C# Image Rotation

If you’re interested in learning more about the mathematical side of image rotation, you might want to explore image transformation matrices.

Creating the Rotation Method

The goal is to create a method that takes a Bitmap object and a rotation angle (in degrees) and returns the rotated image.

private Bitmap RotateImage(Bitmap b, float angle)
{
    // Step-by-step process will be explained below
}

We pass in a Bitmap object (the image we want to rotate) and the angle (the degree of rotation, clockwise).

Create a New Bitmap and Graphics Object

First, we create a new empty Bitmap that will hold the rotated image. This new bitmap will have the same dimensions as the input image. We then create a Graphics object from the empty bitmap to perform the drawing operations.

private Bitmap RotateImage(Bitmap b, float angle)
{
    // Create a new empty bitmap to hold the rotated image
    Bitmap returnBitmap = new Bitmap(b.Width, b.Height);
    // Create a graphics object from the empty bitmap
    Graphics g = Graphics.FromImage(returnBitmap);
}

Move the Image to the Center

By default, an image rotates around its top-left corner. To rotate it around its center, we need to first move the image's rotation point to the center of the image.

g.TranslateTransform((float)b.Width / 2, (float)b.Height / 2);

Rotate the Image

Once the center of the image is set as the rotation point, we can apply the rotation using the RotateTransform method. The angle parameter determines how much the image should be rotated clockwise.

g.RotateTransform(angle);

Move the Image Back to its Original Position

After applying the rotation, we need to move the image back to its original position to prevent it from being displaced. This is done by applying a reverse translation using the TranslateTransform method again.

g.TranslateTransform(-(float)b.Width / 2, -(float)b.Height / 2);

Draw the Image

Finally, we use the Graphics object to draw the rotated image onto the new bitmap. The DrawImage method is used to copy the original image into the graphics object.

g.DrawImage(b, new Point(0, 0));

Complete Code for Rotating an Image in C#

Putting all the steps together, here is the complete code for the RotateImage method:

private Bitmap RotateImage(Bitmap b, float angle)
{
    // Create a new empty bitmap to hold the rotated image
    Bitmap returnBitmap = new Bitmap(b.Width, b.Height);
    // Create a graphics object from the empty bitmap
    Graphics g = Graphics.FromImage(returnBitmap);
    
    // Move the rotation point to the center of the image
    g.TranslateTransform((float)b.Width / 2, (float)b.Height / 2);
    
    // Rotate the image by the specified angle
    g.RotateTransform(angle);
    
    // Move the image back to its original position
    g.TranslateTransform(-(float)b.Width / 2, -(float)b.Height / 2);
    
    // Draw the original image onto the graphics object
    g.DrawImage(b, new Point(0, 0));
    
    // Return the rotated image
    return returnBitmap;
}

In this tutorial, we learned how to rotate an image in C# by using the Graphics object and transformation methods.