How to zoom at a point in picturebox

Question

This is my code.
I am able to zoom the picturebox but not at a point. How to zoom to a mouse point when i rotate the mouse wheel?

The variables are:-

private double m_dZoomscale = 1.0;    //THIS IS THE ZOOM SCALE TO WHICH EACH OBJECT 
                                      //ARE ZOOMED IN THE CANVAS  

public static double s_dScrollValue = .01;
                                      //scale factor value for mouse scroll zooming

The paint code is:-

private void m_Picturebox_Canvas_Paint(object sender, PaintEventArgs e)
{
   Graphics g = e.Graphics;
   g.ScaleTransform((float)m_dZoomscale, (float)m_dZoomscale);
}

The main code is:-

    protected override void OnMouseWheel(MouseEventArgs mea)
    {
        m_Picturebox_Canvas.Focus();
        if (m_Picturebox_Canvas.Focused == true)
        {
            if (mea.Delta > 0)
            {
                ZoomInScroll();
            }
            else if (mea.Delta < 0)
            {
                ZoomOutScroll();
            }
        }
    }

The sub functions are as follows:-

    //FUNCTION FOR MOUSE SCROL ZOOM-IN
    private void ZoomInScroll()
    {
        m_dZoomscale = m_dZoomscale + s_dScrollValue;
        m_Picturebox_Canvas.Invalidate();
    }

    //FUNCTION FOR MOUSE SCROL ZOOM-IN
    private void ZoomOutScroll()
    {
        m_dZoomscale = m_dZoomscale - s_dScrollValue;
        m_Picturebox_Canvas.Invalidate();
    }

Solution

Here is how you achieve this (description below code):

Variables

// this tracks the transformation applied to the PictureBox's Graphics
private Matrix transform = new Matrix();      
public static double s_dScrollValue = 1.01; // zoom factor

Paint code

private void m_Picturebox_Canvas_Paint(object sender, PaintEventArgs e)
{
   Graphics g = e.Graphics;
   g.Transform = transform;
}

Scroll-Event code

protected override void OnMouseWheel(MouseEventArgs mea)
{
    m_Picturebox_Canvas.Focus();
    if (m_Picturebox_Canvas.Focused == true && mea.Delta != 0)
    {
        ZoomScroll(mea.Location, mea.Delta > 0);
    }
}

Zoom function

//FUNCTION FOR MOUSE SCROL ZOOM-IN
private void ZoomScroll(Point location, bool zoomIn)
{
    // make zoom-point (cursor location) our origin
    transform.Translate(-location.X, -location.Y);

    // perform zoom (at origin)
    if(zoomIn)
        transform.Scale(s_dScrollValue, s_dScrollValue);
    else
        transform.Scale(1 / s_dScrollValue, 1 / s_dScrollValue);

    // translate origin back to cursor
    transform.Translate(location.X, location.Y);

    m_Picturebox_Canvas.Invalidate();
}

Description

First of all, as you can see I combined your two zoom methods to one method: ZoomScroll Otherwise we would duplicate a lot of logic...

So what is done here? I guess it is clear that we need to also apply a translation to the Graphics object. We "accumulate" all transformation applied to the PictureBox in a Matrix field.

You successfully scaled your image, but always with the origin (upper-left corner of the PictureBox) as the locical center of your scaling-operation - that is just how Scale/ScaleTransform works! So in order to scale at a different point the following steps are needed:

• translate the world, so that the point you want to scale at is the new origin (e.g. your cursor is at 12|34, so we translate by -12|-34)
• now that the desired spot is our new origin, scale
• translate the world back, so the original point ends up under your cursor again