How to add a semi-transparent gray rectangle on top of a bitmap

My MFC form is covered with a background bitmap.
This is done by adding a bitmap to a CImageList : (1 - See code attached)
and then, at the WM_PAINT handler, calling (2)

Now - sometimed I need to mark a rectangle on the bitmap as inactive/disabled.
I would like to draw a gray rectangle, make it semi - transparent and show it on top of the form.

Is it possible at all?
Any ideas how to do it?
(1):
m_BitmapBG.LoadBitmap(IDB_BACKGROUND);
m_BitmapBG.GetBitmap( &bm );
m_imageListBG.Create(bm.bmWidth,bm.bmHeight,ILC_COLOR24 | ILC_MASK ,1,1);
m_imageListBG.Add(&m_BitmapBG,RED_COLOR);

(2)
CPoint pt(0, 0);
m_imageListBG.Draw(&dc,0,pt,ILD_NORMAL);

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yossikallyAsked:
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ZoppoCommented:
Hi yossikally,

I'm not sure if there's another, easier way, but I would try doing this by creating a bitmap with the rectangles shape/color and draw it transparent using 'AlphaBlend' function. To get this working without flickering you should first copy the background image into a memory DC, then do the 'AlphaBlend' from the rectangle's bitmap into that memory DC, then 'BitBlt' the content of the memory DC into your window's DC.

Hope that helps,

ZOPPO

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yossikallyAuthor Commented:
Thanks.
Wish It had been as easy as it is on MAC/iPhone
ZoppoCommented:
Hm - I just looked around a bit more and found there may be an easier (and maybe more performant) possibility, but will require the use of GDI+.

GDI+ has a 'Graphics::FillRectangle' function which takes a brush as paramter. This brush can be created from a RGBA color, so transparency is supported via Alpha-channel.
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