C++语言 线条的渐变

void CSelRectDlg::OnPaint() 
{
    if (IsIconic())
    {
        CPaintDC dc(this); // device context for painting

        SendMessage(WM_ICONERASEBKGND, (WPARAM) dc.GetSafeHdc(), 0);

        // Center icon in client rectangle
        int cxIcon = GetSystemMetrics(SM_CXICON);
        int cyIcon = GetSystemMetrics(SM_CYICON);
        CRect rect;
        GetClientRect(&rect);
        int x = (rect.Width() - cxIcon + 1) / 2;
        int y = (rect.Height() - cyIcon + 1) / 2;

        // Draw the icon
        dc.DrawIcon(x, y, m_hIcon);
    }
    else
    {
        CPaintDC dc(this);
        CRect rc;
        this->GetWindowRect(rc);
        int y = rc.Height()/4;
        CPen pen, *oldpen;
        oldpen = dc.SelectObject(&pen);
        dc.MoveTo(0, y);
        dc.LineTo(0, y);
        for(int i=1;i<255;i++)
        {
            pen.DeleteObject();
            pen.CreatePen(PS_SOLID, 2, RGB(255, i, 0));
            dc.SelectObject(&pen);
            dc.LineTo(i, y);
        }
        dc.MoveTo(0, y*2);
        dc.LineTo(0, y*2);
        for(int m=1;m<255;m++)
        {
            pen.DeleteObject();
            pen.CreatePen(PS_SOLID, 2, RGB(0, 255,m));
            dc.SelectObject(&pen);
            dc.LineTo(m, y*2);

        }
        dc.MoveTo(0, y*3);
        dc.LineTo(0, y*3);
        for(int n=1;n<255;n++)
        {
            pen.DeleteObject();
            pen.CreatePen(PS_SOLID, 2, RGB(n, 0, 255));
            dc.SelectObject(&pen);
            dc.LineTo(n, y*3);

        }
        pen.DeleteObject();
        dc.SelectObject(oldpen);

        CDialog::OnPaint();
    }
}