控制台程序使用鼠标点击画直线 DDA算法 为什么我画的直线不在面板上，而在屏幕左上角上

#include

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

int WINAPI WinMain(HINSTANCE hInstance,
HINSTANCE hPrevInstance,
PSTR szCmdLine,
int iCmdShow)
{
static TCHAR szAppName[] = TEXT("Line");
HWND hwnd;
MSG msg;
WNDCLASS wndclass;
int cxScreen, cyScreen;

wndclass.style = CS_HREDRAW | CS_VREDRAW;
wndclass.lpfnWndProc = WndProc;
wndclass.cbClsExtra = 0;
wndclass.cbWndExtra = 0;
wndclass.hInstance = hInstance;
wndclass.hIcon = LoadIcon(NULL, IDI_APPLICATION);
wndclass.hCursor = LoadCursor(NULL, IDC_ARROW);
wndclass.hbrBackground = (HBRUSH)GetStockObject(WHITE_BRUSH);
wndclass.lpszMenuName = NULL;
wndclass.lpszClassName = szAppName;

if (!RegisterClass(&wndclass))
{
    MessageBox(NULL, TEXT("This program requires Windows NT!"),
        szAppName, MB_ICONERROR);
    return 0;
}

cxScreen = GetSystemMetrics(SM_CXSCREEN);
cyScreen = GetSystemMetrics(SM_CYSCREEN);
hwnd = CreateWindow(szAppName,
    TEXT("Line Demo"),
    WS_OVERLAPPEDWINDOW,
    cxScreen * 7 / 20,
    cyScreen / 3.5,
    cxScreen * 3 / 10,
    cyScreen / 2,
    NULL,
    NULL,
    hInstance,
    NULL);

ShowWindow(hwnd, iCmdShow);
UpdateWindow(hwnd);

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

return msg.wParam;

}

HDC dc = GetWindowDC(GetDesktopWindow());

int ff(float x)
{
return (int)(x < 0 ? x - 0.5 : x + 0.5);
}

void DDALine(HDC hdc,int x0, int y0, int x1, int y1, COLORREF color)
{
float dx, dy, x, y;
int numb = abs(x1 - x0) > abs(y1 - y0) ? abs(x1 - x0) : abs(y1 - y0);
x = (float)(x0);
y = (float)(y0);
dx = (float)(x1 - x0) / numb;
dy = (float)(y1 - y0) / numb;

for (int i = 0; i <= numb; i++)
{
    SetPixel(dc, ff(x), ff(y), color);
    Sleep(1);
    x += dx;
    y += dy;
}
Sleep(900);

}

 LRESULT CALLBACK WndProc(HWND hwnd, UINT message, WPARAM wParam, LPARAM lParam)
{
    //起点和终点
    static POINT   start, end;
    HPEN            hPen;
    HDC             hdc;
    PAINTSTRUCT     ps;

    switch (message)
    {
    case WM_CREATE:
        //初始化直线起点和终点
        start.x = -1;
        start.y = -1;

        end.x = 0;
        end.y = 0;
        return 0;

        //画直线
    case WM_PAINT:
        hdc = BeginPaint(hwnd, &ps);

        hPen = CreatePen(PS_SOLID, 1, RGB(255, 235, 0));  //画笔格式

        SelectObject(hdc, hPen);

        MoveToEx(hdc, start.x, start.y, NULL);
        DDALine(hdc,start.x, start.y, end.x, end.y, RGB(0, 0, 0));
        DeleteObject(hPen);
        EndPaint(hwnd, &ps);
        return 0;

        //获取起点,使用start记录鼠标落下的位子
    case WM_LBUTTONDOWN:
        start.x = LOWORD(lParam);
        start.y = HIWORD(lParam);
        return 0;

        //获取终点，使用end记录鼠标弹起的位子
    case WM_LBUTTONUP:
        end.x = LOWORD(lParam);
        end.y = HIWORD(lParam);

        InvalidateRect(hwnd, NULL, FALSE);
        return 0;

    case WM_DESTROY:
        PostQuitMessage(0);
        return 0;
    }

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

http://zhidao.baidu.com/link?url=WKmIu0ttj9hh3SERgqbYeA8tQKeuhn3EDtSSDOQQXDHmkFBhkHBFp9MftJDk70uKvqsG-_e5lR7ab1MUawZKcBRLr0H9MUDuLI-fmAli5Ui