ARGB Bild wird nicht transparten WARUM?

  BitmapData bmData = localBitmap.LockBits(new Rectangle(0, 0, localBitmap.Width, localBitmap.Height), ImageLockMode.ReadWrite, PixelFormat.Format32bppArgb);

int stride = bmData.Stride;

            System.IntPtr Scan0 = bmData.Scan0;

 unsafe

            {

                byte* p = (byte*)Scan0;

                int nOffset = stride-localBitmap.Width*4;

for ( int u = 0; u< localBitmap.Height; u++)

                {

                    for( int v= 0; v< localBitmap.Width; v++)

                     {

                            p[0] = 125;

                            p[1] = 125;

                            p[2] = 0;

                            p[3] = 255;

                        p+=4;

                    }

                    p+= nOffset;

                }

            }

            localBitmap.UnlockBits(bmData);

            return localBitmap;

        }