Repainting the Screen Using Device Dependent Bitmaps

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#include <windows.h>
#include <stdlib.h>

LRESULT CALLBACK WndProc(HWND, UINT, WPARAM, LPARAM);

int maxX, maxY;
HDC memDC = NULL;
HBITMAP hBitmap = NULL;
HBITMAP hOldBitmap = NULL;

int WINAPI WinMain(HINSTANCE hInstance,
                   HINSTANCE hPrevInstance,
                   LPSTR lpCmdLine,
                   int nCmdShow)
{
    WNDCLASSEX wc;
    MSG msg;

    wc.cbSize        = sizeof(WNDCLASSEX);
    wc.style         = CS_HREDRAW | CS_VREDRAW;
    wc.lpfnWndProc   = WndProc;
    wc.cbClsExtra    = 0;
    wc.cbWndExtra    = 0;
    wc.hInstance     = hInstance;
    wc.hIcon         = LoadIcon(NULL, IDI_APPLICATION);
    wc.hCursor       = LoadCursor(NULL, IDC_ARROW);
    wc.hbrBackground = (HBRUSH)(COLOR_WINDOW + 1);
    wc.lpszMenuName  = NULL;
    wc.lpszClassName = TEXT("BitmapDemo");
    wc.hIconSm       = LoadIcon(NULL, IDI_APPLICATION);

    RegisterClassEx(&wc);

    CreateWindowEx(
        WS_EX_CLIENTEDGE,
        TEXT("BitmapDemo"),
        TEXT("Repaint Using Bitmap"),
        WS_OVERLAPPEDWINDOW | WS_VISIBLE,
        CW_USEDEFAULT,
        CW_USEDEFAULT,
        700,
        300,
        NULL,
        NULL,
        hInstance,
        NULL);

    while(GetMessage(&msg,NULL,0,0))
    {
        TranslateMessage(&msg);
        DispatchMessage(&msg);
    }

    return msg.wParam;
}

LRESULT CALLBACK WndProc(HWND hwnd,
                         UINT msg,
                         WPARAM wParam,
                         LPARAM lParam)
{
    HDC hdc;
    PAINTSTRUCT ps;

    switch(msg)
    {
        case WM_CREATE:
        {
            RECT rc;
            int i;
            int x1,y1,x2,y2;

            GetClientRect(hwnd,&rc);

            maxX = rc.right;
            maxY = rc.bottom;

            hdc = GetDC(hwnd);

            memDC = CreateCompatibleDC(hdc);

            hBitmap = CreateCompatibleBitmap(
                            hdc,
                            maxX,
                            maxY);

            hOldBitmap = (HBITMAP)SelectObject(
                                memDC,
                                hBitmap);

            PatBlt(
                memDC,
                0,
                0,
                maxX,
                maxY,
                WHITENESS);

            srand(GetTickCount());

            for(i=0;i<200;i++)
            {
                x1 = rand() % maxX;
                y1 = rand() % maxY;
                x2 = rand() % maxX;
                y2 = rand() % maxY;

                MoveToEx(memDC,x1,y1,NULL);
                LineTo(memDC,x2,y2);
            }

            ReleaseDC(hwnd,hdc);

            return 0;
        }

        case WM_PAINT:

            hdc = BeginPaint(hwnd,&ps);

            BitBlt(
                hdc,
                ps.rcPaint.left,
                ps.rcPaint.top,
                ps.rcPaint.right - ps.rcPaint.left,
                ps.rcPaint.bottom - ps.rcPaint.top,
                memDC,
                ps.rcPaint.left,
                ps.rcPaint.top,
                SRCCOPY);

            EndPaint(hwnd,&ps);

            return 0;

        case WM_DESTROY:

            if(memDC)
            {
                SelectObject(memDC,hOldBitmap);
                DeleteObject(hBitmap);
                DeleteDC(memDC);
            }

            PostQuitMessage(0);

            return 0;
    }

    return DefWindowProc(hwnd,msg,wParam,lParam);
}