Changing the Opacity of a Bitmap image

Question

I have a form which has a image. I am using a slider to change the opacity of the image. So in the "ValueChanged" event of the slider I am calling the following method to change the opacity.

//Setting the opacity of the image public static Image SetImgOpacity(Image imgPic, float imgOpac) {         Bitmap bmpPic = new Bitmap(imgPic.Width, imgPic.Height);      Graphics gfxPic = Graphics.FromImage(bmpPic);       ColorMatrix cmxPic = new ColorMatrix();         cmxPic.Matrix33 = imgOpac;         ImageAttributes iaPic = new ImageAttributes();         iaPic.SetColorMatrix(cmxPic, ColorMatrixFlag.Default, ColorAdjustType.Bitmap);        gfxPic.DrawImage(imgPic, new Rectangle(0, 0, bmpPic.Width, bmpPic.Height), 0, 0, imgPic.Width, imgPic.Height, GraphicsUnit.Pixel, iaPic);        gfxPic.Dispose();                   return bmpPic;   } 

The returned Image is set to the original image.

My problem is that the opacity of the image is not changing... If there is any error please be kind enough to point out.. Thnx...

Answer 1

Try this one from CodeProject - Change Opacity of Image in C#:

/// <summary>   /// method for changing the opacity of an image   /// </summary>   /// <param name="image">image to set opacity on</param>   /// <param name="opacity">percentage of opacity</param>   /// <returns></returns>   public Image SetImageOpacity(Image image, float opacity)   {       try       {           //create a Bitmap the size of the image provided           Bitmap bmp = new Bitmap(image.Width, image.Height);            //create a graphics object from the image           using (Graphics gfx = Graphics.FromImage(bmp)) {                //create a color matrix object               ColorMatrix matrix = new ColorMatrix();                    //set the opacity               matrix.Matrix33 = opacity;                //create image attributes               ImageAttributes attributes = new ImageAttributes();                    //set the color(opacity) of the image               attributes.SetColorMatrix(matrix, ColorMatrixFlag.Default, ColorAdjustType.Bitmap);                  //now draw the image               gfx.DrawImage(image, new Rectangle(0, 0, bmp.Width, bmp.Height), 0, 0, image.Width, image.Height, GraphicsUnit.Pixel, attributes);         }         return bmp;       }       catch (Exception ex)       {          MessageBox.Show(ex.Message);           return null;       }   }  

Answer 2

You loop over the pixels and play only the alpha channel. If you do this with Bitmap.LockBits it will actually be very fast.

    private const int bytesPerPixel = 4;      /// <summary>     /// Change the opacity of an image     /// </summary>     /// <param name="originalImage">The original image</param>     /// <param name="opacity">Opacity, where 1.0 is no opacity, 0.0 is full transparency</param>     /// <returns>The changed image</returns>     public static Image ChangeImageOpacity(Image originalImage, double opacity)     {         if ((originalImage.PixelFormat & PixelFormat.Indexed) == PixelFormat.Indexed)         {             // Cannot modify an image with indexed colors             return originalImage;         }          Bitmap bmp = (Bitmap)originalImage.Clone();          // Specify a pixel format.         PixelFormat pxf = PixelFormat.Format32bppArgb;          // Lock the bitmap's bits.         Rectangle rect = new Rectangle(0, 0, bmp.Width, bmp.Height);         BitmapData bmpData = bmp.LockBits(rect, ImageLockMode.ReadWrite, pxf);          // Get the address of the first line.         IntPtr ptr = bmpData.Scan0;          // Declare an array to hold the bytes of the bitmap.         // This code is specific to a bitmap with 32 bits per pixels          // (32 bits = 4 bytes, 3 for RGB and 1 byte for alpha).         int numBytes = bmp.Width * bmp.Height * bytesPerPixel;         byte[] argbValues = new byte[numBytes];          // Copy the ARGB values into the array.         System.Runtime.InteropServices.Marshal.Copy(ptr, argbValues, 0, numBytes);          // Manipulate the bitmap, such as changing the         // RGB values for all pixels in the the bitmap.         for (int counter = 0; counter < argbValues.Length; counter += bytesPerPixel)         {             // argbValues is in format BGRA (Blue, Green, Red, Alpha)              // If 100% transparent, skip pixel             if (argbValues[counter + bytesPerPixel - 1] == 0)                 continue;              int pos = 0;             pos++; // B value             pos++; // G value             pos++; // R value              argbValues[counter + pos] = (byte) (argbValues[counter + pos] * opacity);         }          // Copy the ARGB values back to the bitmap         System.Runtime.InteropServices.Marshal.Copy(argbValues, 0, ptr, numBytes);          // Unlock the bits.         bmp.UnlockBits(bmpData);          return bmp;     }