How to save graphics created on a PictureBox?

问题:

In c# and Visual Studio Windows forms I have loaded an image into a picture box (pictureBox2) and then cropped it and show in another picture box (pictureBox3).

Now I want to save what is inside pictureBox3 as an image file.

How can I do this?

private void crop_bttn_Click(object sender, EventArgs e)
{
    Image crop = GetCopyImage("grayScale.jpg");
    pictureBox2.Image = crop;

    Bitmap sourceBitmap = new Bitmap(pictureBox2.Image, 
                                     pictureBox2.Width, pictureBox2.Height);
    Graphics g = pictureBox3.CreateGraphics();

    g.DrawImage(sourceBitmap, new Rectangle(0, 0, 
                pictureBox3.Width, pictureBox3.Height), rectCropArea, GraphicsUnit.Pixel);
    sourceBitmap.Dispose();
}

回答1:

Never use control.CreateGraphics! Either draw into a Bitmap bmp using a Graphics g = Graphics.FromImage(bmp) or in the Paint event of a control, using the e.Graphics parameter..

Here is a cropping code that draws into a new Bitmap and that makes use of your controls etc but changes a few things:

• It uses a Graphics object that is created from a new Bitmap
• It make use of using clauses to make sure it won't leak
• It takes the size of the pictureBox3.ClientSize so it won't include any borders..

---

private void crop_bttn_Click(object sender, EventArgs e)
{
    Image crop = GetCopyImage("grayScale.jpg");
    pictureBox2.Image = crop;
    Bitmap targetBitmap = new Bitmap(pictureBox3.ClientSize.Width, 
                                    pictureBox3.ClientSize.Height);
    using (Bitmap sourceBitmap = new Bitmap(pictureBox2.Image, 
                 pictureBox2.ClientSize.Width, pictureBox2.ClientSize.Height))
    {
        using (Graphics g = Graphics.FromImage(targetBitmap))
        {
            g.DrawImage(sourceBitmap, new Rectangle(0, 0, 
                        pictureBox3.ClientSize.Width, pictureBox3.ClientSize.Height), 
                        rectCropArea, GraphicsUnit.Pixel);
        }
    }
    if (pictureBox3.Image != null) pictureBox3.Image.Dispose();
    pictureBox3.Image = targetBitmap;
    targetBitmap.Save(somename, someFormat);
}

---

The alternative would be to..:

• move all your code to the Paint event
• replace the Graphics g = pictureBox3.CreateGraphics(); be Graphics g = e.Graphics;
• insert these two lines to the click event:

---

Bitmap targetBitmap = new Bitmap(pictureBox3.ClientSize.Width, 
                                pictureBox3.ClientSize.Height);
pictureBox3.DrawToBitmap(targetBitmap, pictureBox3.ClientRectangle);

回答2:

The method PictureBox.CreateGraphics() should not be used unless you know what you are doing because it can cause some not-so-obvious problems. For example, in you scenario, the image in pictureBox3 will disappear when you minimize or resize the window.

A better way is to draw to a bitmap, which you also can save:

var croppedImage = new Bitmap(pictureBox3.Width, pictureBox3.Height);
var g = Graphics.FromImage(croppedImage);
g.DrawImage(crop, new Point(0, 0), rectCropArea, GraphicsUnit.Pixel);
g.Dispose();
//Now you can save the bitmap
croppedImage.Save(...);
pictureBox3.Image = croppedImage;

Btw, please use more reasonable variable names, especially for pictureBox1..3.