how to set opacity value of an image with a pixel format of 24bit??

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C#

WinForms

i 'm trying to Dissolve 2 image with a pixel format of 24bit.This is my code:

public static Bitmap ghepanh_normal(Bitmap anhnguon, Bitmap mask,Bitmap ketqua, double alpha)
       {
           Rectangle rec1 = new Rectangle(0, 0, anhnguon.Width, anhnguon.Height);
           Rectangle rec2 = new Rectangle(0, 0, mask.Width, mask.Height);
           //Bitmap ketqua = new Bitmap(soure.Width, soure.Height);
           ketqua = new Bitmap(soure);
           int cao = soure.Height;
           int ngang = soure.Width;


              // ketqua.Height = cao;
               //ketqua.Width = ngang;
               //ketqua = null;
           xuli.xamhoa(mask);

               Rectangle rec3 = new Rectangle(0, 0, ketqua.Width, ketqua.Height);
               BitmapData bmdata_nguon = anhnguon.LockBits(rec1, ImageLockMode.ReadWrite, PixelFormat.Format24bppRgb);
               BitmapData bmdata_mask = mask.LockBits(rec2, ImageLockMode.ReadWrite, PixelFormat.Format24bppRgb);
               BitmapData bmdata_ketqua = ketqua.LockBits(rec3, ImageLockMode.ReadWrite, PixelFormat.Format24bppRgb);
               int stride1 = bmdata_nguon.Stride;
               int offset1 = stride1 - anhnguon.Width * 3;
               int stride2 = bmdata_mask.Stride;
               int offset2 = stride2 - mask.Width * 3;
               int stride3 = bmdata_ketqua.Stride;
               int offset3 = stride3 - ketqua.Width * 3;
               IntPtr ptr1 = bmdata_nguon.Scan0;
               IntPtr ptr2 = bmdata_mask.Scan0;
               IntPtr ptr3 = bmdata_ketqua.Scan0;

               unsafe
               {
                   byte* p1 = (byte*)ptr1;
                   byte* p2 = (byte*)ptr2;
                   byte* p3 = (byte*)ptr3;
                   for (int y = 0; y < anhnguon.Height; y++)
                   {
                       for (int x = 0; x < anhnguon.Width * 3; x++)
                       {
                           p3[0] = (byte)(p1[0] * (1 - alpha) + p2[0] * alpha);
                           p1++;
                           p2++;
                           p3++;
                       }
                       p1 += offset1;
                       p2 += offset2;
                       p3 += offset3;
                   }
               }
               anhnguon.UnlockBits(bmdata_nguon);
               mask.UnlockBits(bmdata_mask);
               ketqua.UnlockBits(bmdata_ketqua);
               return (ketqua);



       }

But i not satisfy with my result!
i set 0< alpha <1 but the result :-??
alpha =1: result ketqua = texture
alpha !=1: ketqua = soure

i want to change the opacity of the image texture!! Can you help me??

EDIT ==================

i put the image in a picturebox but i didn't know how to set the opacity because i couldn't set the opacity of an image with a pixel format of 24 bpp

EDIT ==================

i think i will convert the image format to 32bpp then set the opacity of new image and last convert the new image format to 24bpp!!

can i do it??