设置图片透明度的四种方法 .

方法一：像素法

即循环扫描整张图片，设置每一个像素的Alpha通道的值。 由于是一个个像素的处理，所以这种方法效率很低。

/// <summary>
        /// 设置图片的透明度
        /// </summary>
        /// <param name="bitmap">原图</param>
        /// <param name="alpha">透明度（0-255之间）</param>
        /// <returns></returns>
        public static Bitmap SetBitmapAlpha(Bitmap bitmap, int alpha)
        {
            Bitmap bmp = (Bitmap)bitmap.Clone();
            Color color;
            for (int x = 0; x < bmp.Width; x++)
            {
                for (int y = 0; y < bmp.Height; y++)
                {
                    color = bmp.GetPixel(x, y);
                    bmp.SetPixel(x, y, Color.FromArgb(alpha, color));
                }
            }

            bitmap.Dispose();
            return bmp;
        }

方法二：内存法

public static Bitmap SetBitmapAlpha(Bitmap bitmap, int alpha)
        {
            Bitmap bmp = (Bitmap)bitmap.Clone();

            Rectangle rect = new Rectangle(0, 0, bmp.Width, bmp.Height);
            System.Drawing.Imaging.BitmapData bmpData =
                bmp.LockBits(rect, System.Drawing.Imaging.ImageLockMode.ReadWrite,
                PixelFormat.Format32bppArgb);

            IntPtr ptr = bmpData.Scan0;  //获取首地址
            int bytes = Math.Abs(bmpData.Stride) * bmp.Height;
            byte[] argbValues = new byte[bytes];

            //复制ARGB的值到数组
            System.Runtime.InteropServices.Marshal.Copy(ptr, argbValues, 0, bytes);

            for (int count = 3; count < argbValues.Length; count += 4)
            {
                //设置alpha通道的值
                argbValues[count] = (byte)alpha;
            }

            System.Runtime.InteropServices.Marshal.Copy(argbValues, 0, ptr, bytes);
            bmp.UnlockBits(bmpData);


            bitmap.Dispose();
            return bmp;
        }

方法三：指针法

public static Bitmap SetBitmapAlpha(Bitmap bitmap, int alpha)
        {
            Bitmap bmp = (Bitmap)bitmap.Clone();

            Rectangle rect = new Rectangle(0, 0, bmp.Width, bmp.Height);
            BitmapData bmpData = bmp.LockBits(rect,
                ImageLockMode.ReadWrite, PixelFormat.Format32bppArgb);

            unsafe
            {
                byte* p = (byte*)bmpData.Scan0;
                int offset = bmpData.Stride - bmp.Width * 4;
                for (int y = 0; y < bmp.Height; y++)
                {
                    for (int x = 0; x < bmp.Width; x++)
                    {
                        //设置alpha通道的值
                        p[3] = (byte)alpha;
                        p += 4;
                    }
                    p += offset;
                }
            }

            bmp.UnlockBits(bmpData);
            bitmap.Dispose();

            return bmp;
        }

方法四：矩阵法

这个方法的效率最高。

 /// <summary>
        /// 设置图片的透明度
        /// </summary>
        /// <param name="image">原图</param>
        /// <param name="alpha">透明度0-255</param>
        /// <returns></returns>
        private Bitmap SetPictureAlpha(Image image,int alpha)
        {
            //颜色矩阵
            float[][] matrixItems =
            {
                new float[]{1,0,0,0,0},
                new float[]{0,1,0,0,0},
                new float[]{0,0,1,0,0},
                new float[]{0,0,0,alpha/255f,0},
                new float[]{0,0,0,0,1}
            };
            ColorMatrix colorMatrix = new ColorMatrix(matrixItems);
            ImageAttributes imageAtt = new ImageAttributes();
            imageAtt.SetColorMatrix(colorMatrix, ColorMatrixFlag.Default, ColorAdjustType.Bitmap);
            Bitmap bmp = new Bitmap(image.Width, image.Height);
            Graphics g = Graphics.FromImage(bmp);
            g.DrawImage(image, new Rectangle(0, 0, image.Width, image.Height),
                    0, 0, image.Width, image.Height, GraphicsUnit.Pixel, imageAtt);
            g.Dispose();

            return bmp;
        }

注意：需要添加命名空间System.Drawing和ystem.Drawing.Imaging。