Can I create more than one bitmaps for compatible DC? - c++

Question

Is it right to create two bitmaps for one DC?
My situation is I have
dc = CreateCompatibleDC(NULL);
bitmap = CreateDIBSection(dc, pbmi1, DIB_RGB_COLORS, (void **) &bmpBits, NULL, 0);
extra_bitmap = CreateDIBSection(dc, pbmi2, DIB_RGB_COLORS, (void **) &extraBmpBits, NULL, 0);
The difference is: bitmap is width*height , extra_bitmap is width/2*height*2
I need to show both images on the screen according to requirements from users.
I tried these code above, it can run (based only on my limited test). But I am not sure if it is the right way to do it. Any suggestions and/or comments are welcome. Thanks.
By the way, if I can have two bitmaps associated with one DC, what is the right way to release them? Can I just call DeleteDC(dc)? Thanks.

Answer 1

Bitmaps are only associated with a DC when they're selected into them via SelectObject. The DC you pass to the CreateDIBSection is only used to define the layout/palette of the newly created bitmap, and only in some circumstances, but it doesn't irrevocably associate that bitmap with the DC or anything like that.
To draw a bitmap to a window, it needs to be selected into a DC (the "source DC"), and this is where you can only have one at a time.
For example:
HDC hdcTarget = GetDC(hWnd); // or hdcTarget = BeginPaint(...)
HDC hdcSource = CreateCompatibleDC(hdcTarget); // create a "source DC")
HGDIOBJ hOldBmp = SelectObject(dc, bitmap); // select bitmap 1 in
BitBlt(hdcTarget, x, y, w, h, hdcSource, 0, 0, SRCCOPY); // draw bitmap 1
SelectObject(hdcSource, extra_bitmap); // select bitmap 2 in
BitBlt(hdcTarget, x1, y1, w1, h1, hdcSource, 0, 0, SRCCOPY); // draw bitmap 2
SelectObject(hdcSource, hOldBmp); // restore previous bitmap
DeleteDC(hdcSource); // no longer needed
ReleaseDC(hWnd, hdcTarget); // or EndPaint(...)

Answer 2

After search around for a few hours, I get
http://www.ucancode.net/Visual_C_MFC_Samples/CreateDIBSection-BITMAPINFOHEADER-CreateCompatibleDC-BITMAPINFO.htm
In this example:
// Create HDCs to hold our surfaces.
hdcSrc1 = CreateCompatibleDC(hdcDest);
if (!hdcSrc1) goto HANDLEERROR;
hdcSrc2 = CreateCompatibleDC(hdcDest);
if (!hdcSrc2) goto HANDLEERROR;
For delete:
DeleteDC(hdcSrc1);
DeleteDC(hdcSrc2);
DeleteObject(hbmSrc1); // hbmSrc1 and hbmSrc2 are created using hbm = CreateDIBSection(hDC, &bmi, DIB_RGB_COLORS,(void **)&pBits, NULL, 0);
DeleteObject(hbmSrc2);
Hope this can be helpful for others. thanks.

Answer 3

You may create as many in-memory bitmaps as you like.
You may create as many device-dependent DCs as you like.
A bitmap may be selected into only one DC at a time. If you try to select the same bitmap into multiple DCs, then you will get unpredictable behavior.
A DC will never have more that one bitmap selected into it at a time, because the act of selecting one in selects the previous one out.
Don't delete a memory DC while your bitmap is still selected into it. You should select the original bitmap (which is a monochrome 1x1 bitmap) into the DC to get your bitmap out before you delete the DC.
Don't delete an in-memory bitmap while it's still selected into a DC.