CODEWITHSUNDEEP
c++winapigdi
Brian Gradin
Brian Gradin

Reputation: 2275

Win32 - What is the proper way to use GDI objects?

I am making a simple game in Win32. Currently, I have most of my drawing object declared globally in the main file so that I never have to delete them or reload bitmaps. So basically all of my bitmaps are loaded in WM_CREATE, and then nothing ever needs to be deleted - I was told by someone that I could just rely on the system to clean up my resources on project termination. In WM_PAINT (which is called around 10 times a second), I have a lot of BitBlt() calls, as well as quite a few SelectObject() calls. After running for about 10 - 15 minutes, SelectObject(). What would cause SelectObject() to fail, and am I using GDI object incorrectly?

EXAMPLE:

// Top of the file
HDC hdc;
HDC hdcmem;
HDC hbcmem;
HBITMAP colorsprites, blackwhitesprites, nums;
HBITMAP hdcold, hdcbmold, hdcbm;

// Some functions to get the window ready

// More variables used for drawing:
HBITMAP bg, side, mainCont, tmpbm, tmpold, bm_left, bm_right, sidebg, win_bm;
PAINTSTRUCT ps;
RECT rc;
HDC tmphdc;
HDC tmp;
HFONT font;
HBRUSH hbr;
HPEN pen;
BITMAP structBitmapHeader;
HGDIOBJ hBitmap;
HCURSOR crosshairs = LoadCursorW(hInst, IDC_CROSS);
HCURSOR normal = LoadCursorW(hInst, IDC_ARROW);
POINT cursor;
bool addPass = false, ignorePass;

LRESULT CALLBACK WndProc(HWND hWnd, UINT message, WPARAM wParam, LPARAM lParam)
{
    switch (message)
    {
        case WM_CREATE:
            hdc = GetDC(hWnd);
            hdcmem = CreateCompatibleDC(hdc);
            GetClientRect(hWnd, &rc);
            hdcbm = CreateCompatibleBitmap(hdc, rc.right, rc.bottom);
            hbcmem = CreateCompatibleDC(hdcmem);
            hdcbmold = (HBITMAP)SelectObject(hdcmem, hdcbm);

            // Load bitmaps
            bg                  = LoadBitmap(hInst, MAKEINTRESOURCE(IDB_BACKGROUND));
            mainCont            = LoadBitmap(hInst, MAKEINTRESOURCE(IDB_GAME_CONT));
            side                = LoadBitmap(hInst, MAKEINTRESOURCE(IDB_SIDEINFO));
            colorsprites        = LoadBitmap(hInst, MAKEINTRESOURCE(IDB_COLOR_SPRITES));
            blackwhitesprites   = LoadBitmap(hInst, MAKEINTRESOURCE(IDB_BLACKWHITE_SPRITES));
            sidebg              = LoadBitmap(hInst, MAKEINTRESOURCE(IDB_SIDEBG));
            nums                = LoadBitmap(hInst, MAKEINTRESOURCE(IDB_NUMBERS));
            bm_left             = LoadBitmap(hInst, MAKEINTRESOURCE(IDB_LEFT));
            bm_right            = LoadBitmap(hInst, MAKEINTRESOURCE(IDB_RIGHT));
            win_bm              = LoadBitmap(hInst, MAKEINTRESOURCE(IDB_VICTORY));
            if(bg == NULL || mainCont == NULL || side == NULL || colorsprites == NULL 
            || blackwhitesprites == NULL || sidebg == NULL || nums == NULL || bm_left == NULL 
            || bm_right == NULL)
                ThrowError("A bitmap failed to load.");

            break;

        case WM_PAINT:
            BeginPaint(hWnd, &ps);

            // SelectObject() and BitBlt() are called a lot in here

            EndPaint(hWnd, &ps);

            break;

        case WM_DESTROY:
            // System will clean up all GDI stuff - no need to delete anything
            PostQuitMessage(0);
            break;
    }
}

Upvotes: 1

Views: 1699

Answers (2)

ScottMcP-MVP
ScottMcP-MVP

Reputation: 10425

You can make life a lot easier by calling SaveDC right after BeginPaint and RestoreDC just before EndPaint. This will restore the DC to its original state before it is freed.

http://msdn.microsoft.com/en-us/library/windows/desktop/dd162945(v=vs.85).aspx

Upvotes: 2

Mark Ransom
Mark Ransom

Reputation: 308530

Make sure that when you destroy a DC, you've selected the original bitmap back into it. You should never destroy a DC while your bitmap is still selected.

Upvotes: 3

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