MaskBlt (how to draw masked image?)

Posted on 1998-10-16
Last Modified: 2013-11-20
::MaskBlt ( myHDC , 0 ,0 ,width ,height ,myhMemDC ,x ,y ,mymask ,0 ,0 ,raster_operation_code );
myHDC is a HDC of a window
myhMemDC is a HDC with a selected RGB bitmap
my mask is a HBITMAP of a monochrome bitmap.
What do I have to put in raster op. code to draw only in the part where my mask is white?
I tried with all methods listed in BitBlt but nothing.
And I tried with MAKEROP4 macro, but I dont understand what arguments to pass.
Help me peopple please.
Question by:wau

Author Comment

ID: 1323469
Edited text of question

Author Comment

ID: 1323470
Edited text of question

Expert Comment

ID: 1323471
The idea is simple but you have to have an inverted mask too.
First combine the image and the mask using AND operation.
Then combine the background image and the  inverted mask again using AND. In this way you have the image and the clipped background. Then the only thing left to do is to draw them in the DC using OR and everything is OK. If you do not finish this by Monday i will send you some source :)

Hope that helped
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Expert Comment

ID: 1323472
I think bobbym is right. The mask needs to consist of RGB(255,255,255) where the original
pixel should be left, and RGB(0,0,0) where you want to replace it by your own bitmap data.

BTW: mymask is one parameter too much ... What do you mean by it?


Where myhMemDC_MaskBitmap has the mask bitmap selected,
and myhMemDC_RealBitmap has the 'real' bitmap selected. Of course both DC's may be the

Author Comment

ID: 1323473
I done it.
It's working.

BitBlt( image_DC,0,0,width,height,mask_DC,x,y,SRCAND );
BitBlt( wnd_DC,0,0,width,height,mask_invert_DC,0,0,SRCAND );
BitBlt( wnd_DC,0,0,width,height,image_DC,0,0,SRCPAINT );

Thanks very much.

Accepted Solution

AlFa earned 50 total points
ID: 1323474
I use this I think you can transform it.
it draw a bitmap with a colour defined transparent

void DrawTransparent(CDC * pDC, CBitmap *pThisBmp, int x, int y, int W, int H,      int nWidth, int nHeight, COLORREF crColour, CPalette* Palette)
      CBitmap & ThisBmp = *pThisBmp;
      COLORREF crOldBack = pDC->SetBkColor(RGB(255,255,255));
      COLORREF crOldText = pDC->SetTextColor(RGB(0,0,0));
      CDC dcImage, dcTrans;
      // Create two memory dcs for the image and the mask
      // Select the image into the appropriate dc
      CBitmap* pOldBitmapImage = (CBitmap*)dcImage.SelectObject(pThisBmp);
      if (Palette)
            dcImage.SelectPalette(Palette, TRUE);

      // Create the mask bitmap
      CBitmap bitmapTrans;
      bitmapTrans.CreateBitmap(nWidth, nHeight, 1, 1, NULL);
      // Select the mask bitmap into the appropriate dc
      CBitmap* pOldBitmapTrans = dcTrans.SelectObject(&bitmapTrans);
      // Build mask based on transparent colour
      dcTrans.BitBlt(0, 0, nWidth, nHeight, &dcImage, 0, 0, SRCCOPY);
      // Do the work - True Mask method - cool if not actual display
      pDC->SetStretchBltMode( COLORONCOLOR);
      pDC->StretchBlt( x, y, W, H, &dcImage, 0, 0, nWidth, nHeight, SRCINVERT );
      pDC->StretchBlt( x, y, W, H, &dcTrans, 0, 0, nWidth, nHeight, SRCAND );
      pDC->StretchBlt( x, y, W, H, &dcImage, 0, 0, nWidth, nHeight, SRCINVERT );
      // Restore settings


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