Important Update: Not all objects have defaults selected into HDC s, and you can refer to MSDN for the few that don't. Because of this I was previously uncertain as to wether HBITMAP s were one of them, since there doesn't seem to be any definitive documentation on it, and examples (even those by Microsoft) often ignored the default bitmap. Since the writing of the original tutorial several years ago, it was confirmed to me that there was in fact a default bitmap that needs releasing. This information is courtesy of Shaun Ivory, a software engineer for MS and a friend of mine from #winprog.
Apparently there was a bug in a screensaver written at MS, and it turns out it was because the default bitmap wasn't getting replaced or destroyed, and it eventually ran out of GDI resources. Be warned! It's an easy mistake to make.
Displaying Bitmaps
Ok, down to business. The simplest drawing operations on a window occure by handling WM_PAINT. When your window is first displayed, restored from being minimised, or uncovered from having another window on top of it, Windows sends the WM_PAINT message to the window to let it know that it needs to redraw it's contents. When you draw something on the screen it is NOT permanent, it's only there untill something else draws over it, and at that point you need to draw it again when the time comes.
HBITMAP g_hbmBall = NULL;
case WM_CREATE:
g_hbmBall = LoadBitmap(GetModuleHandle(NULL), MAKEINTRESOURCE(IDB_BALL));
if (g_hbmBall == NULL) MessageBox(hwnd, "Could not load IDB_BALL!", "Error", MB_OK | MB_ICONEXCLAMATION);
break;
The first step is of course loading the bitmap, this is quite simple with a bitmap resource, there are no significant differences from loading other resource types. Then we can get down to drawing…
case WM_PAINT:
{
BITMAP bm;
PAINTSTRUCT ps;
HDC hdc = BeginPaint(hwnd, &ps);
HDC hdcMem = CreateCompatibleDC(hdc);
HBITMAP hbmOld = SelectObject(hdcMem, g_hbmBall);
GetObject(g_hbmBall, sizeof(bm), &bm);
BitBlt(hdc, 0, 0, bm.bmWidth, bm.bmHeight, hdcMem, 0, 0, SRCCOPY);
SelectObject(hdcMem, hbmOld);
DeleteDC(hdcMem);
EndPaint(hwnd, &ps);
}
break;
Getting the Window DC
To start off we declare a couple of variables we need. Notice that the first one is a BITMAP, not an HBITMAP. BITMAP is a struct that holds information about an HBITMAP which is the actual GDI object. We need a way to get the height and width of the HBITMAP so we use GetObject() which contrary to it's name doesn't really get an object, but rather information about an existing one. "GetObjectInfo" would have been a more appropriate label. GetObject() works for various GDI object types which it can distinguish based on the value of the second parameter, the size of the structure.
The PAINTSTRUCT is a structure that contains information about the window being painted and what exactly is going on with the paint message. For most simple tasks, you can simply ignore the information it contains, but it's required for the call to BeginPaint(). BeginPaint() as it's name suggests is designed specifically for handling the WM_PAINT message. When not handling a WM_PAINT message you would use GetDC() which we will see in the timer animation examples in a while… but in WM_PAINT, it's important to use BeginPaint() and EndPaint().
BeginPaint() returns us an HDC that represents the HWND that we pass to it, the one that WM_PAINT is being handled for. Any drawing operation we perform on this HDC will immediately display on the screen.
Setting up a Memory DC for the Bitmap
As I mention above, in order to draw on or with bitmaps, we need to create a DC in memory… the easiest way to do that here is to CreateCompatibleDC() with the one we already have. This gives us a Memory DC that is compatible with the color depth and display properties of the HDC for the window.
Now we call SelectObject() to select the bitmap into the DC being careful to store the default bitmap so that we can replace it later on and not leak GDI objects.
Drawing
Once we've gotten the dimentions of the bitmap filled into the BITMAP struct, we can call BitBlt() to copy the image from our Memory DC to the Window DC, thus displaying on the screen. As always, you can look up each parameter in MSDN, but in short they are: The destination, the position and size, the source and source position, and finally the Raster Operation (ROP code), which specifies how to do the copy. In this case, we want a simple exact copy of the source made, no fancy stuff.
BitBlt() is probably the all time happiest function in all of the Win32 API and is the staple diet of anyone learning to write games or other graphics applications in windows. It was probably the first API that I memorised all the parameters to.
Cleanup
At this point the bitmap should be on the screen, and we need to clean up after ourselves. The first thing to do is restore the Memory DC to the state it was when we got it, which means replacing our bitmap with the default one that we saved. Next we can delete it altogether with DeleteDC().
Finally we release the Window DC we got from BeginPaint() using EndPaint().
Destroying an HDC is a little confusing sometimes because there are at least 3 ways to do it depending on how you got it in the first place. Here's a list of the common methods of gaining an HDC, and how to release it when you're done.
• GetDC() — ReleaseDC()
• BeginPaint() — EndPaint()
• CreateCompatibleDC() — DeleteDC()
And finally, at the termination of our program, we want to free any resources that we allocated. Technically speaking this isn't absolutely required, since modern Windows platforms are pretty good at freeing everything when your program exists, but it's always a good idea to keep track of your own objects because if get lazy and don't delete them they have a habit of getting loose. And no doubt, there are still bugs in windows especially older versions that won't clean up all of your GDI objects if you don't do a thorough job.