New Post: No alpha blending in the FillXXX methods

  public static void FillRectangleBlend2(this WriteableBitmap bmp, int x1, int y1, int x2, int y2, Color colore)
  {
      var color = ConvertColor(colore);
      using (var context = bmp.GetBitmapContext())
      {
          // Use refs for faster access (really important!) speeds up a lot!
          var w = context.Width;
          var h = context.Height;
          var pixels = context.Pixels;

          // Check boundaries
          if ((x1 < 0 && x2 < 0) || (y1 < 0 && y2 < 0)
           || (x1 >= w && x2 >= w) || (y1 >= h && y2 >= h))
          {
              return;
          }

          // Clamp boundaries
          if (x1 < 0) { x1 = 0; }
          if (y1 < 0) { y1 = 0; }
          if (x2 < 0) { x2 = 0; }
          if (y2 < 0) { y2 = 0; }
          if (x1 >= w) { x1 = w - 1; }
          if (y1 >= h) { y1 = h - 1; }
          if (x2 >= w) { x2 = w - 1; }
          if (y2 >= h) { y2 = h - 1; }

          int alpha = 1 + (int)((uint)color >> 24);
          if (alpha == 1) return;

          int alphac = 256 - alpha, p, pal, par, pag, pab;
          if (alphac > 0)
          {
              color &= 0xffffff;
              par = alpha * ((color >> 16) & 255);
              pag = alpha * ((color >> 8) & 255);
              pab = alpha * (color & 255);
              color = ((par >> 8) << 16)
                  | ((pag >> 8) << 8)
                  | (pab >> 8);
          }

          unchecked
          {
              for (int y = y1; y <= y2; y++)
              {
                  for (int i = y * w + x1; i < y * w + x2; i++)
                  {
                      p = pixels[i];
                      pal = 1 + (p >> 24);
                      par = alphac * pal * ((p >> 16) & 255);
                      pag = alphac * pal * ((p >> 8) & 255);
                      pab = alphac * pal * (p & 255);
                      pixels[i] = (Math.Min(255, alpha + pal - 1) << 24)
                          | (par & (255 * 256 * 256))
                          | (pag & (255 * 256 * 256)) >> 8
                          | (pab >> 16) + color;

                  }
              }
          }

      }
  }