面板显示缩放的图像

Question

I am using this code to draw a scrollable panel on win form. The ImageBox is 512x512 and image I am using is 1024x768 (added as resource):

imageBox1.Image = Properties.Resources.test;

Unfortunately, it seems like image is scaled for some reasons - I cannot scroll to it's border. If I use 512x512 image, it doesn't fit the ImageBox, it seems cropped. Any ideas what is going on here?

using System;
using System.Drawing;
using System.Windows.Forms;

class ImageBox : Panel {
    public ImageBox() {
        this.AutoScroll = true;
        this.DoubleBuffered = true;
    }
    private Image mImage;
    public Image Image {
        get { return mImage; }
        set {
            mImage = value;
            if (mImage != null) this.AutoScrollMinSize = mImage.Size;
            else this.AutoScrollMinSize = new Size(0, 0);
            this.Invalidate();
        }
    }
    protected override void OnPaint(PaintEventArgs e) {
        e.Graphics.TranslateTransform(this.AutoScrollPosition.X, this.AutoScrollPosition.Y);
        if (mImage != null) e.Graphics.DrawImage(mImage, 0, 0);
        base.OnPaint(e);
    }
}

Answer 1

DrawImage has many variants and it is well worth checking them all out carefully. You have chosen the wrong one for your purpose. Look at the doc at MSDN:

 Graphics.DrawImage Method (Image, Int32, Int32) ... Draws the specified image, using its original physical size, at the location specified by a coordinate pair. 

At first glance this sounds good. 'Physical size' - isn't that pixels? But read on at MSDN:

 Remarks An Image stores a value for pixel width and a value for horizontal resolution (dots per inch). The physical width, measured in inches, of an image is the pixel width divided by the horizontal resolution. For example, an image with a pixel width of 216 and a horizontal resolution of 72 dots per inch has a physical width of 3 inches. Similar remarks apply to pixel height and physical height. The DrawImage method draws an image using its physical size, so the image will have its correct size in inches regardless of the resolution (dots per inch) of the display device. For example, suppose an image has a pixel width of 216 and a horizontal resolution of 72 dots per inch. If you call DrawImage to draw that image on a device that has a resolution of 96 dots per inch, the pixel width of the rendered image will be (216/72)*96 = 288. 

Ouch, that's not about pixels after all! It is about displays and the resolution the image has embedded in it..This is good if you want to get the image eg printed right on all printers.

But you want the pixels of the image to match the pixels of the display. You could adapt the resolution of the image to your screen; but that wouldn't work for a different screen. So this DrawImage call will not work for you..

So you should, quite simply, use the number of pixels your image has and feed them into the right DrawImage call:

e.Graphics.DrawImage(mImage, 0, 0, mImage.Width, mImage.Height);

Now it will not distort the image but put one image pixel onto one screen pixel..

Edit: Note: I had misquoted MSDN in my OP; now the right (but wrong for your purpose) method call is quoted in the first part..

Answer 2

It is an issue with the resolution of the image, it is less than the resolution of your display. Pretty unusual.

There is more than one workaround for this. @TaW's approach works but favors the monitor resolution. You'll get a sharper image but it will not be close to the image size as originally recorded. The other approach is to keep the physical size, like DrawImage() does, and adjust the scrollbars accordingly. Change the Image property setter to:

    set {
        mImage = value;
        if (value == null) this.AutoScrollMinSize = new Size(0, 0);
        else {
            var size = value.Size;
            using (var gr = this.CreateGraphics()) {
                size.Width = (int)(size.Width * gr.DpiX / value.HorizontalResolution);
                size.Height = (int)(size.Height * gr.DpiY / value.VerticalResolution);
            }
            this.AutoScrollMinSize = size;
        }
        this.Invalidate();
    }

Picking the "right" approach is not so obvious, you probably ought to consider adding another property so you can change it as needed.