Cropping Images

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Introduction

In this article, I'll give a demo of how to crop an image by selecting a region with the mouse.

Background

We select a region of an image, create another image of the selected region, and zoom the new image to the size of the PictureBox.

Although the PictureBox has its the SizeMode = PictureBoxSizeMode.Zoom, a MemoryOutOfRange exception may be thrown when cropping the image several times. So, I assume this property is just for fitting an image to a PictureBox once when loading the form.

Extension methods for Image

I've implemented the cropping and fitting to the PictureBox as an extension to the Image class.

Cropping

The image is cropped by cloning a region of the original image.

/// <summary>
/// Crops an image according to a selection rectangel
/// </summary>
/// <param name="image">
/// the image to be cropped
/// </param>
/// <param name="selection">
/// the selection
/// </param>
/// <returns>
/// cropped image
/// </returns>
public static Image Crop(this Image image, Rectangle selection)
{
    Bitmap bmp = image as Bitmap;

    // Check if it is a bitmap:
    if (bmp == null)
        throw new ArgumentException("No valid bitmap");

    // Crop the image:
    Bitmap cropBmp = bmp.Clone(selection, bmp.PixelFormat);

    // Release the resources:
    image.Dispose();

    return cropBmp;
}

Fitting the image to the PictureBox

As mentioned under the Background section, fitting can't be done by the PictureBox itself. So, I've coded this myself.

As the first step, the scale factors in the vertical and horizontal directions are calculated. To scale (or zoom) the image so that the original ratio of width to height is maintained, the bigger ratio of the scale is used. The interpolation-mode is set to produce high quality pictures.

/// <summary>
/// Fits an image to the size of a picturebox
/// </summary>
/// <param name="image">
/// image to be fit
/// </param>
/// <param name="picBox">
/// picturebox in that the image should fit
/// </param>
/// <returns>
/// fitted image
/// </returns>
/// <remarks>
/// Although the picturebox has the SizeMode-property that offers
/// the same functionality an OutOfMemory-Exception is thrown
/// when assigning images to a picturebox several times.
/// 
/// AFAIK the SizeMode is designed for assigning an image to
/// picturebox only once.
/// </remarks>
public static Image Fit2PictureBox(this Image image, PictureBox picBox)
{
    Bitmap bmp = null;
    Graphics g;

    // Scale:
    double scaleY = (double)image.Width / picBox.Width;
    double scaleX = (double)image.Height / picBox.Height;
    double scale = scaleY < scaleX ? scaleX : scaleY;

    // Create new bitmap:
    bmp = new Bitmap(
        (int)((double)image.Width / scale),
        (int)((double)image.Height / scale));

    // Set resolution of the new image:
    bmp.SetResolution(
        image.HorizontalResolution,
        image.VerticalResolution);

    // Create graphics:
    g = Graphics.FromImage(bmp);

    // Set interpolation mode:
    g.InterpolationMode = InterpolationMode.HighQualityBicubic;

    // Draw the new image:
    g.DrawImage(
        image,
        new Rectangle(            // Destination
            0, 0,
            bmp.Width, bmp.Height),
        new Rectangle(            // Source
            0, 0,
            image.Width, image.Height),
        GraphicsUnit.Pixel);

    // Release the resources of the graphics:
    g.Dispose();
    
    // Release the resources of the origin image:
    image.Dispose();

    return bmp;
}

The user interface (for the demo)

Restore original image

To restore the original image, it is saved in the form-load event. Notice, save a copy of the image and not a reference. In the latter case, the copy would point to the (cropped) image in the PictureBox. Also, on restoring, a copy is assigned to the PictureBox.

private Image _originalImage;

private void Form1_Load(object sender, System.EventArgs e)
{
    // Save just a copy of the image on no reference!
    _originalImage = pictureBox1.Image.Clone() as Image;
}

private void button1_Click(object sender, System.EventArgs e)
{
    pictureBox1.Image = _originalImage.Clone() as Image;
}

Selecting the region

Selecting a region is straightforward. Just use the MouseDown and MouseMove events.

private bool _selecting;
private Rectangle _selection;

private void pictureBox1_MouseDown(object sender, MouseEventArgs e)
{
    // Starting point of the selection:
    if (e.Button == MouseButtons.Left)
    {
        _selecting = true;
        _selection = new Rectangle(new Point(e.X, e.Y), new Size());
    }
}

private void pictureBox1_MouseMove(object sender, MouseEventArgs e)
{
    // Update the actual size of the selection:
    if (_selecting)
    {
        _selection.Width = e.X - _selection.X;
        _selection.Height = e.Y - _selection.Y;

        // Redraw the picturebox:
        pictureBox1.Refresh();
    }
}

To display the selection, a rectangle is drawn on the picture.

private void pictureBox1_Paint(object sender, PaintEventArgs e)
{
    if (_selecting)
    {
        // Draw a rectangle displaying the current selection
        Pen pen = Pens.GreenYellow;
        e.Graphics.DrawRectangle(pen, _selection);
    }
}

Cropping

The end of selection is indicated by a MouseUp. Before cropping, the size of the selection is validated because it could be zero in the case of double-clicking in the PictureBox.

private void pictureBox1_MouseUp(object sender, MouseEventArgs e)
{
    if (e.Button == MouseButtons.Left &&
        _selecting &&
        _selection.Size != new Size())
    {
        // Create cropped image:
        Image img = pictureBox1.Image.Crop(_selection);

        // Fit image to the picturebox:
        pictureBox1.Image = img.Fit2PictureBox(pictureBox1);

        _selecting = false;
    }
    else
        _selecting = false;
}

History

• 5^th November 2008: Initial release.