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hi all:-)

I have the following question:

I have two CBitMap objects, which represent 2 .png-Files.

Now i want to overlay them the following way, where the 2nd cbitmap is over the first Bitmap:

if a pixel of the 2nd cbitmap is white, i want to display the pixel of the first cbitmap. if it is not white, i want to display the pixel of the 2nd cbitmap.


Is this possible to achieve?


I tried it with the Alphablend() function, where i set the alpha-value of the white pixels of the 2nd cbitmap to zero.

Always, only the second png is displayed...

Any ideas?

Thanks for any help!

[edit]
Here is my code, for future visitors:

void PngImage::DrawOverlayedBitmap(CBitmap* pBitmapUnder, CBitmap* pBitmapUpper, CWnd* pWnd, CPoint Offset)
{
	// get bitmap information
	BITMAP bmpInfo;
	pBitmapUnder->GetObject(sizeof(bmpInfo), &bmpInfo);

	// get size
	CSize size;
	size.cx = bmpInfo.bmWidth;
	size.cy = bmpInfo.bmHeight;

	// get window's client device context
	CClientDC* pDC = new CClientDC(pWnd);

	// create memory device context
	CDC* memDC = new CDC; memDC->CreateCompatibleDC(pDC);

	// buffer bitmap
	CBitmap* old = memDC->SelectObject(pBitmapUnder);

	// copy bitmap bits
	pDC->BitBlt
	(
		Offset.x,		// specifies the x-coordinate (in logical units) of the upper-left corner of the destination rectangle
		Offset.y,		// specifies the y-coordinate (in logical units) of the upper-left corner of the destination rectangle
		size.cx,		// specifies the width (in logical units) of the destination rectangle
		size.cy,		// specifies the height (in logical units) of the destination rectangle
		memDC,			// specifies the source device context
		0,				// specifies the x-coordinate (in logical units) of the upper-left corner of the source rectangle
		0,				// specifies the x-coordinate (in logical units) of the upper-left corner of the source rectangle
		SRCCOPY			// specifies the raster operation to be performed
	);

	// reselect first bitmap
	memDC->SelectObject(pBitmapUpper);

	// copy bitmap bits
	pDC->BitBlt
	(
		Offset.x,		// specifies the x-coordinate (in logical units) of the upper-left corner of the destination rectangle
		Offset.y,		// specifies the y-coordinate (in logical units) of the upper-left corner of the destination rectangle
		size.cx,		// specifies the width (in logical units) of the destination rectangle
		size.cy,		// specifies the height (in logical units) of the destination rectangle
		memDC,			// specifies the source device context
		0,				// specifies the x-coordinate (in logical units) of the upper-left corner of the source rectangle
		0,				// specifies the x-coordinate (in logical units) of the upper-left corner of the source rectangle
		SRCAND		// specifies the raster operation to be performed
	);

	// delete device context and reset pointer
	delete memDC; memDC = 0;

	// delete device context and reset pointer
	delete pDC; pDC = 0;
}


[/edit]
Posted
Updated 18-Feb-15 23:20pm
v2

1 solution

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Solution 1

You should load your first image into a DC then load the second into a memory DC, and then blit the second one into the first, using the appropriate operation code as described in https://msdn.microsoft.com/en-us/library/windows/desktop/dd183370%28v=vs.85%29.aspx[^].
   
v2
Comments
Member 10781325 19-Feb-15 4:52am
   
thx very much!! that does it:-)
Member 10781325 19-Feb-15 5:17am
   
Moved to the original question.
Member 10781325 19-Feb-15 5:18am
   
i know, its ugly, thats just test code;-)
Richard MacCutchan 19-Feb-15 5:21am
   
Thanks for posting, will maybe help someone else in the future.

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