Qml Oscilloscope

The oscilloscope application demonstrates how to use the Qt Charts QML API to implement an application with strict performance requirements. The application uses generated data with configurable characteristics to mimic a simple oscilloscope user interface.

To get information about actual rendering speed shown in the application output console, you can set QSG_RENDER_TIMING = 1 to your run environment settings. To do so go to Projects - Run - Run environment in Qt Creator and select Add. Then you can experiment with the different configurable options of the example application to find the configuration that gives you the best performance in your environment.

Running the Example

To run the example from Qt Creator, open the Welcome mode and select the example from Examples. For more information, visit Building and Running an Example.

Creating Views

The application window is shared by control and scope views:

 Item {
     id: main
     width: 600
     height: 400

     ControlPanel {
         id: controlPanel
         anchors.top: parent.top
         anchors.topMargin: 10
         anchors.bottom: parent.bottom
         anchors.left: parent.left
         anchors.leftMargin: 10
     ...
     ScopeView {
         id: scopeView
         anchors.top: parent.top
         anchors.bottom: parent.bottom
         anchors.right: parent.right
         anchors.left: controlPanel.right
         height: main.height
     }

ControlView implements the buttons used for configuring. ScopeView uses a ChartView to show a chart with two line series:

 ChartView {
     id: chartView

     property bool openGL: openGLSupported

     animationOptions: ChartView.NoAnimation
     theme: ChartView.ChartThemeDark

     onOpenGLChanged: {
         if (openGLSupported) {
             var series1 = series("signal 1")
             if (series1)
                 series1.useOpenGL = openGL;
             var series2 = series("signal 2")
             if (series2)
                 series2.useOpenGL = openGL;
         }
     }

     ValueAxis {
         id: axisY1
         min: -1
         max: 4
     }

     ValueAxis {
         id: axisY2
         min: -10
         max: 5
     }

     ValueAxis {
         id: axisX
         min: 0
         max: 1024
     }

     LineSeries {
         id: lineSeries1
         name: "signal 1"
         axisX: axisX
         axisY: axisY1
         useOpenGL: chartView.openGL
     }
     LineSeries {
         id: lineSeries2
         name: "signal 2"
         axisX: axisX
         axisYRight: axisY2
         useOpenGL: chartView.openGL
     }
     ...

The data of the line series is updated with a QML timer. In a real life application the updating could be triggered with a signal from Qt C++ code.

 Timer {
     id: refreshTimer
     interval: 1 / 60 * 1000 // 60 Hz
     running: true
     repeat: true
     onTriggered: {
         dataSource.update(chartView.series(0));
         dataSource.update(chartView.series(1));
     }
 }

The oscilloscope also allows you to switch the type of the series used for visualizing the signal sources. This is implemented by dynamically destroying and creating series:

 function changeSeriesType(type) {
     chartView.removeAllSeries();

     // Create two new series of the correct type. Axis x is the same for both of the series,
     // but the series have their own y-axes to make it possible to control the y-offset
     // of the "signal sources".
     var series1
     var series2
     if (type === "line") {
         series1 = chartView.createSeries(ChartView.SeriesTypeLine, "signal 1",
                                          axisX, axisY1);
         series1.useOpenGL = chartView.openGL

         series2 = chartView.createSeries(ChartView.SeriesTypeLine, "signal 2",
                                          axisX, axisY2);
         series2.useOpenGL = chartView.openGL
     } else {
         series1 = chartView.createSeries(ChartView.SeriesTypeScatter, "signal 1",
                                          axisX, axisY1);
         series1.markerSize = 2;
         series1.borderColor = "transparent";
         series1.useOpenGL = chartView.openGL

         series2 = chartView.createSeries(ChartView.SeriesTypeScatter, "signal 2",
                                          axisX, axisY2);
         series2.markerSize = 2;
         series2.borderColor = "transparent";
         series2.useOpenGL = chartView.openGL
     }
 }

 function createAxis(min, max) {
     // The following creates a ValueAxis object that can be then set as a x or y axis for a series
     return Qt.createQmlObject("import QtQuick 2.0; import QtCharts 2.0; ValueAxis { min: "
                               + min + "; max: " + max + " }", chartView);
 }

Example project @ code.qt.io