grid

Env: WindowsXP, VC++6.0

In my application, I need to draw grid on scaled(x8) bitmap in
CMyScrollView::OnDraw(CDC* pDC).

In the method No.1(below), I draw directly grid on the current hDC. But it
has a problem. When we move the scrollbar button, the view of grid is
changed(gray to white on some places).

In the method No.2(below), I firstly draw grid on another hdcScale. Then
copy it to the current hDC. There is no such problem. But I have to consume
memory.

So, I am wondering how to work it around by method No.1.

TIA
William

***********************************************************************************
Method No.1
CMyScrollView::OnDraw(CDC* pDC)
{
 CRect rcClient;
 GetClientRect(&rcClient);

 int iWindowWid, iWindowHei;
 iWindowWid = rcClient.right / iZoom; //iZoom=8
 iWindowHei = rcClient.bottom / iZoom;

 //add one point
 iWindowWid++;
 iWindowHei++;

 //when original bitmap is small than view window
 iWindowWid = min(iWindowWid, cxImage);
 iWindowHei = min(iWindowHei, cyImage);

 ::StretchBlt( pDC->m_hDC, 0, 0, iWindowWid * iZoom , iWindowHei * iZoom,
    hdcImage, 0, 0, iWindowWid, iWindowHei, SRCCOPY);

 //
 // Draw grid
 //
 DrawGrid(pDC->m_hDC);
}

 void CMyScrollView::DrawGrid(HDC hdcTemp)
{
const int brush_width = 8, brush_height = 8;
 COLORREF colors[] = {RGB(128,128,128), RGB(192,192,192)};

 HDC hDCPattern = ::CreateCompatibleDC(hdcTemp);
 HBITMAP hBMP = ::CreateCompatibleBitmap(hdcTemp, brush_width,
brush_height);
 HBITMAP hBMPOld = (HBITMAP)SelectObject(hDCPattern, hBMP);

 for (int i=0; i<brush_width; i++)
  for (int j=0; j<brush_height; j++)
   SetPixelV(hDCPattern, i, j, colors[(i+1)*(j+1)&1]);

 HBRUSH hBrPattern = ::CreatePatternBrush(hBMP);
 HBRUSH hOldBrush = (HBRUSH)SelectObject(hdcTemp, hBrPattern );

 for (int x = iScale; x < gi.cxScale; x += iScale)
  PatBlt(hdcTemp, x, 0, 1, gi.cyScale, PATCOPY);

 for (int y = iScale; y < gi.cyScale; y += iScale)
  PatBlt(hdcTemp, 0, y, gi.cxScale, 1, PATCOPY);

 SelectObject(hDCPattern, hBMPOld);
 SelectObject(hdcTemp, hOldBrush);
 DeleteObject(hBrPattern);
}

***********************************************************************************
Method No.2

CMyScrollView::OnDraw(CDC* pDC)
{
 CRect rcClient;
 GetClientRect(&rcClient);

 int iWindowWid, iWindowHei;
 iWindowWid = rcClient.right / iZoom; // //iZoom=8
 iWindowHei = rcClient.bottom / iZoom;

 //add one point
 iWindowWid++;
 iWindowHei++;

 //when original bitmap is small than view window
 iWindowWid = min(iWindowWid, cxImage);
 iWindowHei = min(iWindowHei, cyImage);

 HDC hdcScale;
 HBITMAP hbmScale;
 hdcScale = CreateCompatibleDC(hdcImage);
 hbmScale = CreateCompatibleBitmap(hdcImage, iWindowWid * iZoom, iWindowHei
* iZoom);
 SelectObject(hdcScale, hbmScale);

 ::StretchBlt( hdcScale, 0, 0, iWindowWid * iZoom , iWindowHei * iZoom,
    hdcImage, 0, 0, iWindowWid, iWindowHei, SRCCOPY);

 //
 // Draw grid on hdcShow.
 //
  DrawGrid(hdcScale);

 //
 // Copy to the screen.
 //
 ::BitBlt(pDC->m_hDC, 0, 0, iWindowWid * iZoom, iWindowHei * iZoom,
hdcScale, 0, 0, SRCCOPY);

 DeleteDC(hdcScale);
 DeleteObject(hbmScale);
}

*********************************************************************
NOTE:
hdcImage is made at initiation period as follows,
{
 HDC hdcParent;
 hdcParent = GetDC(m_hWnd);

 hdcImage = CreateCompatibleDC(hdcParent);
 hbmImage = Create256DIBitmap(hdcParent, cxImage, cyImage, FALSE ); //at
it CreateDIBSection() is called.
 SelectObject(hdcImage, hbmImage);

}