuleRenderer',
u'pointDisplacement',
u'invertedPolygonRenderer',
u'heatmapRenderer']

It is possible to obtain a dump of a renderer contents in text form — can be useful for debugging

print rendererV2.dump()

Single Symbol Renderer

Puoi ottenere il simbolo usato per la visualizzazione chiamando il metodo symbol() e cambiarlo con il metodo setSymbol() (nota per gli sviluppatori C++: il visualizzatore assume la proprietà del simbolo.)

You can change the symbol used by a particular vector layer by calling setSymbol() passing an instance of the appropriate symbol instance. Symbols for point, line and polygon layers can be created by calling the createSimple() function of the corresponding classes QgsMarkerSymbolV2, QgsLineSymbolV2 and QgsFillSymbolV2.

The dictionary passed to createSimple() sets the style properties of the symbol.

For example you can replace the symbol used by a particular point layer by calling setSymbol() passing an instance of a QgsMarkerSymbolV2 as in the following code example:

symbol = QgsMarkerSymbolV2.createSimple({'name': 'square', 'color': 'red'})
layer.rendererV2().setSymbol(symbol)

name indicates the shape of the marker, and can be any of the following:

  • circle
  • square
  • cross
  • rectangle
  • diamond
  • pentagon
  • triangle
  • equilateral_triangle
  • star
  • regular_star
  • arrow
  • filled_arrowhead
  • x

To get the full list of properties for the first symbol layer of a simbol instance you can follow the example code:

print layer.rendererV2().symbol().symbolLayers()[0].properties()
# Prints
{u'angle': u'0',
u'color': u'0,128,0,255',
u'horizontal_anchor_point': u'1',
u'name': u'circle',
u'offset': u'0,0',
u'offset_map_unit_scale': u'0,0',
u'offset_unit': u'MM',
u'outline_color': u'0,0,0,255',
u'outline_style': u'solid',
u'outline_width': u'0',
u'outline_width_map_unit_scale': u'0,0',
u'outline_width_unit': u'MM',
u'scale_method': u'area',
u'size': u'2',
u'size_map_unit_scale': u'0,0',
u'size_unit': u'MM',
u'vertical_anchor_point': u'1'}

This can be useful if you want to alter some properties:

# You can alter a single propery...
layer.rendererV2().symbol().symbolLayer(0).setName('square')
# ... but not all properties are accessible from methods,
# you can also replace the symbol completely:
props = layer.rendererV2().symbol().symbolLayer(0).properties()
props['color'] = 'yellow'
props['name'] = 'square'
layer.rendererV2().setSymbol(QgsMarkerSymbolV2.createSimple(props))

Categorized Symbol Renderer

Puoi interrogare e impostare il nome dell’attributo che è usato per la classificazione: usa i metodi classAttribute() e setClassAttribute().

To get a list of categories

for cat in rendererV2.categories():
    print "%s: %s :: %s" % (cat.value().toString(), cat.label(), str(cat.symbol()))

Dove value() è il valore usato per la distinzione tra le categorie, label() è un testo usato per la descrizione della categoria e il metodo symbol() restituisce il simbolo assegnato.

Il visualizzatore di solito memorizza anche il simbolo di origine e la scala di colori utilizzati per la classificazione: metodi sourceColorRamp() e sourceSymbol().

Graduated Symbol Renderer

Questo visualizzatore è molto simile al visualizzatore simbolo categorizzato descritto sopra, ma invece di un valore di attributo per classe esso lavora con intervalli di valori e quindi può essere usato solo con attributi di tipo numerico.

To find out more about ranges used in the renderer

for ran in rendererV2.ranges():
    print "%f - %f: %s %s" % (
        ran.lowerValue(),
        ran.upperValue(),
        ran.label(),
        str(ran.symbol())
      )

puoi usare di nuovo classAttribute() per trovare il nome attributo di classificazione, i metodi sourceSymbol() e sourceColorRamp(). Inoltre, è presente il metodo mode() che determina come gli intervalli siano creati: usando intervalli uguali, quantili o qualche altro metodo.

If you wish to create your own graduated symbol renderer you can do so as illustrated in the example snippet below (which creates a simple two class arrangement)

from qgis.core import *

myVectorLayer = QgsVectorLayer(myVectorPath, myName, 'ogr')
myTargetField = 'target_field'
myRangeList = []
myOpacity = 1
# Make our first symbol and range...
myMin = 0.0
myMax = 50.0
myLabel = 'Group 1'
myColour = QtGui.QColor('#ffee00')
mySymbol1 = QgsSymbolV2.defaultSymbol(myVectorLayer.geometryType())
mySymbol1.setColor(myColour)
mySymbol1.setAlpha(myOpacity)
myRange1 = QgsRendererRangeV2(myMin, myMax, mySymbol1, myLabel)
myRangeList.append(myRange1)
#now make another symbol and range...
myMin = 50.1
myMax = 100
myLabel = 'Group 2'
myColour = QtGui.QColor('#00eeff')
mySymbol2 = QgsSymbolV2.defaultSymbol(
     myVectorLayer.geometryType())
mySymbol2.setColor(myColour)
mySymbol2.setAlpha(myOpacity)
myRange2 = QgsRendererRangeV2(myMin, myMax, mySymbol2 myLabel)
myRangeList.append(myRange2)
myRenderer = QgsGraduatedSymbolRendererV2('', myRangeList)
myRenderer.setMode(QgsGraduatedSymbolRendererV2.EqualInterval)
myRenderer.setClassAttribute(myTargetField)

myVectorLayer.setRendererV2(myRenderer)
QgsMapLayerRegistry.instance().addMapLayer(myVectorLayer)

Working with Symbols

Per la rappresentazione di simboli, è presente la classe di base QgsSymbolV2 con tre classi derivate:

  • QgsMarkerSymbolV2 — for point features
  • QgsLineSymbolV2 — for line features
  • QgsFillSymbolV2 — for polygon features

Ogni simbolo consiste in uno o più vettori simbolo (classi derivate da QgsSymbolLayerV2). I vettori simbolo creano la visualizzazione effettiva, la classe simbolo stessa serve solo come contenitore per i vettori simbolo.

Having an instance of a symbol (e.g. from a renderer), it is possible to explore it: type() method says whether it is a marker, line or fill symbol. There is a dump() method which returns a brief description of the symbol. To get a list of symbol layers

for i in xrange(symbol.symbolLayerCount()):
    lyr = symbol.symbolLayer(i)
    print "%d: %s" % (i, lyr.layerType())

Per trovare il colore del simbolo usa il metodo color() e setColor() per cambiarlo. Con i simboli singoli inoltre è possibile interrogare per la dimensione del simbolo e la rotazione con i metodi size() e angle(), per i simboli linea è presente il metodo width() che restituisce la larghezza della linea.

La dimensione e la larghezza sono in millimetri per impostazione predefinita, gli angoli sono in gradi.

Working with Symbol Layers

Come detto prima, i vettori simbolo (sottoclassi di QgsSymbolLayerV2) determinano l’aspetto delle geometrie. Ci sono sono diverse classi di vettori simbolo di base per un uso generale. È possibile implementare nuovi tipi di vettori simbolo e quindi personalizzare arbitrariamente il modo con cui le geometria saranno visualizzate. Il metodo layerType() identifica unicamente la classe di vettore simbolo — quelli di base e di default sono i tipi di vettori simbolo SimpleMarker, SimpleLine e SimpleFill.

You can get a complete list of the types of symbol layers you can create for a given symbol layer class like this

from qgis.core import QgsSymbolLayerV2Registry
myRegistry = QgsSymbolLayerV2Registry.instance()
myMetadata = myRegistry.symbolLayerMetadata("SimpleFill")
for item in myRegistry.symbolLayersForType(QgsSymbolV2.Marker):
    print item

Output

EllipseMarker
FontMarker
SimpleMarker
SvgMarker
VectorField

La classe QgsSymbolLayerV2Registry gestisce un database di tutti i tipi di vettore simbolo disponibili.

Per accedere ai dati di vettore simbolo, usa il suo metodo properties() che restituisce un dizionario chiave-valore di proprietà che determinano l’aspetto. Ogni tipo di vettore simbolo ha un set specifico di proprietà che usa. Inoltre, ci sono i metodi generali color(), size(), angle(), width() con i loro set corrispondenti. Naturalmente la dimensione e l’angolo sono disponibili solo per vettori simbolo singolo e lo spessore per vettori simbolo linea.

Creating Custom Symbol Layer Types

Imagine you would like to customize the way how the data gets rendered. You can create your own symbol layer class that will draw the features exactly as you wish. Here is an example of a marker that draws red circles with specified radius

class FooSymbolLayer(QgsMarkerSymbolLayerV2):

  def __init__(self, radius=4.0):
      QgsMarkerSymbolLayerV2.__init__(self)
      self.radius = radius
      self.color = QColor(255,0,0)

  def layerType(self):
     return "FooMarker"

  def properties(self):
      return { "radius" : str(self.radius) }

  def startRender(self, context):
    pass

  def stopRender(self, context):
      pass

  def renderPoint(self, point, context):
      # Rendering depends on whether the symbol is selected (QGIS >= 1.5)
      color = context.selectionColor() if context.selected() else self.color
      p = context.renderContext().painter()
      p.setPen(color)
      p.drawEllipse(point, self.radius, self.radius)

  def clone(self):
      return FooSymbolLayer(self.radius)

Il metodo layerType() determina il nome del vettore simbolo e deve essere unico tra tutti i vettori simbolo. Le proprietà sono usate per la persistenza degli attributi. Il metodo clone() deve restituire una copia del vettore simbolo con tutti gli stessi esatti attributi. Infine ci sono i metodi di visualizzazione: startRender è chiamato prima della visualizzazione della prima geoemtria, :func:`stopRender() quando la visualizzazione è stata fatta. E il metodo renderPoint() che fa la visualizzazione. Le coordinate del punto(i) sono già trasformate nelle coordinate in uscita.

For polylines and polygons the only difference would be in the rendering method: you would use renderPolyline() which receives a list of lines, resp. renderPolygon() which receives list of points on outer ring as a first parameter and a list of inner rings (or None) as a second parameter.

Usually it is convenient to add a GUI for setting attributes of the symbol layer type to allow users to customize the appearance: in case of our example above we can let user set circle radius. The following code implements such widget

class FooSymbolLayerWidget(QgsSymbolLayerV2Widget):
    def __init__(self, parent=None):
        QgsSymbolLayerV2Widget.__init__(self, parent)

        self.layer = None

        # setup a simple UI
        self.label = QLabel("Radius:")
        self.spinRadius = QDoubleSpinBox()
        self.hbox = QHBoxLayout()
        self.hbox.addWidget(self.label)
        self.hbox.addWidget(self.spinRadius)
        self.setLayout(self.hbox)
        self.connect(self.spinRadius, SIGNAL("valueChanged(double)"), \
            self.radiusChanged)

    def setSymbolLayer(self, layer):
        if layer.layerType() != "FooMarker":
            return
        self.layer = layer
        self.spinRadius.setValue(layer.radius)

    def symbolLayer(self):
        return self.layer

    def radiusChanged(self, value):
        self.layer.radius = value
        self.emit(SIGNAL("changed()"))

This widget can be embedded into the symbol properties dialog. When the symbol layer type is selected in symbol properties dialog, it creates an instance of the symbol layer and an instance of the symbol layer widget. Then it calls setSymbolLayer() method to assign the symbol layer to the widget. In that method the widget should update the UI to reflect the attributes of the symbol layer. symbolLayer() function is used to retrieve the symbol layer again by the properties dialog to use it for the symbol.

On every change of attributes, the widget should emit changed() signal to let the properties dialog update the symbol preview.

Now we are missing only the final glue: to make QGIS aware of these new classes. This is done by adding the symbol layer to registry. It is possible to use the symbol layer also without adding it to the registry, but some functionality will not work: e.g. loading of project files with the custom symbol layers or inability to edit the layer’s attributes in GUI.

We will have to create metadata for the symbol layer

class FooSymbolLayerMetadata(QgsSymbolLayerV2AbstractMetadata):

  def __init__(self):
    QgsSymbolLayerV2AbstractMetadata.__init__(self, "FooMarker", QgsSymbolV2.Marker)

  def createSymbolLayer(self, props):
    radius = float(props[QString("radius")]) if QString("radius") in props else 4.0
    return FooSymbolLayer(radius)

  def createSymbolLayerWidget(self):
    return FooSymbolLayerWidget()

QgsSymbolLayerV2Registry.instance().addSymbolLayerType(FooSymbolLayerMetadata())

You should pass layer type (the same as returned by the layer) and symbol type (marker/line/fill) to the constructor of parent class. createSymbolLayer() takes care of creating an instance of symbol layer with attributes specified in the props dictionary. (Beware, the keys are QString instances, not “str” objects). And there is createSymbolLayerWidget() method which returns settings widget for this symbol layer type.

The last step is to add this symbol layer to the registry — and we are done.

Creating Custom Renderers

It might be useful to create a new renderer implementation if you would like to customize the rules how to select symbols for rendering of features. Some use cases where you would want to do it: symbol is determined from a combination of fields, size of symbols changes depending on current scale etc.

The following code shows a simple custom renderer that creates two marker symbols and chooses randomly one of them for every feature

import random

class RandomRenderer(QgsFeatureRendererV2):
  def __init__(self, syms=None):
    QgsFeatureRendererV2.__init__(self, "RandomRenderer")
    self.syms = syms if syms else [QgsSymbolV2.defaultSymbol(QGis.Point), QgsSymbolV2.defaultSymbol(QGis.Point)]

  def symbolForFeature(self, feature):
    return random.choice(self.syms)

  def startRender(self, context, vlayer):
    for s in self.syms:
      s.startRender(context)

  def stopRender(self, context):
    for s in self.syms:
      s.stopRender(context)

  def usedAttributes(self):
    return []

  def clone(self):
    return RandomRenderer(self.syms)

The constructor of parent QgsFeatureRendererV2 class needs renderer name (has to be unique among renderers). symbolForFeature() method is the one that decides what symbol will be used for a particular feature. startRender() and stopRender() take care of initialization/finalization of symbol rendering. usedAttributes() method can return a list of field names that renderer expects to be present. Finally clone() function should return a copy of the renderer.

Like with symbol layers, it is possible to attach a GUI for configuration of the renderer. It has to be derived from QgsRendererV2Widget. The following sample code creates a button that allows user to set symbol of the first symbol

class RandomRendererWidget(QgsRendererV2Widget):
  def __init__(self, layer, style, renderer):
    QgsRendererV2Widget.__init__(self, layer, style)
    if renderer is None or renderer.type() != "RandomRenderer":
      self.r = RandomRenderer()
    else:
      self.r = renderer
    # setup UI
    self.btn1 = QgsColorButtonV2()
    self.btn1.setColor(self.r.syms[0].color())
    self.vbox = QVBoxLayout()
    self.vbox.addWidget(self.btn1)
    self.setLayout(self.vbox)
    self.connect(self.btn1, SIGNAL("clicked()"), self.setColor1)

  def setColor1(self):
    color = QColorDialog.getColor(self.r.syms[0].color(), self)
    if not color.isValid(): return
    self.r.syms[0].setColor(color);
    self.btn1.setColor(self.r.syms[0].color())

  def renderer(self):
    return self.r

The constructor receives instances of the active layer (QgsVectorLayer), the global style (QgsStyleV2) and current renderer. If there is no renderer or the renderer has different type, it will be replaced with our new renderer, otherwise we will use the current renderer (which has already the type we need). The widget contents should be updated to show current state of the renderer. When the renderer dialog is accepted, widget’s renderer() method is called to get the current renderer — it will be assigned to the layer.

The last missing bit is the renderer metadata and registration in registry, otherwise loading of layers with the renderer will not work and user will not be able to select it from the list of renderers. Let us finish our RandomRenderer example

class RandomRendererMetadata(QgsRendererV2AbstractMetadata):
  def __init__(self):
    QgsRendererV2AbstractMetadata.__init__(self, "RandomRenderer", "Random renderer")

  def createRenderer(self, element):
    return RandomRenderer()
  def createRendererWidget(self, layer, style, renderer):
    return RandomRendererWidget(layer, style, renderer)

QgsRendererV2Registry.instance().addRenderer(RandomRendererMetadata())

Similarly as with symbol layers, abstract metadata constructor awaits renderer name, name visible for users and optionally name of renderer’s icon. createRenderer() method passes QDomElement instance that can be used to restore renderer’s state from DOM tree. createRendererWidget() method creates the configuration widget. It does not have to be present or can return None if the renderer does not come with GUI.

To associate an icon with the renderer you can assign it in QgsRendererV2AbstractMetadata constructor as a third (optional) argument — the base class constructor in the RandomRendererMetadata __init__() function becomes

QgsRendererV2AbstractMetadata.__init__(self,
       "RandomRenderer",
       "Random renderer",
       QIcon(QPixmap("RandomRendererIcon.png", "png")))

The icon can be associated also at any later time using setIcon() method of the metadata class. The icon can be loaded from a file (as shown above) or can be loaded from a Qt resource (PyQt4 includes .qrc compiler for Python).

Further Topics

TODO:
creating/modifying symbols working with style (QgsStyleV2) working with color ramps (QgsVectorColorRampV2) rule-based renderer (see this blogpost) exploring symbol layer and renderer registries