如何在转换后获得pictureBox1的XY

Question

I have a list of Points that have been drawn on pictureBox1.

pictureBox1 has been transformed.

Now, I want to get XY coordinates of the point that was drawn as I hover over any drawn point.

When I hover over the pictureBox1, I am getting the XY of the pictureBox -- not a transformed XY.

Can you help me get to the transformed XY?

Thanks

    private void pictureBox1_Paint(object sender, PaintEventArgs e)
    {
        int height = pictureBox1.ClientSize.Height / 2;
        int width = pictureBox1.ClientSize.Width / 2;            

        //=====
        //scale
        //=====

        e.Graphics.SmoothingMode = SmoothingMode.AntiAlias;
        e.Graphics.TranslateTransform(-width, -height);
        e.Graphics.ScaleTransform(2f, 2f);

        //===========
        //draw center
        //===========

        e.Graphics.DrawLine(new Pen(Color.Black, 0.5f), new Point(width - 2, height), new Point(width + 2, height));
        e.Graphics.DrawLine(new Pen(Color.Black, 0.5f), new Point(width, height - 2), new Point(width, height + 2));

        //===========
        //draw points
        //===========

        foreach (var p in Points)
        {
            Point[] pts = new Point[] { new Point(p.X, p.Y) };
            Rectangle rc = new Rectangle(pts[0], new Size(1, 1));
            e.Graphics.DrawRectangle(Pens.Red, rc);
        }
    }

Answer 1

You can create a Matrix with necessary transformations and apply it in pictureBox1_Paint(...) via MultiplyTransform(...):

https://msdn.microsoft.com/en-us/library/bt34tx5d(v=vs.110).aspx

Then you can use Matrix::TransformPoints(...) to get transformed XY

Answer 2

As a variation to @Vitaly's answer you can do this:

After transforming the Graphics object you can save its transformation matrix e.Graphics.Transform in a variable:

Matrix matrix = null;
private void pictureBox1_Paint(object sender, PaintEventArgs e)
{
    int height = pictureBox1.ClientSize.Height / 2;
    int width = pictureBox1.ClientSize.Width / 2;

    //=====
    //scale
    //=====

    e.Graphics.SmoothingMode = SmoothingMode.AntiAlias;
    e.Graphics.TranslateTransform(-width, -height);
    e.Graphics.ScaleTransform(2f, 2f);

    matrix = e.Graphics.Transform;   // save the transformation matrix!
    ...

This is necessary as the transfomation data are lost after the Paint event! Note that the GraphicsState graphics.Save()&Restore() functions can't be used very well for this purpose, as it only puts the state on the stack for using it once , meaning it doesn't save these data in a persistent way.

Later you can use the Matrix and this function to either transform Points with the same matrix or reverse the transformation, eg for mouse coordinates:

PointF transformed(Point p0, bool forward)
{
    Matrix m = matrix.Clone();
    if (!forward)  m.Invert(); 

    var pt = new Point[] { p0 };
    m.TransformPoints(pt);
    return pt[0];
}

Now my MouseMove event shows the location both raw and re-transformed:

private void pictureBox1_MouseMove(object sender, MouseEventArgs e)
{
    label1.Text = e.Location + " <-> " + transformed(e.Location, false) ;
}

And to test the forward transformation you could add this to the end of the Paint event:

e.Graphics.ResetTransform();
for (int i = 0; i < Points.Count; i++)
{
    Point[] pts = new Point[] { Point.Round(transformed(Points[i], true)) };  
    Rectangle rc = new Rectangle(pts[0], new Size(19, 19));
    e.Graphics.DrawRectangle(Pens.Red, rc);
}

This first clears all the transformations and then paints larger Rectangles at the same locations by calling the transformed function.

Note that this will also work with a rotated Graphics object. (Although the last test does not draw the larger rectangles rotated, just moved to the right locations.)

Also note that I return PointF for better precision when scaling with fractions. You can use Point.Round (or Point.Truncate ) to get Point .

Do have a look the the Matrix.Elements : They contain the numbers you have used:

float scaleX = matrix.Elements[0];
float scaleY = matrix.Elements[3];
float transX = matrix.Elements[4];
float transY = matrix.Elements[5];

Finally: It is well worth studying the many methods of Matrix ..!