你根本不需要渲染的Canvas或任何其他可见的Panel。

只需使用Visual层上可用的基本图形基元即可。

DrawGeometry下面的方法Geometry使用位图的渲染大小（即将其DPI考虑在内的大小）绘制到BitmapSource上，并返回生成的BitmapSource。

public static BitmapSource DrawGeometry(
    BitmapSource source, Pen pen, Geometry geometry)
{
    var visual = new DrawingVisual();
    var rect = new Rect(0, 0, source.Width, source.Height);

    using (var dc = visual.RenderOpen())
    {
        dc.DrawImage(source, rect);
        dc.DrawGeometry(null, pen, geometry);
    }

    var target = new RenderTargetBitmap(
        (int)rect.Width, (int)rect.Height, 96, 96, PixelFormats.Default);

    target.Render(visual);
    return target;
}

为了绘制位图的像素单位并因此忽略其DPI，请按如下所示修改方法：

var rect = new Rect(0, 0, source.PixelWidth, source.PixelHeight);

using (var dc = visual.RenderOpen())
{
    dc.DrawRectangle(new ImageBrush(source), null, rect);
    dc.DrawGeometry(null, pen, geometry);
}

以下方法使用上述方法绘制多段线作为IEnumerable<Point>。

public static BitmapSource DrawPolyline(
    BitmapSource source, Pen pen, IEnumerable<Point> points)
{
    var geometry = new PathGeometry();

    if (points.Count() >= 2)
    {
        var figure = new PathFigure { StartPoint = points.First() };
        figure.Segments.Add(new PolyLineSegment(points.Skip(1), true));
        geometry.Figures.Add(figure);
    }

    return DrawGeometry(source, pen, geometry);
}

它会像

var source = new BitmapImage(new Uri(...));

var pen = new Pen
{
    Brush = Brushes.Blue,
    Thickness = 2,
};

var points = new List<Point>
{
    new Point(100, 100),
    new Point(1000, 100),
    new Point(1000, 1000),
    new Point(100, 1000),
    new Point(100, 100),
};

image.Source = DrawPolyline(source, pen, points);