How to make a portion of a bitmap transparent using GDI+?

//=============== TBitmap ======================================================// Draws a transparent bitmap to the back buffer, using// a passed in mask. White is transparent.// uses SRCAND and SRCPAINT bit blit operators// HBITMAP bmp - bitmap we want to draw// HBITMAP mask - the mask we want to use for the blit// int x  - the x coordinate at which we want to draw// int y  - the y coordinate at which we want to draw// int wx - width of the bitmap rectangle which we want to draw// int wy - height of the bitmap rectangle which we want to draw// int dx - the x coordinate of the source bitmap from which we want to start drawing// int dy - the y coordinate of the source bitmap from which we want to start drawing//==============================================================================void KScrBuffer::TBitmap(HBITMAP bmp,HBITMAP mask, int x, int y, int wx, int wy,int dx, int dy)//displays transparent bitmap to backbuffer{                                                                                            //using a pre-made maks bitmap                                                                    //if the width and height of the draw rectangle are 0 (default) then set them //to our client rectangle (i.e. we want to draw as much of the bitmap as fits //on the screen)    if(wx==0) 	wx=ClRect.right; if(wy==0)   wy=ClRect.bottom; //The following code blits the mask to the screen with AND (so that only the  //black portions are blitted), then blits the bitmap with PAINT (so that the  //bitmap gets pained over only the previously blitted black portions) OldBitmap = (HBITMAP)SelectObject(HDCbitmap, mask);            //select mask BitBlt(HDCback, x, y, wx, wy, HDCbitmap, dx, dy, SRCAND);   //blit with AND SelectObject(HDCbitmap, bmp);                               //select bitmap BitBlt(HDCback, x, y, wx, wy, HDCbitmap, dx, dy, SRCPAINT); //blit with PAINT //select old bitmap to HDC SelectObject(HDCbitmap, OldBitmap);}

//=============== CreateBitmapMask ==============================================// Creates a mask for a given bitmap. White is transparent, black is solid// HBITMAP hbmColour - the bitmap we want to create the mask for// COLORREF crTransparent  - the color we should consider as transparent in the //                           bitmap (default black)//============================================================================== HBITMAP CreateBitmapMask(HBITMAP bmp, COLORREF crTransparent){												    HBITMAP mask;		 //our mask bitmap																			    HDC hdcMem, hdcMem2; //two temporary HDCs we will use for blitting    BITMAP rawBitmap;    //raw bitmap information    // Create monochrome (1 bit) mask bitmap    GetObject(bmp, sizeof(BITMAP), &rawBitmap);    mask = CreateBitmap(rawBitmap.bmWidth, rawBitmap.bmHeight, 1, 1, NULL);    // Create HDCs that are compatible with the display driver    hdcMem = CreateCompatibleDC(0);    hdcMem2 = CreateCompatibleDC(0);    //select the bitmap and mask to the HDCs    SelectObject(hdcMem, bmp);    SelectObject(hdcMem2, mask);    // Set the background color of the bitmap to the color    // we want to be transparent.    SetBkColor(hdcMem, crTransparent);    // Copy the bits from the color image to the B+W mask... everything    // with the background color ends up white while everythig else ends up    // black...Just what we wanted.    BitBlt(hdcMem2, 0, 0, rawBitmap.bmWidth, rawBitmap.bmHeight, hdcMem, 0, 0, SRCCOPY);    // Clean up.    DeleteDC(hdcMem);    DeleteDC(hdcMem2);        //return the mask    return mask;}//=============== MakeTransparent =============================================// Adjusts a bitmap so that it can be used for transparent // drawing using a mask. // HBITMAP * bmp - the bitmap we want to adjust// UINT TransparentColor - the color we want to make transparent// RETURN: the created mask bitmap//==============================================================================   void MakeTransparent(HBITMAP* bmp, UINT TransparentColor)//makes a bitmap transparent{    HDC hdcMem, hdcMem2; //two temporary HDCs we will use for blitting    HBITMAP hbmMask;     //a mask for our bitmap    BITMAP rawBitmap;    //raw bitmap information    // Create monochrome (1 bit) mask bitmap.    GetObject(bmp, sizeof(BITMAP), &rawBitmap);    hbmMask = CreateBitmap(rawBitmap.bmWidth, rawBitmap.bmHeight, 1, 1, NULL);    // Create HDCs that are compatible with the display driver    hdcMem = CreateCompatibleDC(0);    hdcMem2 = CreateCompatibleDC(0);    //select the bitmap and mask to the HDCs    SelectObject(hdcMem, bmp);    SelectObject(hdcMem2, hbmMask);    // Set the background color of the bitmap to the color    // we want to be transparent.    SetBkColor(hdcMem, TransparentColor);    // Copy the bits from the color image to the B+W mask... everything    // with the background color ends up white while everythig else ends up    // black...Just what we wanted.    BitBlt(hdcMem2, 0, 0, rawBitmap.bmWidth, rawBitmap.bmHeight, hdcMem, 0, 0, SRCCOPY);    // Take our new mask and use it to turn the transparent color in our    // original bitmap to black so the transparency effect will    // work right.    BitBlt(hdcMem, 0, 0, rawBitmap.bmWidth, rawBitmap.bmHeight, hdcMem2, 0, 0, SRCINVERT);    // Clean up.    DeleteDC(hdcMem);    DeleteDC(hdcMem2);    DeleteObject(hbmMask);}