flip coordinates when drawing to control

Question

I am drawing a graph on a control, but 0,0 is at the top-left hand corner of the control. Is there a way to flip the coordinates so that 0,0 is at the lower left corner of the control?

Answer 1

If you are using WinForms, then you might find that you can flip the Y-Axis using Graphics.ScaleTransform:

private void ScaleTransformFloat(PaintEventArgs e)
{
    // Begin graphics container
    GraphicsContainer containerState = e.Graphics.BeginContainer();

    // Flip the Y-Axis
    e.Graphics.ScaleTransform(1.0F, -1.0F);

    // Translate the drawing area accordingly
    e.Graphics.TranslateTransform(0.0F, -(float)Height);

    // Whatever you draw now (using this graphics context) will appear as
    // though (0,0) were at the bottom left corner
    e.Graphics.DrawRectangle(new Pen(Color.Blue, 3), 50, 0, 100, 40);

    // End graphics container
    e.Graphics.EndContainer(containerState);

    // Other drawing actions here...
}

You only need to include the begin/end container calls if you want to do additional drawing using the regular coordinate system as well. More information on graphics containers is available on MSDN.

As mentioned by Tom in the comments, this approach requires the Height value to be available with a correct value. If you try this and see nothing being drawn, ensure that value is correct in a debugger.

Answer 2

Here's a simple UserControl that demonstrates how to do this:

public partial class UserControl1 : UserControl
{
    public UserControl1()
    {
        SetStyle(ControlStyles.AllPaintingInWmPaint | ControlStyles.UserPaint | ControlStyles.DoubleBuffer, true);

        InitializeComponent();
    }

    protected override void OnPaint(PaintEventArgs e)
    {
        e.Graphics.ScaleTransform(1.0F, -1.0F);
        e.Graphics.TranslateTransform(0.0F, -(float)Height);
        e.Graphics.DrawLine(Pens.Black, new Point(0, 0), new Point(Width, Height));

        base.OnPaint(e);
    }
}