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POBitBlt two images atop one another
 

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    <p>I am attempting to write a program that, when a checkbox is ticked, displays a grid over a large area of the screen. I'm attempting to do this in WM_PAINT using BitBlt, and my display function is definitely called with the right boolean value. However, it doesn't make any change to the window. Am I doing something stupid, or is there no practical way of achieving this? If this is unachievable, is there any way of getting similar results without specifying a different set of images containing the grid?</p> <pre><code>void DisplayRoom(HWND hwnd, char TileList[][100], POINT pMaxDisplay, POINT pMinDisplay, HBITMAP ahbmTileset[], bool bDisplayBars) { HBITMAP hbmSprite, hbmMask; BITMAP bm; PAINTSTRUCT ps; HINSTANCE hinNULL = GetModuleHandle(NULL); HDC hdc = BeginPaint(hwnd, &amp;ps); HDC hdcMem = CreateCompatibleDC(hdc); HBITMAP hbmOld; POINT Position; hbmSprite = LoadBitmap(hinNULL, MAKEINTRESOURCE(IDB_CLEAR_GROUND)); hbmOld = (HBITMAP)SelectObject(hdcMem, hbmSprite); GetObject(hbmSprite, sizeof(bm), &amp;bm); for (int iii = pMinDisplay.x; iii &lt; pMaxDisplay.x; iii++) for (int jjj = pMinDisplay.y; jjj &lt; pMaxDisplay.y; jjj++) { switch(TileList[iii][jjj]) { case 'g': hbmSprite = ahbmTileset[0]; break; case 'd': hbmSprite = ahbmTileset[1]; break; case 'i': hbmSprite = ahbmTileset[2]; break; case 'l': hbmSprite = ahbmTileset[3]; break; } assert(hbmSprite != NULL); SelectObject(hdcMem, hbmSprite); BitBlt(hdc, (iii - pMinDisplay.x) * 34 + 90, (jjj - pMinDisplay.y) * 34 + 60, 34, 34, hdcMem, 0, 0, SRCCOPY); } DeleteDC(hdcMem); EndPaint(hwnd, &amp;ps); if (bDisplayBars) { HDC hdc = BeginPaint(hwnd, &amp;ps); HDC hdcMem = CreateCompatibleDC(hdc); hbmSprite = LoadBitmap(hinNULL, MAKEINTRESOURCE(IDB_GRID)); assert(hbmSprite != NULL); hbmMask = CreateBitmapMask(hbmSprite, RGB(0,0,0)); for (int iii = pMinDisplay.x; iii &lt; pMaxDisplay.x; iii++) for (int jjj = pMinDisplay.y; jjj &lt; pMaxDisplay.y; jjj++) { SelectObject(hdcMem, hbmMask); BitBlt(hdc, (iii - pMinDisplay.x) * 34 + 90, (jjj - pMinDisplay.y) * 34 + 60, 34, 34, hdcMem, 0, 0, SRCAND); SelectObject(hdcMem, hbmSprite); BitBlt(hdc, (iii - pMinDisplay.x) * 34 + 90, (jjj - pMinDisplay.y) * 34 + 60, 34, 34, hdcMem, 0, 0, SRCPAINT); } DeleteDC (hdcMem); EndPaint(hwnd, &amp;ps); } } </code></pre> <p>The dialog procedure is shown below.</p> <pre><code>BOOL CALLBACK CreateRoom(HWND hwnd, UINT Message, WPARAM wParam, LPARAM lParam) { static char achTileDetails[100][100]; static POINT pDimensions, pMonsterLocations, pMinDisplay, pMaxDisplay; static HBITMAP ahbmTileset[4]; static bool bShowBars; switch(Message) { case WM_INITDIALOG: SetFocus(hwnd); for (int iii = 0; iii &lt; 100; iii++) for(int jjj = 0; jjj &lt; 100; jjj++) achTileDetails[iii][jjj] = 'g'; pDimensions.x = 20; pDimensions.y = 10; pMinDisplay.x = 0; pMinDisplay.y = 0; pMaxDisplay.x = 20; pMaxDisplay.y = 10; for (int iii = 0; iii &lt; 4; iii++) ahbmTileset[iii] = LoadBitmap(GetModuleHandle(NULL), MAKEINTRESOURCE(IDB_CLEAR_GROUND + iii)); return TRUE; case WM_PAINT: DisplayRoom(hwnd, achTileDetails, pMaxDisplay, pMinDisplay, ahbmTileset, bShowBars); break; case WM_COMMAND: switch(LOWORD(wParam)) { case IDCANCEL: EndDialog(hwnd, IDCANCEL); break; case IDC_DISPLAY_GRID: switch(HIWORD(wParam)) { case BN_CLICKED: bShowBars = SendMessage(GetDlgItem(hwnd, IDC_DISPLAY_GRID), BM_GETCHECK, 0, 0); InvalidateRect(hwnd, NULL, TRUE); UpdateWindow(hwnd); break; } break; } break; case WM_LBUTTONDOWN: break; default: return FALSE; } } </code></pre>
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    1. COI guess it would help if you showed the calling code. You could also strip out 95% of the code here and still illustrate the fault. I note that you do no error checking at all. Finally, a complete compilable program would make it trivial for us to solve your problem. Please provide that.
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      1. USDavid Heffernan
    2. CO@DavidHeffernan I've added the dialog procedure and some asserts to ensure that the bitmaps are loaded correctly. The asserts have thrown up nothing on running the program.
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    3. COI see @john has spotted the problem. Good for him. If you had stripped the code down to the bare minimum, you would have seen that the painting started to behave once you got down to a single `Begin/EndPaint` pair. As a general point of advice, creating a small compilable reproduction does not just help us, it will 9 times out of 10 allow you to solve the problem yourself.
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      1. USDavid Heffernan
 

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