8.2 A single symbol for vector layer point elements uses one

Article directory

• Preface
• Single symbol rendering
• • Simple Marker
  • • QGis
    • Code
  • SVG marker
  • • QGis
    • Code
• Summarize

Foreword

• The previous tutorial gave an overall introduction to vector layer symbolization, and took point layers as an example to introduce the renderers that can be used:
  • No symbols
  • Single symbol
  • Category (Categorized)
  • Graduated
  • Rule-based
  • Point displacement
  • Point cluster
  • Heatmap
• This article introduces how to use a single symbol
• Note: The sample codes in the article are all from the open source project qgis_cpp_api_apps

Single symbol rendering

• QGIS uses a single symbol and Simple Marker for rendering by default. Taking places_33S.shp as an example, after adding the places_33S layer, the default display is as shown below
  
• In the layer properties of the point feature layer, select the “Symbology” tab and you can see the default display, as shown below
  
• In addition to Simple Marker, there are many other options as shown below
  

Simple Marker

Simple marker symbol layer, consisting of a rendered shape with solid fill color and an stroke.

• Simple markers refer to using some simple geometric shapes as markers, and can set their fill color and brush color

QGis

• Set the Symbol layer type to Simple marker Layer and set the properties as shown below.
  
  

Code implementation

• The QgsSimpleMarkerSymbolLayer class is a simple marker layer (Simple marker Layer). The class diagram is as follows
  
• Its constructor is as follows, the parameters of the constructor are its corresponding parameters

QgsSimpleMarkerSymbolLayer (Qgis::MarkerShape shape=Qgis::MarkerShape::Circle, double size=DEFAULT_SIMPLEMARKER_SIZE, double angle=DEFAULT_SIMPLEMARKER_ANGLE, Qgis::ScaleMethod scaleMethod=DEFAULT_SCALE_METHOD, const QColor & amp;color=DEFAULT_SIMPLEMARK ER_COLOR, const QColor & amp ;strokeColor=DEFAULT_SIMPLEMARKER_BORDERCOLOR, Qt::PenJoinStyle penJoinStyle=DEFAULT_SIMPLEMARKER_JOINSTYLE)

• The steps to set the point symbol to a simple mark are as follows:

1. First get the renderer from the layer and convert it to a Single Symbol Renderer code as follows

QgsFeatureRenderer * layerRenderer= layer->renderer();
QgsSingleSymbolRenderer *singleRenderer = QgsSingleSymbolRenderer::convertFromRenderer(layerRenderer);

2. Construct a Simple Marker SymbolLayer, the code is as follows

auto markerSymbolLayer = new QgsSimpleMarkerSymbolLayer(Qgis::MarkerShape::Heart,4.0);
QgsSymbolLayerList layerList;
    layerList << markerSymbolLayer;

3. Construct Marker Symbol and set symbol for renderer. The code is as follows

 auto markerSymbol = new QgsMarkerSymbol(layerList);
    singleRenderer->setSymbol(markerSymbol);
    layer->setRenderer(singleRenderer);

SVG marker

provides you with images from your SVG paths to render as marker symbol.
Each SVG file colors and stroke can also be adapted.

• svg tag refers to using svg pictures as tags, and you can set the fill color and brush color of svg

QGis

• Set the Symbol layer type to SVG marker (SVG marker). See the following figure for the properties.
  
  

Code implementation

• The QgsSvgMarkerSymbolLayer class is an SVG marker layer. The class diagram is as follows
  
• Its constructor is as follows, the parameters of the constructor are its corresponding parameters

QgsSvgMarkerSymbolLayer (const QString & amp;path, double size=DEFAULT_SVGMARKER_SIZE, double angle=DEFAULT_SVGMARKER_ANGLE, Qgis::ScaleMethod scaleMethod=DEFAULT_SCALE_METHOD)

• The steps to set the point symbol as an svg tag are as follows:

1. First get the renderer from the layer and convert it to a Single Symbol Renderer code as follows

QgsFeatureRenderer * layerRenderer= layer->renderer();
QgsSingleSymbolRenderer *singleRenderer = QgsSingleSymbolRenderer::convertFromRenderer(layerRenderer);

2. Construct the Svg Marker SymbolLayer, the code is as follows

 //Construct Svg Marker SymbolLayer method one
    //As an example, two parameters, path and size, are used. Other parameters are used similarly.
    QString path = QStringLiteral("resources/plane.svg");
    auto markerSymbolLayer = new QgsSvgMarkerSymbolLayer(path,4.0);

3. Construct Marker Symbol and set symbol for renderer. The code is as follows

 //QgsSymbolLayerList needs to be passed in to the QgsMarkerSymbol constructor
    //Multiple Symbol Layers form a Symbol
    QgsSymbolLayerList layerList;
    layerList << markerSymbolLayer;
    auto markerSymbol = new QgsMarkerSymbol(layerList);
    singleRenderer->setSymbol(markerSymbol);
    layer->setRenderer(singleRenderer);

Summary

• The methods of using a single symbol for the point layer and setting it as a simple mark and svg mark are introduced respectively.