How to bitblt transparently ??

Anybody have a good example on how to bitblt bitmaps transparently using GDI ?

[David Giovannini]

This came out of CMenuSpawn, available from this site.

void CMenuSpawn::TransparentBlt(CDC * pDestDc, int x, int y, int w, int h, CBitmap * pBmp, int sx, int sy, COLORREF crTransparent)
{
CDC memDC, maskDC, tempDC;
maskDC.CreateCompatibleDC(pDestDc);
CBitmap maskBitmap;

//add these to store return of SelectObject() calls
CBitmap* pOldMemBmp = NULL;
CBitmap* pOldMaskBmp = NULL;

memDC.CreateCompatibleDC(pDestDc);
tempDC.CreateCompatibleDC(pDestDc);
CBitmap bmpImage;
bmpImage.CreateCompatibleBitmap( pDestDc, w, h);
pOldMemBmp = memDC.SelectObject( &bmpImage );

CBitmap * oldBmp = tempDC.SelectObject(pBmp);

memDC.BitBlt( 0,0,w, h, &tempDC, sx, sy, SRCCOPY );

// Create monochrome bitmap for the mask
maskBitmap.CreateBitmap(w, h, 1, 1, NULL);
pOldMaskBmp = maskDC.SelectObject( &maskBitmap );
memDC.SetBkColor(crTransparent);

// Create the mask from the memory DC
maskDC.BitBlt(0, 0, w, h, &memDC, 0, 0, SRCCOPY);

memDC.SetBkColor(RGB(0,0,0));
memDC.SetTextColor(RGB(255,255,255));
memDC.BitBlt(0, 0, w, h, &maskDC, 0, 0, SRCAND);

// Set the foreground to black. See comment above.
pDestDc->SetBkColor(RGB(255,255,255));
pDestDc->SetTextColor(RGB(0,0,0));
pDestDc->BitBlt(x, y, w, h, &maskDC, 0, 0, SRCAND);

// Combine the foreground with the background
pDestDc->BitBlt(x, y, w, h, &memDC, 0, 0, SRCPAINT);

tempDC.SelectObject(oldBmp);
if (pOldMaskBmp) maskDC.SelectObject( pOldMaskBmp );
if (pOldMemBmp) memDC.SelectObject( pOldMemBmp );
}

// this variation does a color replace (useful for scrolling graphics)
void CMenuSpawn::BackgroundBlt(CDC * pDestDc, int x, int y, int w, int h, CBitmap * pBmp, int sx, int sy, COLORREF crTransparent)
{
COLORREF backGround = ::GetSysColor(COLOR_BTNFACE);

CDC memDC, maskDC, tempDC;
maskDC.CreateCompatibleDC(pDestDc);
CBitmap maskBitmap;

//add these to store return of SelectObject() calls
CBitmap* pOldMemBmp = NULL;
CBitmap* pOldMaskBmp = NULL;

memDC.CreateCompatibleDC(pDestDc);
tempDC.CreateCompatibleDC(pDestDc);
CBitmap bmpImage;
bmpImage.CreateCompatibleBitmap( pDestDc, w, h);
pOldMemBmp = memDC.SelectObject( &bmpImage );

CBitmap * oldBmp = tempDC.SelectObject(pBmp);

memDC.BitBlt( 0,0,w, h, &tempDC, sx, sy, SRCCOPY );

// Create monochrome bitmap for the mask
maskBitmap.CreateBitmap(w, h, 1, 1, NULL);
pOldMaskBmp = maskDC.SelectObject( &maskBitmap );
memDC.SetBkColor(crTransparent);

// Create the mask from the memory DC
maskDC.BitBlt(0, 0, w, h, &memDC, 0, 0, SRCCOPY);

memDC.SetBkColor(backGround);
memDC.SetTextColor(RGB(255,255,255));
memDC.BitBlt(0, 0, w, h, &maskDC, 0, 0, SRCAND);

// Set the foreground to black. See comment above.
pDestDc->SetBkColor(RGB(255,255,255));
pDestDc->SetTextColor(RGB(0,0,0));
pDestDc->BitBlt(x, y, w, h, &maskDC, 0, 0, SRCAND);

// Combine the foreground with the background
pDestDc->BitBlt(x, y, w, h, &memDC, 0, 0, SRCPAINT);

tempDC.SelectObject(oldBmp);
if (pOldMaskBmp) maskDC.SelectObject( pOldMaskBmp );
if (pOldMemBmp) memDC.SelectObject( pOldMemBmp );
}