RotateFlip似乎只能90度为单位旋转，可以任意角度旋转图像吗？
img.RotateFlip似乎只能90度为单位旋转？

------解决方案--------------------
private void button1_Click(object sender, EventArgs e)
{
Bitmap a = new Bitmap(pictureBox1.Image);//得到图片框中的图片
pictureBox1 .Image = Rotate(a, 30);//这里的30为度数，
pictureBox1.Refresh();//最后刷新图片框
}


#region 图片旋转函数
/// <summary>
/// 以逆时针为方向对图像进行旋转
/// </summary>
/// <param name="b">位图流</param>
/// <param name="angle">旋转角度[0,360](前台给的)</param>
/// <returns></returns>
public Bitmap Rotate(Bitmap b, int angle)
{
angle = angle % 360;

//弧度转换
double radian = angle * Math.PI / 180.0;
double cos = Math.Cos(radian);
double sin = Math.Sin(radian);

//原图的宽和高
int w = b.Width;
int h = b.Height;
int W = (int)(Math.Max(Math.Abs(w * cos - h * sin), Math.Abs(w * cos + h * sin)));
int H = (int)(Math.Max(Math.Abs(w * sin - h * cos), Math.Abs(w * sin + h * cos)));

//目标位图
Bitmap dsImage = new Bitmap(W, H);
System.Drawing.Graphics g = System.Drawing.Graphics.FromImage(dsImage);

g.InterpolationMode = System.Drawing.Drawing2D.InterpolationMode.Bilinear;

g.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.HighQuality;

//计算偏移量
Point Offset = new Point((W - w) / 2, (H - h) / 2);

//构造图像显示区域：让图像的中心与窗口的中心点一致
Rectangle rect = new Rectangle(Offset.X, Offset.Y, w, h);
Point center = new Point(rect.X + rect.Width / 2, rect.Y + rect.Height / 2);

g.TranslateTransform(center.X, center.Y);
g.RotateTransform(360 - angle);

//恢复图像在水平和垂直方向的平移
g.TranslateTransform(-center.X, -center.Y);
g.DrawImage(b, rect);

//重至绘图的所有变换
g.ResetTransform();

g.Save();
g.Dispose();
//dsImage.Save("yuancd.jpg", System.Drawing.Imaging.ImageFormat.Jpeg);
return dsImage;
}
#endregion 图片旋转函数