Zoom an image in ASP.NET with GDI+ using Bitmap's OutStream option

The following snippet is the heart of the operation. This snippet resides in our ASPX page that we will use as the URL image of our image button. We have the following code which grabs some parameters passed in through the query string and a few through the session (this is not a great way to do this, but this is only a quic k demo).

Then, it will take a look at where the user clicked and create a region around that based on a zoom level. It will then grab a smaller source rect and draw it to the destination. It will then output it to the stream, which is ImageButton1 in page ZoomZoom.

Once you have the Graphics object, you are able to draw lines or whatever you wish, so you can do anything GDI+ allows.

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protected void Page_Load(object sender, EventArgs e)
{
   Mouse mouse = (Mouse)Session[Mouse.SessionName];

   ImageViewParameters Pageparams = 
     (ImageViewParameters)Session[ImageViewParameters.SessionName];

   if (mouse == null || Pageparams == null )
   {
      Server.Transfer("Zoomzoom.aspx");
   }

   string loc = Request.QueryString[PageVariables.ImagePath].ToString();
   // this.ImageButton1.ImageUrl.Replace("~", "");

   if (loc == null)
   { 
      return;
   }

   if (loc.Length < 5)
   {
      return;
   }

   Bitmap bitmap = new Bitmap(loc);

   MouseToImageCoordinates(mouse, Pageparams, bitmap);
   // Dimentions of the zoomed square
   float squareSizeX = (int)( (float)bitmap.Width / (float)Pageparams.ZoomLevel );
   float squareSizeY = (int)((float) bitmap.Height / (float)Pageparams.ZoomLevel );
   // Create a new bitmap which we will draw on
   Bitmap bitmap2 = new Bitmap(bitmap.Width, bitmap.Height);
   // Get the graphics device from the blank image
   Graphics g = Graphics.FromImage( bitmap2 );
   // Make a rect around where the user clicked
   float desX = ( mouse.X - (squareSizeX / 2.0f) ) ;
   float desY = ( mouse.Y - (squareSizeY / 2.0f) );

   RectangleF source = new RectangleF(desX, desY, squareSizeX, squareSizeY);
   RectangleF destination = new RectangleF(0, 0, bitmap.Width, bitmap.Height);

   g.DrawImage(bitmap, destination, source, GraphicsUnit.Pixel);
   // Output the image to the stream
   bitmap2.Save(Response.OutputStream, ImageFormat.Jpeg);

}

Enhancements

The first image at the top navigates where the user clicks, but it might be better if it worked more like the Windows image viewer. I did it that way at first, but it was too slow to get to the farthest zoom level, and would cause needless server utilization. I would recommend drawing a box in the navigation image showing the viewable portion of the zoomed in image below. (Just draw the source rect of the zoomed image on the other image.)
If you click on the edge, it sometimes shows black in a low zoom level.

Points of Interest

I did spend quite some time researching the best way to do this, but not much time on the code. A few things to watch out for: it will probably hammer the server's CPU pretty good, so I'm not sure how many users can view the page at once.

Alternatives

I also tried embedding a Windows Forms control into a webpage which does work (Google for Andrew Ma's articles) without the user having to do anything; however, it seemed like sometimes the user might get a blank box depending on how the security was set up. It also felt really hackish. If the zone was not trusted, then no errors are reported, it just doesn't work. The other way to do this would be an ActiveX control, but then again, you have security issues and upgrade issues. I also viewed a bunch of AJAX samples, but they also run fairly slow (at least, the ones I saw). The AJAX samples didn't run faster than this, but IIS is on my machine with just one user, so that's not a fair test.

This is the best way I have seen this done from a deployment perspective .