Capture game window using wm_paint - c++

Question

I'm trying to capture a game window using SendMessage with wm_paint and wm_printclient.
I already did it successfully using PrintWindow but the game can change between graphic engines and for some of them I get a white rectangle. I was hoping using SendMessage would not have this problem.
The problem is I'm getting a black rectangle as result of SendMessage, for any graphic engine and even for any program/window.
void capture::captureProgramScreen(HWND hwnd, tImage* res)
{
RECT rc;
GetWindowRect(hwnd, &rc);
//create
HDC hdcScreen = GetDC(NULL);
HDC hdc = CreateCompatibleDC(hdcScreen);
HBITMAP hbmp = CreateCompatibleBitmap(hdcScreen, rc.right - rc.left, rc.bottom - rc.top);
res->width = rc.right - rc.left - 17;
res->height = rc.bottom - rc.top - 39;
res->absoluteTop = rc.top;
res->absoluteLeft = rc.left;
SelectObject(hdc, hbmp);
SendMessage(hwnd, WM_PRINTCLIENT, (int)hdc, PRF_CHILDREN | PRF_CLIENT | PRF_ERASEBKGND | PRF_NONCLIENT | PRF_OWNED);
BITMAPINFO MyBMInfo = { 0 };
MyBMInfo.bmiHeader.biSize = sizeof(MyBMInfo.bmiHeader);
if (0 == GetDIBits(hdc, hbmp, 0, 0, NULL, &MyBMInfo, DIB_RGB_COLORS))
{
res->error = true;
res->errorcode = 2;
return;
}
res->v = std::vector<BYTE>(MyBMInfo.bmiHeader.biSizeImage);
MyBMInfo.bmiHeader.biBitCount = 32;
MyBMInfo.bmiHeader.biCompression = BI_RGB;
MyBMInfo.bmiHeader.biHeight = abs(MyBMInfo.bmiHeader.biHeight);
if (0 == GetDIBits(hdc, hbmp, 0, MyBMInfo.bmiHeader.biHeight, &(res->v[0]), &MyBMInfo, DIB_RGB_COLORS))
{
res->error = true;
res->errorcode = 3;
res->width = 0;
res->height = 0;
res->v.clear();
return;
}
//4 Bytes per pixel order (B G R A) from [left to right] [bottom to top]
return;
}
Thank you!

Answer 1

There are at least a few possible issues:
Not all programs/windows implement WM_PRINTCLIENT. Many games don't even implement WM_PAINT, as they draw continuously at their desired frame rate rather than in response to a need to update themselves. Many games use newer graphics APIs that don't really draw to a Device Context.
I'm not sure why you have two calls to GetDIBits. The first one happens before you initialize all the fields of the BITMAPINFO, so that one will fail. It's still not completely filled out by the time you make the second call.