The TransformNormal method is used to transform a normal vector by a matrix. This method is particularly useful in graphics programming where you need to transform normals for lighting calculations. Unlike transforming a position vector, transforming a normal vector does not involve translation, which is why this method is distinct from the Transform method.

To use the TransformNormal method, you need an instance of the Matrix struct and a Vector3 representing the normal you wish to transform. The method returns a new Vector3 that is the result of the transformation.

Example usage:

Matrix matrix = Matrix.CreateRotationX(45.0f);
Vector3 normal = new Vector3(0, 1, 0);
Vector3 transformedNormal = matrix.TransformNormal(normal);

In this example, a rotation matrix is created to rotate around the X-axis by 45 degrees. The normal vector (0, 1, 0) is then transformed by this matrix, resulting in a new normal vector.