Highlight point near cursor with pixel space awareness

Hi,

I have an issue that visually it appears that the mouse is closer to a point but is highlighting another point because of the way the calculation is made and doesn’t take into account the axis scale. Looking into the method HighlightPointNearest the formula is only looking at the actual coordinates of the points (x1 - x) * (x1 - x) + (y1 - y) * (y1 - y) and not looking how the points are drawn on the screen.

Note how visually the mouse appears closure to (2, 200) point

As I move the mouse vertically the highlighted point eventually moves from (1, 100) to (2, 200)

Is there way to factor in the pixel location or the scale of the X and Y axis when different so that the point is highlighting correctly?

Sample Project

• TestHighlight.zip

MainWindow.xaml.cs

/// <summary>
/// Interaction logic for MainWindow.xaml
/// </summary>
public partial class MainWindow : Window
{
    ScottPlot.PlottableScatterHighlight sphTest;
    PlottableVLine vLine;
    PlottableHLine hLine;

    public MainWindow()
    {
        InitializeComponent();

        double[] xs = new double[] { 1, 2 };
        double[] ys = new double[] { 100, 200 };

        sphTest = pltTest.plt.PlotScatterHighlight(xs, ys, lineWidth: 0, color: System.Drawing.Color.Blue, markerSize: 10);

        vLine = pltTest.plt.PlotVLine(1, color: System.Drawing.Color.Red, lineStyle: LineStyle.Dash);
        hLine = pltTest.plt.PlotHLine(100, color: System.Drawing.Color.Red, lineStyle: LineStyle.Dash);

        pltTest.Render();
    }

    private void pltTest_MouseMove(object sender, MouseEventArgs e)
    {

        (double mouseX, double mouseY) = pltTest.GetMouseCoordinates();
        sphTest.HighlightClear();

        var (x, y, index) = sphTest.HighlightPointNearest(mouseX, mouseY);


        int pixelX = (int)e.MouseDevice.GetPosition(pltTest).X;
        int pixelY = (int)e.MouseDevice.GetPosition(pltTest).Y;

        (double coordinateX, double coordinateY) = pltTest.GetMouseCoordinates();

        XPixelLabel.Content = $"{pixelX:0.000}";
        YPixelLabel.Content = $"{pixelY:0.000}";

        XCoordinateLabel.Content = $"{coordinateX:0.00000000}";
        YCoordinateLabel.Content = $"{coordinateY:0.00000000}";

        vLine.position = coordinateX;
        hLine.position = coordinateY;

        pltTest.Render(skipIfCurrentlyRendering: true);
    }
}

[oszymczak]

@swharden thanks for the solution… tested it on my data set works perfect.

[swharden]

Hi @olofszymczak, @StendProg, and @bclehmann, this is an interesting discussion! There’s a lot to unpack here, and there are multiple ways to solve this problem. I think I found an efficient working solution to the primary issue. A few different topics are addressed here, so I’ll mention them separately:

Pixel-Aware GetPointNearest()

I’m still working on the PR, but briefly I modified the code that measures distance between two points so it now optionally incorporates the ratio of scales (pixels per unit) of X and Y axes. See #722 for details.

// Use it like this
double xyRatio = formsPlot1.Plot.XAxis.Dims.PxPerUnit / formsPlot1.Plot.YAxis.Dims.PxPerUnit;
(double x, double y, int index) = MyScatterPlot.GetPointNearest(mouseCoordX, mouseCoordY, xyRatio);

// This is how it works
public (double x, double y, int index) GetPointNearest(double x, double y, double xyRatio = 1)
{
    double pointDistanceSquared(double x1, double y1) =>
        (x1 - x) * (x1 - x) * (xyRatio * xyRatio) + (y1 - y) * (y1 - y);

    /* rest of the function is unchanged */
}

Full code is here: ScatterPlot.cs#L324-L351

I’ll work to make this simpler, better-documented, and add a working demo. For now though, this seems to work.

Optimization may be improved

As discussed above, it may be possible to further optimize this method for speed. I haven’t benchmarked it, but if LINQ and tuples are replaced with discrete statements it may run faster. This is outside the scope of the original issue, but worth considering. I added it as a low-priority triaged task in #716 and I probably won’t do it soon, but if someone wants to do this and make a PR I’d be happy to take a look.

ScatterPlotHighlight is Deprecated in ScottPlot 4.1

This plot type was useful for a while, but it has a highly undesirable design: it mixes point distance detection with rendering. This design limits what the user can do. For example, the user could not use this plot type to render something different (like an arrow) as a highlight. Also this plot type cannot be used for Signal plots or other plot types.

In ScottPlot 4.1 a IHasPoints interface was created that has a few GetPointNearest() methods. Plottables where highlight functionality is desired can simply implement this interface. Since it does not control rendering, users can display the highlight however they want.

The recommended way to display a highlighted point is to use AddPoint() to place a point (a scatter plot with a single X/Y value), set its marker shape to filled circle, then set its color and size how you like. When the mouse moves you can find the point nearest the mouse, then change this point’s X/Y position and visibility, and request a render. This method is demonstrated verbosely in this WinForms app: Form1.cs