The Qt Quick 2 Spectrogram example demonstrates how to show a polar and cartesian spectrograms and how to utilize orthographic projection to show them in 2D.

Spectrogram is simply a surface graph with a range gradient used to emphasize the different values. Typically spectrograms are shown with two dimensional surfaces, which we simulate with a top down orthographic view of the graph. To enforce the 2D effect, we disable the graph rotation via mouse or touch when in the orthographic mode.

The focus in this example is on showing how to display spectrograms, so the basic functionality is not explained. For more detailed QML example documentation, see Qt Quick 2 Scatter Example.

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 a Spectrogram

To create a 2D spectrogram, we define a Surface3D item:

Surface3D {
    id: surfaceGraph
    width: surfaceView.width
    height: surfaceView.height

    shadowQuality: AbstractGraph3D.ShadowQualityNone
    selectionMode: AbstractGraph3D.SelectionSlice | AbstractGraph3D.SelectionItemAndColumn
    axisX: xAxis
    axisY: yAxis
    axisZ: zAxis

    theme: customTheme
    inputHandler: customInputHandler

    // Remove the perspective and view the graph from top down to achieve 2D effect
    orthoProjection: true
    scene.activeCamera.cameraPreset: Camera3D.CameraPresetDirectlyAbove

    flipHorizontalGrid: true

    radialLabelOffset: 0.01

    horizontalAspectRatio: 1
    scene.activeCamera.zoomLevel: 85

    Surface3DSeries {
        id: surfaceSeries
        flatShadingEnabled: false
        drawMode: Surface3DSeries.DrawSurface
        baseGradient: surfaceGradient
        colorStyle: Theme3D.ColorStyleRangeGradient
        itemLabelFormat: "(@xLabel, @zLabel): @yLabel"

        ItemModelSurfaceDataProxy {
            itemModel: surfaceData.model
            rowRole: "radius"
            columnRole: "angle"
            yPosRole: "value"
        }
    }
}

The key properties for enabling the 2D effect are orthoProjection and scene.activeCamera.cameraPreset. We remove the perspective by enabling orthographic projection for the graph, and then we eliminate the Y-dimension by viewing the graph directly from above:

orthoProjection: true
scene.activeCamera.cameraPreset: Camera3D.CameraPresetDirectlyAbove

Since this viewpoint causes the horizontal axis grid to be mostly obscured by the surface, we also specify that the horizontal grid should be drawn on top of the graph:

flipHorizontalGrid: true

Polar Spectrogram

Depending on the data, it is sometimes more natural to use a polar graph instead of a cartesian one. Qt Data Visualization supports this via polar property. In this example we provide a button to switch between polar and cartesian modes:

NewButton {
    id: polarToggle
    Layout.fillWidth: true
    Layout.fillHeight: true
    text: "Switch to polar"
    onClicked: {
        if (surfaceGraph.polar === false) {
            surfaceGraph.polar = true
            text = "Switch to cartesian"
        } else {
            surfaceGraph.polar = false
            text = "Switch to polar"
        }
    }
}

In the polar mode, the X-axis is converted into the angular polar axis, and the Z-axis is converted into the radial polar axis. The surface points are recalculated according to new axes.

The radial axis labels are drawn outside the graph by default, but in this example we want to draw them right next to the 0 degree angular axis inside the graph, so we define only a tiny offset for them:

radialLabelOffset: 0.01

To enforce the 2D effect, graph rotation via user input is disabled when in orthographic mode. We do this by specifying a new input handler:

TouchInputHandler3D {
    id: customInputHandler
    rotationEnabled: false
}
Surface3D {
...
inputHandler: customInputHandler
...

When the projection mode changes, we adjust the value of the rotationEnabled property of the customInputHandler to control the rotation.

Example Contents

Example project @ code.qt.io