裁剪图像C＃失真

Question

I am trying to crop an image with C# but i have some problem.

I need to crop this image and remove 15 pixels from top:

I have used this code:

Bitmap myBitmap = new Bitmap(outputFileName);
Rectangle destRectangle = new Rectangle(new Point(0, 15),
new Size(myBitmap.Width, myBitmap.Height));
Bitmap bmp = new Bitmap(myBitmap.Width, myBitmap.Height - 15);
Graphics g = Graphics.FromImage(bmp);
g.DrawImage(myBitmap, 0, 0, destRectangle, GraphicsUnit.Pixel);
bmp.Save(outputFileNameCut, ImageFormat.Png);

This is the first image quality with a zoom:

and this the second:

How can i have the same image quality?

Answer 1

The problem is that you are grabbing a rectangle higher than what fits in the bitmap (the second Size parameter is the size, not the bottom-right coordinates), so it's scaling it:

Rectangle destRectangle = new Rectangle(
       new Point(0, 15), new Size(myBitmap.Width, myBitmap.Height-15));

This should work... since it's not actually the dest rectangle, but the source rectangle on the DrawImage call

Other way to crop, which doesn't even need a Graphics object might be:

Bitmap myBitmap = new Bitmap(outputFileName);
Rectangle srcRectangle = new Rectangle(
       new Point(0, 15), new Size(myBitmap.Width, myBitmap.Height-15));
Bitmap croppedBitmap = myBitmap.Clone(srcRectangle, myBitmap.PixelFormat);
croppedBitmap.Save(outputFileNameCut, ImageFormat.Png);

If you use this method, make 100% sure that the crop rectangle doesn't cross the bounds of the original image, since Clone will throw an exception if it does.

Answer 2

Try to paste

g.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.None;
g.InterpolationMode = System.Drawing.Drawing2D.InterpolationMode.NearestNeighbor;

before calling DrawImage

or use

g.DrawImageUnscaled(myBitmap, new Point(0, -15));

Answer 3

Try to use ImageProcessor framework. It can handle this easily using .Quality() method.