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Introduction

A big question in the Windows Forms world is "How can I zoom and scroll the PictureBox?" The answer is to use the ImagePanel, which enables you to simply zoom and pan an image.

I use Microsoft Visual C# 2005 Express Edition to build it. It's easy and simple. You can start a new project and add a user control. In the Design window, you drag a vertical scrollbar and a horizontal scrollbar to the user control, then go to the code window and add the code as in the source file, ImagePanel.cs. That's all, a zoomable and scrollable PictureBox replacement has been built and you can use it now.

Zooming is done by a Matrix object:

Matrix mx=new Matrix(); // create an identity matrix
mx.Scale(zoom,zoom); // zoom image

Panning is done by the values returned by the scrollbars. We tell the scrollbar the image size, which is its real size multiplied by the zoom factor:

if ((canvasSize.Width * zoom - viewRectWidth) > 0)
{
    this.hScrollBar1.Maximum =(int)( canvasSize.Width * zoom) - viewRectWidth;
}
if ((canvasSize.Height * zoom - viewRectHeight) > 0)
{
    this.vScrollBar1.Maximum = (int)(canvasSize.Height * zoom) - viewRectHeight;
}

So, the correct position within the real image is the scrollbar values divided by the zoom factor:

Point pt=new Point((int)(hScrollBar1.Value/zoom),(int)(vScrollBar1.Value/zoom));

To display the image on the center of this control, the Matrix object is used again:

mx.Translate(viewRectWidth/2.0f,viewRectHeight/2.0f, MatrixOrder.Append);

This component has a good feature: during image zoom out, only a portion of the image is drawn. It saves a lot of computer resources:

if (canvasSize.Width * zoom < viewRectWidth && 
            canvasSize.Height * zoom < viewRectHeight)
    srcRect = new Rectangle(0, 0, canvasSize.Width, canvasSize.Height);
    // view all image
else srcRect = new Rectangle(pt, new Size((int)(viewRectWidth / zoom), 
                            (int)(viewRectHeight / zoom)));
// view a portion of image

To test this little but strong control, I put it on a form and added a trackbar to provide the zoom factor. Double click the ImagePanel to load a picture. Try it, how's it?

Thanks.

A Zoomable and Scrollable PictureBox