9. マップキャンバスを使う

ヒント

The code snippets on this page need the following imports if you're outside the pyqgis console:

 1from qgis.PyQt.QtGui import (
 2    QColor,
 3)
 4
 5from qgis.PyQt.QtCore import Qt, QRectF
 6
 7from qgis.core import (
 8    QgsVectorLayer,
 9    QgsPoint,
10    QgsPointXY,
11    QgsProject,
12    QgsGeometry,
13    QgsMapRendererJob,
14    QgsWkbTypes,
15)
16
17from qgis.gui import (
18    QgsMapCanvas,
19    QgsVertexMarker,
20    QgsMapCanvasItem,
21    QgsRubberBand,
22)

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 qgis.gui 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 QGraphicsItem objects.

要約すると、地図キャンバスアーキテクチャは3つのコンセプトからなります:

  • map canvas --- 地図の可視化

  • map canvas items --- additional items that can be displayed on the map canvas

  • map tools --- for interaction with the map canvas

9.1. 地図キャンバスを埋め込む

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 QgsMapCanvas as 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.

デフォルトでは、地図キャンバスの背景色は黒でありアンチエイリアスは使用されません。背景を白に設定し、投影をなめらかにするためのアンチエイリアスを有効にするには

canvas.setCanvasColor(Qt.white)
canvas.enableAntiAliasing(True)

(In case you are wondering, Qt comes from PyQt.QtCore module and Qt.white is one of the predefined QColor instances.)

それでは、地図レイヤを追加していきましょう。まずレイヤを開いて、現在のプロジェクトに追加します。次に、キャンバスの範囲を設定し、キャンバスのレイヤのリストを設定します。

 1vlayer = QgsVectorLayer('testdata/airports.shp', "Airports layer", "ogr")
 2if not vlayer.isValid():
 3    print("Layer failed to load!")
 4
 5# add layer to the registry
 6QgsProject.instance().addMapLayer(vlayer)
 7
 8# set extent to the extent of our layer
 9canvas.setExtent(vlayer.extent())
10
11# set the map canvas layer set
12canvas.setLayers([vlayer])

これらのコマンドを実行した後、キャンバスには読み込んだレイヤーが表示されているはずです。

9.2. ラバーバンドと頂点マーカー

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, QgsWkbTypes.LineGeometry)  # line
points = [QgsPoint(-100, 45), QgsPoint(10, 60), QgsPoint(120, 45)]
r.setToGeometry(QgsGeometry.fromPolyline(points), None)

ポリゴンを表示するには

r = QgsRubberBand(canvas, QgsWkbTypes.PolygonGeometry)  # polygon
points = [[QgsPointXY(-100, 35), QgsPointXY(10, 50), QgsPointXY(120, 35)]]
r.setToGeometry(QgsGeometry.fromPolygonXY(points), None)

ポリゴンの点が普通のリストではないことに注意してください。実際には、ポリゴンの線状のリングを含有するリングのリストです:最初のリングは外側の境界であり、さらに(オプションの)リングはポリゴンの穴に対応します。

ラバーバンドはいくらかカスタマイズできます、すなわち、その色と線幅を変更することがが可能です

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 hide() and show() combo. To completely remove the item, you have to remove it from the scene of the canvas

canvas.scene().removeItem(r)

(C ++ではアイテムを削除することだけ可能ですが、Pythonでは del r は参照を削除するだけでありオブジェクトはキャンバスの所有物なのでそのまま残ります)

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.

9.3. 地図キャンバスで地図ツールを使用する

以下の例では、地図キャンバスと、地図のパンニングとズームのための基本的な地図ツールを含むウィンドウを作成します。パンニングは QgsMapToolPan で行い、ズームイン/ズームアウトは QgsMapToolZoom インスタンスのペアで行います。アクションはチェック可能に設定されており、後からツールに割り当てられ、アクションのチェック済み/チェック解除状態の自動処理を可能にします -- 地図ツールがアクティブになると、そのアクションは選択されたと印が付き、前の地図ツールのアクションは選択解除されます。地図ツールは setMapTool() メソッドを使って起動します。

 1from qgis.gui import *
 2from qgis.PyQt.QtWidgets import QAction, QMainWindow
 3from qgis.PyQt.QtCore import Qt
 4
 5class MyWnd(QMainWindow):
 6    def __init__(self, layer):
 7        QMainWindow.__init__(self)
 8
 9        self.canvas = QgsMapCanvas()
10        self.canvas.setCanvasColor(Qt.white)
11
12        self.canvas.setExtent(layer.extent())
13        self.canvas.setLayers([layer])
14
15        self.setCentralWidget(self.canvas)
16
17        self.actionZoomIn = QAction("Zoom in", self)
18        self.actionZoomOut = QAction("Zoom out", self)
19        self.actionPan = QAction("Pan", self)
20
21        self.actionZoomIn.setCheckable(True)
22        self.actionZoomOut.setCheckable(True)
23        self.actionPan.setCheckable(True)
24
25        self.actionZoomIn.triggered.connect(self.zoomIn)
26        self.actionZoomOut.triggered.connect(self.zoomOut)
27        self.actionPan.triggered.connect(self.pan)
28
29        self.toolbar = self.addToolBar("Canvas actions")
30        self.toolbar.addAction(self.actionZoomIn)
31        self.toolbar.addAction(self.actionZoomOut)
32        self.toolbar.addAction(self.actionPan)
33
34        # create the map tools
35        self.toolPan = QgsMapToolPan(self.canvas)
36        self.toolPan.setAction(self.actionPan)
37        self.toolZoomIn = QgsMapToolZoom(self.canvas, False) # false = in
38        self.toolZoomIn.setAction(self.actionZoomIn)
39        self.toolZoomOut = QgsMapToolZoom(self.canvas, True) # true = out
40        self.toolZoomOut.setAction(self.actionZoomOut)
41
42        self.pan()
43
44    def zoomIn(self):
45        self.canvas.setMapTool(self.toolZoomIn)
46
47    def zoomOut(self):
48        self.canvas.setMapTool(self.toolZoomOut)
49
50    def pan(self):
51        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()

9.3.1. Select a feature using QgsMapToolIdentifyFeature

You can use the map tool QgsMapToolIdentifyFeature for asking to the user to select a feature that will be sent to a callback function.

 1def callback(feature):
 2  """Code called when the feature is selected by the user"""
 3  print("You clicked on feature {}".format(feature.id()))
 4
 5canvas = iface.mapCanvas()
 6feature_identifier = QgsMapToolIdentifyFeature(canvas)
 7
 8# indicates the layer on which the selection will be done
 9feature_identifier.setLayer(vlayer)
10
11# use the callback as a slot triggered when the user identifies a feature
12feature_identifier.featureIdentified.connect(callback)
13
14# activation of the map tool
15canvas.setMapTool(feature_identifier)

9.4. カスタム地図ツールを書く

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 QgsMapTool, class or any derived class, and selected as active tools in the canvas using the setMapTool() method as we have already seen.

キャンバスをクリックしてドラッグすることで矩形範囲を定義できる地図ツールの例を次に示します。矩形が定義されると、境界座標がコンソールに表示されます。前述のラバーバンド要素を使用して、選択されている矩形が定義されていることを示します。

 1class RectangleMapTool(QgsMapToolEmitPoint):
 2  def __init__(self, canvas):
 3    self.canvas = canvas
 4    QgsMapToolEmitPoint.__init__(self, self.canvas)
 5    self.rubberBand = QgsRubberBand(self.canvas, QgsWkbTypes.PolygonGeometry)
 6    self.rubberBand.setColor(Qt.red)
 7    self.rubberBand.setWidth(1)
 8    self.reset()
 9
10  def reset(self):
11    self.startPoint = self.endPoint = None
12    self.isEmittingPoint = False
13    self.rubberBand.reset(QgsWkbTypes.PolygonGeometry)
14
15  def canvasPressEvent(self, e):
16    self.startPoint = self.toMapCoordinates(e.pos())
17    self.endPoint = self.startPoint
18    self.isEmittingPoint = True
19    self.showRect(self.startPoint, self.endPoint)
20
21  def canvasReleaseEvent(self, e):
22    self.isEmittingPoint = False
23    r = self.rectangle()
24    if r is not None:
25      print("Rectangle:", r.xMinimum(),
26            r.yMinimum(), r.xMaximum(), r.yMaximum()
27           )
28
29  def canvasMoveEvent(self, e):
30    if not self.isEmittingPoint:
31      return
32
33    self.endPoint = self.toMapCoordinates(e.pos())
34    self.showRect(self.startPoint, self.endPoint)
35
36  def showRect(self, startPoint, endPoint):
37    self.rubberBand.reset(QgsWkbTypes.PolygonGeometry)
38    if startPoint.x() == endPoint.x() or startPoint.y() == endPoint.y():
39      return
40
41    point1 = QgsPoint(startPoint.x(), startPoint.y())
42    point2 = QgsPoint(startPoint.x(), endPoint.y())
43    point3 = QgsPoint(endPoint.x(), endPoint.y())
44    point4 = QgsPoint(endPoint.x(), startPoint.y())
45
46    self.rubberBand.addPoint(point1, False)
47    self.rubberBand.addPoint(point2, False)
48    self.rubberBand.addPoint(point3, False)
49    self.rubberBand.addPoint(point4, True)    # true to update canvas
50    self.rubberBand.show()
51
52  def rectangle(self):
53    if self.startPoint is None or self.endPoint is None:
54      return None
55    elif (self.startPoint.x() == self.endPoint.x() or \
56          self.startPoint.y() == self.endPoint.y()):
57      return None
58
59      return QgsRectangle(self.startPoint, self.endPoint)
60
61  def deactivate(self):
62    QgsMapTool.deactivate(self)
63    self.deactivated.emit()

9.5. カスタム地図キャンバスアイテムを書く

Here is an example of a custom canvas item that draws a circle:

 1class CircleCanvasItem(QgsMapCanvasItem):
 2  def __init__(self, canvas):
 3    super().__init__(canvas)
 4    self.center = QgsPoint(0, 0)
 5    self.size   = 100
 6
 7  def setCenter(self, center):
 8    self.center = center
 9
10  def center(self):
11    return self.center
12
13  def setSize(self, size):
14    self.size = size
15
16  def size(self):
17    return self.size
18
19  def boundingRect(self):
20    return QRectF(self.center.x() - self.size/2,
21      self.center.y() - self.size/2,
22      self.center.x() + self.size/2,
23      self.center.y() + self.size/2)
24
25  def paint(self, painter, option, widget):
26    path = QPainterPath()
27    path.moveTo(self.center.x(), self.center.y());
28    path.arcTo(self.boundingRect(), 0.0, 360.0)
29    painter.fillPath(path, QColor("red"))
30
31
32# Using the custom item:
33item = CircleCanvasItem(iface.mapCanvas())
34item.setCenter(QgsPointXY(200,200))
35item.setSize(80)