Win32 - why is nothing being drawn to the screen? - c++

Question

I'm trying to use an off-screen buffer so that I can keep track of changes to the screen before/after WM_PAINT and just copy them through one line in WM_PAINT. Here's some code I have to set up the graphics:
hdc = GetDC(hWnd);
hdcmem = CreateCompatibleDC(hdc);
hbcmem = CreateCompatibleDC(hdcmem);
// Load bitmaps
bg = LoadBitmap(hInst, MAKEINTRESOURCE(IDB_BACKGROUND));
side = LoadBitmap(hInst, MAKEINTRESOURCE(IDB_SIDEINFO));
mainCont = LoadBitmap(hInst, MAKEINTRESOURCE(IDB_GAME_CONT));
if(bg == NULL || side == NULL || mainCont == NULL)
ThrowError("A bitmap failed to load.");
// Background
hdcold = (HBITMAP)SelectObject(hbcmem, bg);
BitBlt(hdcmem, 0, 0, 237, 196, hbcmem, 0, 0, SRCCOPY);
BitBlt(hdcmem, 237, 0, 237, 196, hbcmem, 0, 0, SRCCOPY);
BitBlt(hdcmem, 237 * 2, 0, 237, 196, hbcmem, 0, 0, SRCCOPY);
BitBlt(hdcmem, 0, 196, 237, 196, hbcmem, 0, 0, SRCCOPY);
BitBlt(hdcmem, 237, 196, 237, 196, hbcmem, 0, 0, SRCCOPY);
BitBlt(hdcmem, 237 * 2, 196, 237, 196, hbcmem, 0, 0, SRCCOPY);
// Side Info
hdcold = (HBITMAP)SelectObject(hbcmem, side);
BitBlt(hdcmem, 339, 26, 154, 300, hbcmem, 0, 0, SRCCOPY);
// Main Game Container
hdcold = (HBITMAP)SelectObject(hbcmem, mainCont);
BitBlt(hdcmem, 26, 26, 300, 300, hbcmem, 0, 0, SRCCOPY);
hdc, hdcmem, hbcmem, hdcold, bg, side, and mainCont are declared previously. Their scope includes everything in this file (including this code and the code in WM_PAINT).
Here's the code in WM_PAINT:
PAINTSTRUCT ps;
BeginPaint(hWnd, &ps);
BitBlt(hdc, 0, 0, 518, 401, hdcmem, 0, 0, SRCCOPY);
EndPaint(hWnd, &ps);
For some reason, nothing is being drawn to the screen. I'm racking my brain trying to figure it out. A pointer in the right direction would be much appreciated.

Answer 1

Create a compatible bitmap for your memory device context first, then select that bitmap to the memory dc and it should work !
hdc = GetDC(hWnd); // used only to create compatibles.
hdcmem = CreateCompatibleDC(hdc);
hbcmem = CreateCompatibleDC(hdc);
// Create client-area-sized compatible bitmap.
RECT rc;
GetClientRect(hWnd, &rc);
HBITMAP hbm_memdc = CreateComptibleBitmap(hdc, rc.right, rc.bottom);
HBITMAP hbm_memdc_old = (HBITMAP)SelectObject(hdcmem, hbm_memdc)
ReleaseDC(hdc); // this no longer needed
// now start rendering into hdcmem...
You'll want to keep the old bitmap handle selected out to put it back before destroying your custom one on shutdown. How you manage that is entirely up to you.