Setting a background color

The approach we will take to get a uniform background color is that whenever the control needs painting, we will paint the background, we will let the control draw the items in a memory device context and then paint this image transparently onto the control surface.

Step 1: Add handler function for WM_PAINT

Next we create a mask bitmap using yet another device context. A mask bitmap is a monochrome bitmap in which one color indicates the background color in the source bitmap and the other color indicates all the bits that are not the background color. We need the mask bitmap to copy only the foreground color from the memory DC to the paint DC.

Once we have the mask bitmap, we draw the background color using the paint DC and then draw the image in the memory DC transparently over the paint DC. I had initially used MaskBlt() for drawing the image transparently but found out that it was supported on NT only and not Windows 95. Here's what we do. The image in memDC is the foreground image. When drawing this image we have to somehow make the background color have no effect. We achieve this by setting the background to black using the mask bitmap. When we later use the SRCPAINT raster operation, the black color has no effect on the destination color. Similarly we use the mask bitmap to set the foreground color of the image in the paint DC to black. We finally combine the two images.

Thanks to Matthias Kerkhoff from Germany for pointing out the problem with MaskBlt() and suggesting the new method for drawing the image transparently.

void CTreeCtrlX::OnPaint() 
{
	CPaintDC dc(this);
	
	// Create a memory DC compatible with the paint DC
	CDC memDC;
	memDC.CreateCompatibleDC( &dc );

	CRect rcClip, rcClient;
	dc.GetClipBox( &rcClip );
	GetClientRect(&rcClient);

	// Select a compatible bitmap into the memory DC
	CBitmap bitmap;
	bitmap.CreateCompatibleBitmap( &dc, rcClient.Width(), rcClient.Height() );
	memDC.SelectObject( &bitmap );
	
	// Set clip region to be same as that in paint DC
	CRgn rgn;
	rgn.CreateRectRgnIndirect( &rcClip );
	memDC.SelectClipRgn(&rgn);
	rgn.DeleteObject();
	
	// First let the control do its default drawing.
	CWnd::DefWindowProc( WM_PAINT, (WPARAM)memDC.m_hDC, 0 );


	// Now create a mask
	CDC maskDC;
	maskDC.CreateCompatibleDC(&dc);
	CBitmap maskBitmap;

	// Create monochrome bitmap for the mask
	maskBitmap.CreateBitmap( rcClip.Width(), rcClip.Height(), 1, 1, NULL );
	maskDC.SelectObject( &maskBitmap );
	memDC.SetBkColor( ::GetSysColor( COLOR_WINDOW ) );

	// Create the mask from the memory DC
	maskDC.BitBlt( 0, 0, rcClip.Width(), rcClip.Height(), &memDC, 
				rcClip.left, rcClip.top, SRCCOPY );

	// Fill the background with custom color
	// Use a protected member variable to save the color
	// rather than hard coding it.
	dc.FillRect(rcClip, &CBrush(RGB(255,255,192)) );
	
	// Copy the image in memDC transparently

	// MaskBlt works in NT only - so we use another method
	//	dc.MaskBlt( rcClip.left, rcClip.top, rcClip.Width(), rcClip.Height(), &memDC, 
	//			rcClip.left, rcClip.top, maskBitmap, 0, 0, 
	//			MAKEROP4(SRCAND,SRCCOPY) );


	// Set the background in memDC to black. Using SRCPAINT with black and any other
	// color results in the other color, thus making black the transparent color
	memDC.SetBkColor(RGB(0,0,0));          
	memDC.SetTextColor(RGB(255,255,255));  
	memDC.BitBlt(rcClip.left, rcClip.top, rcClip.Width(), rcClip.Height(), &maskDC, rcClip.left, rcClip.top, SRCAND);

	// Set the foreground to black. See comment above.
	dc.SetBkColor(RGB(255,255,255));
	dc.SetTextColor(RGB(0,0,0));
	dc.BitBlt(rcClip.left, rcClip.top, rcClip.Width(), rcClip.Height(), &maskDC, rcClip.left, rcClip.top, SRCAND);
	
	// Combine the foreground with the background
	dc.BitBlt(rcClip.left, rcClip.top, rcClip.Width(), rcClip.Height(), &memDC, 
					rcClip.left, rcClip.top,SRCPAINT);
}

Step 2: Add handler for WM_ERASEBKGND

In actual production code, you'd probably check for a non default background color before returning TRUE;

BOOL CTreeCtrlX::OnEraseBkgnd(CDC* pDC) 
{
 	// To prevent flickering when using non default bkcolor
	return TRUE;
}