Is HDC and Memory DC different? - winapi

Question

I am studying Double buffering in windows winapi.
When I draw text on HDC direct using DrawText function, it works well like under code.
case WM_PAINT:
hDC = BeginPaint(hwnd,&ps);
DrawText(hDC,"test",4,&rt,DT_CENTER | DT_WORDBREAK);
DeleteDC(hMemDC);
ReleaseDC(hwnd,hDC);
EndPaint(hwnd,&ps);
break;
But, I would like to want to use double buffering, so I make memory dc and bitblt function.
Under code is that,It don't works.I can show white empty screen.
case WM_PAINT:
hDC = BeginPaint(hwnd,&ps);
hMemDC = CreateCompatibleDC(hDC);
//GetClientRect(hwnd, &crt);
//hBitmap = CreateCompatibleBitmap(hDC, crt.right, crt.bottom);
//OldBitmap = (HBITMAP)SelectObject(hMemDC, hBitmap);
DrawText(hMemDC,"test",4,&rt,DT_CENTER | DT_WORDBREAK);
BitBlt(hDC,0,0,800,800,hMemDC,0,0,SRCCOPY);
DeleteDC(hMemDC);
ReleaseDC(hwnd,hDC);
EndPaint(hwnd,&ps);
break;
Is memory dc different original dc ?
If I use CreateCompatibleBitmap function, it work well.
What is concept am i missing ?
Is there website well-organized website?

Answer 1

A typical non-double-buffered drawing routine looks like this:
case WM_PAINT:
{
PAINTSTRUCT ps;
HDC hdc = BeginPaint(hWnd, &ps);
RECT rcWnd;
GetClientRect(hWnd, &rcWnd);
{
DrawText(hdc, _T("Hello, world!"), -1,
&rcWnd, DT_CENTER | DT_VCENTER | DT_SINGLELINE);
}
EndPaint(hWnd, &ps);
}
break;
A typical double-buffered drawing routine looks like this:
case WM_PAINT:
{
PAINTSTRUCT ps;
HDC hdc = BeginPaint(hWnd, &ps);
RECT rcWnd;
GetClientRect(hWnd, &rcWnd);
{
const int width = rcWnd.right - rcWnd.left
const int height = rcWnd.bottom - rcWnd.top;
// create a new DC based on the target HDC
HDC hDCMem = CreateCompatibleDC(hdc);
// create a bitmap that is compatible with the target DC
HBITMAP hMemBmp = CreateCompatibleBitmap(hdc, width, height);
// select the new bitmap in to the DC, saving the old bitmap
HBITMAP hOldBmp = (HBITMAP)SelectObject(hDCMem, hMemBmp);
// do your drawing
HBRUSH hBr = CreateSolidBrush(RGB(255, 255, 255));
FillRect(hDCMem, &rcWnd, hBr);
DeleteObject(hBr);
DrawText(hDCMem, _T("Hello, world!"), -1,
&rcWnd, DT_CENTER | DT_VCENTER | DT_SINGLELINE);
// copy all the bits from our new DC over to the target DC
BitBlt(hdc, rcWnd.left, rcWnd.top, width, height, hDCMem, 0, 0, SRCCOPY);
// select the original bitmap the DC came with
SelectObject(hDCMem, hOldBmp);
// delete our bitmap
DeleteObject(hMemBmp);
// delete the DC
DeleteDC(hDCMem);
}
EndPaint(hWnd, &ps);
}
break;
case WM_ERASEBKGND:
// since we are drawing the entire area because we are double-buffering, there is
// no need to erase the background. This will speed up your drawing.
return TRUE;
HDC objects are all the same -- there is no special things that happen when you create another one in memory. I mean, they are all in memory, really. You are just creating another canvas to do your drawing on, then copying that canvas over in one shot.