Incorrect scaling of Pen when using Graphics.ScaleTransform

Question

I'm seeing strange behaviour when drawing a line with a scale transform (Graphics.ScaleTransform() - see MSDN) in my OnPaint() method.

When using a large y-scale factor for the ScaleTransform method, then if the x-scale is set above 1x, the line suddenly becomes much larger.

Setting the width of pen with which the line is drawn to -1 seems to get round the problem, but I do not want to draw a very thin line (the line must be printed later, 1px is too thin).

Here's some sample code to demonstrate the problem:

public class GraphicsTestForm : Form
{
    private readonly float _lineLength = 300;
    private readonly Pen _whitePen;

    private Label _debugLabel;

    public GraphicsTestForm()
    {
        ClientSize = new Size(300, 300);

        Text = @"GraphicsTest";
        SetStyle(ControlStyles.ResizeRedraw, true);

        _debugLabel = new Label
        {
            ForeColor = Color.Yellow,
            BackColor = Color.Transparent
        };
        Controls.Add(_debugLabel);

        _lineLength = ClientSize.Width;
        _whitePen = new Pen(Color.White, 1f); // can change pen width to -1
    }

    protected override void OnPaint(PaintEventArgs e)
    {
        base.OnPaint(e);

        float scaleX = ClientSize.Width / _lineLength;
        const int ScaleY = 100;

        e.Graphics.Clear(Color.Black);

        _debugLabel.Text = @"x-scale: " + scaleX;

        // scale the X-axis so the line exactly fits the graphics area
        // scale the Y-axis by scale factor
        e.Graphics.ScaleTransform(scaleX, ScaleY);

        float y = ClientSize.Height / (ScaleY * 2f);
        e.Graphics.DrawLine(_whitePen, 0, y, _lineLength, y);

        e.Graphics.ResetTransform();
    }
}

I would like the line/pen to scale gracefully, without jumping in size so dramatically.

(Additionally, I noticed that when the line is very large, it is not drawn continuously across multiple monitors. Perhaps this is related?)

Answer 1

Try to change the pen width according to the scale:

 _whitePen = new Pen(Color.White, 1f / ScaleY); 
 e.Graphics.DrawLine(_whitePen, 0, y, _lineLength, y);