如何从矩形获取图像？

Question

I'm making a program that's cropping images. I have two PictureBoxes and a Button named 'crop'. One picture box contains an image and when I select a rectangle in it and press 'Crop' the selected area appears in the other picture box; so the program is working when I press crop. The problem is: How can I get the image from crop area into picture box Image?

 Rectangle rectCropArea;
 Image srcImage = null;

 TargetPicBox.Refresh();
 //Prepare a new Bitmap on which the cropped image will be drawn
 Bitmap sourceBitmap = new Bitmap(SrcPicBox.Image, SrcPicBox.Width, SrcPicBox.Height);
 Graphics g = TargetPicBox.CreateGraphics();

 g.DrawImage(sourceBitmap, new Rectangle(0, 0, TargetPicBox.Width, TargetPicBox.Height), 
 rectCropArea, GraphicsUnit.Pixel);

 //Good practice to dispose the System.Drawing objects when not in use.
 sourceBitmap.Dispose();

 Image x = TargetPicBox.Image;

The problem is that x = null and the image is showing in the picture box so how can I get the Image from this picture box into the Image variable ?

Answer 1

A couple of issues:

• First and most important: You are being confused about the relationship between PictureBox.Image (a Property) and the Graphics you associate with the PictureBox 's surface . The Graphics object you get from Control.CreateGraphics is only able to paint onto the surface of the control; usually not what you want; and even when you do, you usually want to do it in a Paint event using e.Graphics ..

So, while your code seems to work, it only paints non-persistent pixels onto the surface. Minimize/maximize and you'll see what non-persistent means..!

To change a Bitmap bmp you need to associate it with a Grahics object like this:

Graphics g = Graphics.FromImage(bmp);

Now you can draw into it:

g.DrawImage(sourceBitmap, targetArea, sourceArea, GraphicsUnit.Pixel);

After that you can assign the Bitmap to the Image Property of the TargetPicBox ..

Finally dispose of the Graphics , or better, put it into a using clause..

I am assuming that you have managed to give the rectCropArea meaningful values.

• Also note that the way you copy the source bitmap has an error: If you want the full image, do use its Size (*), not the one of the PictureBox !!

• And instead of creating a target rectangle, with the same error, simply use the TargetPicBox.ClientRectangle !

Here is an example code for the crop Button:

 // a Rectangle for testing
 Rectangle rectCropArea = new Rectangle(22,22,55,99);
 // see the note below about the aspect ratios of the two rectangles!!
 Rectangle targetRect = TargetPicBox.ClientRectangle;
 Bitmap targetBitmap = new Bitmap(targetRect.Width, targetRect.Height);
 using (Bitmap sourceBitmap = new Bitmap(SrcPicBox.Image,
                              SrcPicBox.Image.Width, SrcPicBox.Image.Height) )
 using (Graphics g = Graphics.FromImage(targetBitmap))
        g.DrawImage(sourceBitmap, targetRect, rectCropArea, GraphicsUnit.Pixel);

 if (TargetPicBox.Image != null) TargetPicBox.Dispose();
 TargetPicBox.Image = targetBitmap;

• Of course you should have prepared the Rectangle in the proper mouse events!
• Here you would want to decide on the aspect ratio of the result; you probably don't want to distort the result! So you need to decide whether to crop the source cropping rectangle or whether to expand the target rectangle..!
• Unless you are sure about the dpi resolution you should use SetResolution to make sure the new image has the same!

Note that since I assign targetBitmap to TargetPicBox.Image I must not dipose of it! Instead, before assigning a new Image , I first Dispose the old one..