Memory DC, How?

I've been though a ton of sample source, and still am so confused on this.  What I need to do seems so simple, yet I cannot seem to get the syntax down.

I have two DCs, hCurrentPage and hNextPage. Both DCs have the same dimensions.  Here is what I need to do.

1.  Create a memory DC
        //set drawing rectangle to the dimensions of the pages
        CRect rcDraw(0,0,sizeImage.cx,sizeImage.cy);
       
        //create the memory DC
        HDC hdcScreen = CreateDC("DISPLAY", NULL, NULL, NULL);
        HDC hdcCompatible = CreateCompatibleDC(hdcScreen);
        HBITMAP hbmScreen = CreateCompatibleBitmap(hdcScreen,
             GetDeviceCaps(hdcScreen, HORZRES), GetDeviceCaps(hdcScreen, VERTRES));
       
        SelectObject(hdcCompatible, hbmScreen);

2.  Copy the content of hNextPage into a memory DC using BitBlt SRCCOPY
        ::BitBlt(hdcCompatible, 0,0, rcDraw.Width(), rcDraw.Height()
                , hNextPage, 0, 0, SRCCOPY);

3.  Copy the content of hCurrentPage into the memory DC using BitBlt SRCCOPY
        //In the actual program I deflate the rectangle, but for clarity I have ommitte that code
        ::BitBlt(hdcCompatible,rcDraw.left,rcDraw.top,rcDraw.Width(),rcDraw.Height()
                , hCurrentPage,rcDraw.left,rcDraw.top,SRCCOPY);

4.  Copy the combined content in the memory DC to hCurrentPage  using BitBlt SRCCOPY
        ::BitBlt(hCurrentPage,rcNew.left,rcNew.top,rcNew.Width(),rcNew.Height()
                , hdcCompatible, rcNew.left, rcNew.top, SRCCOPY);

5.  Delete the memory DC
       DeleteDC(hdcCompatible);


What am I doing wrong?  If it makes any difference, the above code is in a loop to perform a transition between hCurrentPage and hNextPage.

Thanks.
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BWarmuskerkenAsked:
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SteHCommented:
Hi there,

First of all: in what function are you calling this loop? Is it part of a message handling for a know window?

Depending on this knowlegde one could decide to get a compatible DC for *that* window. For me it is not fully clear, that hCurrentPage, hNextPage and hdcCompatible are all compatible. The first 2 are but the third perhaps not.

Since like this I would suggest to create the compatible bitmap from hNextPage and copy that content as well:
(relating to http://www.experts-exchange.com/Programming/Programming_Languages/Cplusplus/Q_20803968.html)

   HDC hdcCompatible = CreateCompatibleDC (hNextPage);
   HBITMAP hbmDC = CreateCompatibleBitmap (hNextPage);
   ::BitBlt (hdcCompatible, 0, 0, rcDraw.Width(), rcDraw.Height(), hNextPage, 0, 0, SRCCOPY);

   Do steps 3,4,5 now.
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BWarmuskerkenAuthor Commented:
Hi SteH,

Thanks for the help, again.  I guess I didn't realize I could use CreateCompatibleDC on the DC I already had.  I have to admit that I feel a little stupd now.  Hopefully, I won't be back to ask any more questions, but if I do, I know who to ask now.
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SteHCommented:
I think everone is starting with question which turn out to be stupid if one knows more. Thats just learning.
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BWarmuskerkenAuthor Commented:
SteH,  

I know this is after the fact, but...

Im doing this in a For loop

for ()
{
HDC hdcCompatible = CreateCompatibleDC (hNextPage);
   HBITMAP hbmDC = CreateCompatibleBitmap (hNextPage);
   ::BitBlt (hdcCompatible, 0, 0, rcDraw.Width(), rcDraw.Height(), hNextPage, 0, 0, SRCCOPY);
   // Other code here to BitBlt

  DeleteDC(hdcCompatible);
}

Is creating and deleteing the object going to cause me hassles?
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SteHCommented:
It should not harm if all memory is properly released.

You could change it to:

HDC hdcCompatible = CreateCompatibleDC (hNextPage);
HBITMAP hbmDC = CreateCompatibleBitmap (hNextPage);
for ()
{
   ::BitBlt (hdcCompatible, 0, 0, rcDraw.Width(), rcDraw.Height(), hNextPage, 0, 0, SRCCOPY);
   // Other code here to BitBlt
}
DeleteDC(hdcCompatible);

What could be a problem is that the content of hNextPage and hCurrentPage change during the loop and the starting point in the second iteration might not be as desired. Perhaps you can store the images after each iteration and have a look at them.



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BWarmuskerkenAuthor Commented:
Putting those lines outside of the loop made all the difference in the world.  It's smoothe.
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