Using the Map Canvas¶
Despite our constant efforts, information beyond this line may not be updated for QGIS 3. Refer to https://qgis.org/pyqgis/master for the python API documentation or, give a hand to update the chapters you know about. Thanks.
The Map canvas widget is probably the most important widget within QGIS because it shows the map composed from overlaid map layers and allows interaction with the map and layers. The canvas always shows a part of the map defined by the current canvas extent. The interaction is done through the use of map tools: there are tools for panning, zooming, identifying layers, measuring, vector editing and others. Similar to other graphics programs, there is always one tool active and the user can switch between the available tools.
The map canvas is implemented with the
QgsMapCanvas class in the
module. The implementation is based on the Qt Graphics View framework.
This framework generally provides a surface and a view where custom graphics
items are placed and user can interact with them. We will assume that you are
familiar enough with Qt to understand the concepts of the graphics scene, view
and items. If not, please read the overview of the framework.
Whenever the map has been panned, zoomed in/out (or some other action that triggers
a refresh), the map is rendered again within the current extent. The layers are
rendered to an image (using the
QgsMapRendererJob class) and that image is
displayed on the canvas. The
QgsMapCanvas class also controls refreshing
of the rendered map. Besides this item which acts as a background, there may be more map canvas items.
Typical map canvas items are rubber bands (used for measuring, vector editing
etc.) or vertex markers. The canvas items are usually used to give visual
feedback for map tools, for example, when creating a new polygon, the map tool
creates a rubber band canvas item that shows the current shape of the polygon.
All map canvas items are subclasses of
QgsMapCanvasItem which adds
some more functionality to the basic
To summarize, the map canvas architecture consists of three concepts:
map canvas — for viewing of the map
map canvas items — additional items that can be displayed on the map canvas
map tools — for interaction with the map canvas
Embedding Map Canvas¶
Map canvas is a widget like any other Qt widget, so using it is as simple as creating and showing it
canvas = QgsMapCanvas() canvas.show()
This produces a standalone window with map canvas. It can be also embedded into
an existing widget or window. When using
.ui files and Qt Designer, place a
QWidget on the form and promote it to a new class: set
class name and set
qgis.gui as header file. The
pyuic5 utility will
take care of it. This is a very convenient way of embedding the canvas. The
other possibility is to manually write the code to construct map canvas and
other widgets (as children of a main window or dialog) and create a layout.
By default, map canvas has black background and does not use anti-aliasing. To set white background and enable anti-aliasing for smooth rendering
(In case you are wondering,
Qt comes from
PyQt.QtCore module and
Qt.white is one of the predefined
Now it is time to add some map layers. We will first open a layer and add it to the current project. Then we will set the canvas extent and set the list of layers for canvas
path_to_ports_layer = os.path.join(QgsProject.instance().homePath(), "data", "ports", "ports.shp") vlayer = QgsVectorLayer(path_to_ports_layer, "Ports layer", "ogr") if not vlayer.isValid(): print("Layer failed to load!") # add layer to the registry QgsProject.instance().addMapLayer(vlayer) # set extent to the extent of our layer canvas.setExtent(vlayer.extent()) # set the map canvas layer set canvas.setLayers([vlayer])
After executing these commands, the canvas should show the layer you have loaded.
Rubber Bands and Vertex Markers¶
To show some additional data on top of the map in canvas, use map canvas items.
It is possible to create custom canvas item classes (covered below), however
there are two useful canvas item classes for convenience:
QgsRubberBand for drawing polylines or polygons, and
QgsVertexMarker for drawing points. They both work with map
coordinates, so the shape is moved/scaled automatically when the canvas is
being panned or zoomed.
To show a polyline
r = QgsRubberBand(canvas, False) # False = not a polygon points = [QgsPoint(-100, 45), QgsPoint(10, 60), QgsPoint(120, 45)] r.setToGeometry(QgsGeometry.fromPolyline(points), None)
To show a polygon
r = QgsRubberBand(canvas, True) # True = a polygon points = [[QgsPointXY(-100, 35), QgsPointXY(10, 50), QgsPointXY(120, 35)]] r.setToGeometry(QgsGeometry.fromPolygonXY(points), None)
Note that points for polygon is not a plain list: in fact, it is a list of rings containing linear rings of the polygon: first ring is the outer border, further (optional) rings correspond to holes in the polygon.
Rubber bands allow some customization, namely to change their color and line width
r.setColor(QColor(0, 0, 255)) r.setWidth(3)
The canvas items are bound to the canvas scene. To temporarily hide them (and
show them again), use the
show() combo. To completely remove
the item, you have to remove it from the scene of the canvas
(in C++ it’s possible to just delete the item, however in Python
would just delete the reference and the object will still exist as it is owned
by the canvas)
Rubber band can be also used for drawing points, but the
QgsVertexMarker class is better suited for this
QgsRubberBand would only draw a rectangle around the desired point).
You can use the vertex marker like this:
m = QgsVertexMarker(canvas) m.setCenter(QgsPointXY(10,40))
This will draw a red cross on position [10,45]. It is possible to customize the icon type, size, color and pen width
m.setColor(QColor(0, 255, 0)) m.setIconSize(5) m.setIconType(QgsVertexMarker.ICON_BOX) # or ICON_CROSS, ICON_X m.setPenWidth(3)
For temporary hiding of vertex markers and removing them from canvas, use the same methods as for rubber bands.
Using Map Tools with Canvas¶
The following example constructs a window that contains a map canvas and basic
map tools for map panning and zooming. Actions are created for activation of
each tool: panning is done with
QgsMapToolPan, zooming in/out with a
QgsMapToolZoom instances. The actions are set as checkable and
later assigned to the tools to allow automatic handling of checked/unchecked
state of the actions – when a map tool gets activated, its action is marked as
selected and the action of the previous map tool is deselected. The map tools
are activated using
from qgis.gui import * from qgis.PyQt.QtWidgets import QAction, QMainWindow from qgis.PyQt.QtCore import Qt class MyWnd(QMainWindow): def __init__(self, layer): QMainWindow.__init__(self) self.canvas = QgsMapCanvas() self.canvas.setCanvasColor(Qt.white) self.canvas.setExtent(layer.extent()) self.canvas.setLayers([layer]) self.setCentralWidget(self.canvas) self.actionZoomIn = QAction("Zoom in", self) self.actionZoomOut = QAction("Zoom out", self) self.actionPan = QAction("Pan", self) self.actionZoomIn.setCheckable(True) self.actionZoomOut.setCheckable(True) self.actionPan.setCheckable(True) self.actionZoomIn.triggered.connect(self.zoomIn) self.actionZoomOut.triggered.connect(self.zoomOut) self.actionPan.triggered.connect(self.pan) self.toolbar = self.addToolBar("Canvas actions") self.toolbar.addAction(self.actionZoomIn) self.toolbar.addAction(self.actionZoomOut) self.toolbar.addAction(self.actionPan) # create the map tools self.toolPan = QgsMapToolPan(self.canvas) self.toolPan.setAction(self.actionPan) self.toolZoomIn = QgsMapToolZoom(self.canvas, False) # false = in self.toolZoomIn.setAction(self.actionZoomIn) self.toolZoomOut = QgsMapToolZoom(self.canvas, True) # true = out self.toolZoomOut.setAction(self.actionZoomOut) self.pan() def zoomIn(self): self.canvas.setMapTool(self.toolZoomIn) def zoomOut(self): self.canvas.setMapTool(self.toolZoomOut) def pan(self): self.canvas.setMapTool(self.toolPan)
You can try the above code in the Python console editor. To invoke the canvas window,
add the following lines to instantiate the
MyWnd class. They will render the currently
selected layer on the newly created canvas
w = MyWnd(iface.activeLayer()) w.show()
Writing Custom Map Tools¶
You can write your custom tools, to implement a custom behavior to actions performed by users on the canvas.
Map tools should inherit from the
class or any derived class, and selected as active tools in the canvas using
method as we have already seen.
Here is an example of a map tool that allows to define a rectangular extent by clicking and dragging on the canvas. When the rectangle is defined, it prints its boundary coordinates in the console. It uses the rubber band elements described before to show the selected rectangle as it is being defined.
class RectangleMapTool(QgsMapToolEmitPoint): def __init__(self, canvas): self.canvas = canvas QgsMapToolEmitPoint.__init__(self, self.canvas) self.rubberBand = QgsRubberBand(self.canvas, True) self.rubberBand.setColor(Qt.red) self.rubberBand.setWidth(1) self.reset() def reset(self): self.startPoint = self.endPoint = None self.isEmittingPoint = False self.rubberBand.reset(True) def canvasPressEvent(self, e): self.startPoint = self.toMapCoordinates(e.pos()) self.endPoint = self.startPoint self.isEmittingPoint = True self.showRect(self.startPoint, self.endPoint) def canvasReleaseEvent(self, e): self.isEmittingPoint = False r = self.rectangle() if r is not None: print("Rectangle:", r.xMinimum(), r.yMinimum(), r.xMaximum(), r.yMaximum() ) def canvasMoveEvent(self, e): if not self.isEmittingPoint: return self.endPoint = self.toMapCoordinates(e.pos()) self.showRect(self.startPoint, self.endPoint) def showRect(self, startPoint, endPoint): self.rubberBand.reset(QGis.Polygon) if startPoint.x() == endPoint.x() or startPoint.y() == endPoint.y(): return point1 = QgsPoint(startPoint.x(), startPoint.y()) point2 = QgsPoint(startPoint.x(), endPoint.y()) point3 = QgsPoint(endPoint.x(), endPoint.y()) point4 = QgsPoint(endPoint.x(), startPoint.y()) self.rubberBand.addPoint(point1, False) self.rubberBand.addPoint(point2, False) self.rubberBand.addPoint(point3, False) self.rubberBand.addPoint(point4, True) # true to update canvas self.rubberBand.show() def rectangle(self): if self.startPoint is None or self.endPoint is None: return None elif (self.startPoint.x() == self.endPoint.x() or \ self.startPoint.y() == self.endPoint.y()): return None return QgsRectangle(self.startPoint, self.endPoint) def deactivate(self): QgsMapTool.deactivate(self) self.deactivated.emit()
Writing Custom Map Canvas Items¶
how to create a map canvas item
import sys from qgis.core import QgsApplication from qgis.gui import QgsMapCanvas def init(): a = QgsApplication(sys.argv, True) QgsApplication.setPrefixPath('/home/martin/qgis/inst', True) QgsApplication.initQgis() return a def show_canvas(app): canvas = QgsMapCanvas() canvas.show() app.exec_() app = init() show_canvas(app)