c# - Cropping images sometimes a wrong area

Question

I'm using jQuery-cropbox to crop images on a web application.

Everything is running smoothly most of the time, but from time to time, the generated image is wrong, as if the cropping was applied with wrong coordinates.

As an example, consider this image :

In cropbox, I zoom and drag until I get this :

But when I click on the crop button, it gives me this:

Here is the cropping code:

private Bitmap CropImage(Image img, Rectangle rect)
{
    Bitmap newImg = new Bitmap(rect.Width, rect.Height);
    Graphics g = Graphics.FromImage(newImg);
    g.DrawImage(img, -rect.X, -rect.Y);

    return newImg;
}

//...

// save image
var streamOut = new MemoryStream();
Bitmap newImg = CropImage(oldImg, rect);
newImg.Save(streamOut, ImageFormat.Png);

oldImg is the image previously posted, rect the coordinates I use to crop.

I checked the coordinates in rect of course, they are correct.

For reference, to obtain the result provided the coordinates are (those may not be exactly accurate, I took them from a second test, so I certainly placed the picture a bit differently):

x: 47
y: 94
w: 150
h: 150

What is happening ?

Answer 1

Your image (as posted) has 72dpi; your screen and therefore the Graphics you use probably have 96dpi.

This will lead to the effect you see.

To avoid the effect simply set the resolution of img before drawing it to the one in your Graphics object:

img.SetResolution((int)g.DpiX, (int)g.DpiY);