Changing resolution or cropping an image

One of the simpler processes in GDI+ image manipulation is changing the resolution of an image or cropping an image. This all depends on the DrawImage method as illustrated in the GDI+ FAQ article "Drawing an image" which you may wish to read before continuing.

To change the resolution of an image you need to go through the following steps.

• Load the original image
• Create a blank image of the resolution you desire.
• Get a Graphics object for the new image
• Draw the original image onto the new one
• Save the new image, perhaps with a different filename.

Cropping an image is similar in operation.

• Load the original image
• Create a blank image the same size as the area you want to crop to
• Get the Graphics object for the new image
• Draw the desired portion of the original image onto the new image
• Save the new image, perhaps with a different filename.

Listing 1 shows how to change the resolution of an image to make it 30% of the original's width and height.

Bitmap bm = (Bitmap)Image.FromFile("myimage.jpg");

Bitmap resized = new Bitmap((int)(0.3f*bm.Width),(int)(0.3f*bm.Height));

Graphics g=Graphics.FromImage(resized);

g.DrawImage(bm, new Rectangle(0,0,resized.Width,resized.Height),0,0,bm.Width,bm.Height,GraphicsUnit.Pixel);

g.Dispose();

resized.Save("resizedimage.jpg",ImageFormat.Jpeg);

Listing 2 shows how to crop an image..

Bitmap bm = (Bitmap)Image.FromFile("myimage.jpg"); // assumes a 400 * 300 image from which a 160 * 120 chunk will be taken

Bitmap cropped = new Bitmap(160,120);

Graphics g=Graphics.FromImage(cropped);

g.DrawImage(bm, new Rectangle(0,0,cropped.Width,cropped.Height),100,50,cropped.Width,cropped.Height,GraphicsUnit.Pixel);

g.Dispose();

cropped.Save("croppedimage.jpg",ImageFormat.Jpeg);

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