A Simple, Flicker-Free 2D Animation Demo

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Environment: VC6

This project generates a simple, flicker-free 2D animation demo. It loads two pictures—a background (big one) and a foreground (small one). The user can select and move the small picture over the big one without any flickering. Resizing the window does not produce any flickering of its contents, either. The idea is very simple and it could be used for different purposes. It could be implemented in different programming languages as well. I used it with Borland C++ Builder to create a multimedia picture slider for a catalogue program and Delphi to create a program for creating moving and resizing dialog box controls. Of course, it is better there to use “Canvas” rather than “Device context,” but the principle is the same.

All the interesting code is located in xdrawView.h and xdrawView.cpp. It creates two memory dc and three bitmap objects:

• CDC* m_PicDC;
• CDC* m_CanvasDC;
• CBitmap* m_BkgBitmap;
• CBitmap* m_Bitmap;
• CBitmap* m_CanvasBitmap;

The coordinates and size of the foreground picture on the screen are in the following code:

struct PICTURE
{
  CPoint topleft;       // top-left corner of the pic.
  int height, width;    // pic. width, height
};
PICTURE m_picture;

Background picture width and height:

int m_hBkg, m_wBkg;

When a user moves the foreground picture over the background one, the program calculates a “dirty rectangle.” The “dirty rectangle” combines the new and old position of the foreground picture and sends it to be invalidated. The OnDraw() function is doing this flicker-free invalidation.

//—————————————————————

// Draws a flicker-free content of the window—when requested.

//—————————————————————

void CXdrawView::OnDraw(CDC* pDC)

{

// CXdrawDoc* pDoc = GetDocument();

// ASSERT_VALID(pDoc);

  // TODO: add draw code for native data here

  CBitmap* pOldBmp1;

  CBitmap* pOldBmp2;

  CRect cr;

  //selects canvas

  pOldBmp1 = m_CanvasDC->SelectObject(m_CanvasBitmap);

  //selects background, blits into canvas

  pOldBmp2 = m_PicDC->SelectObject(m_BkgBitmap);

  m_CanvasDC->BitBlt(0, 0, m_wBkg, m_hBkg, m_PicDC, 0, 0, SRCCOPY);

  //selects small pic, blits into canvas

  m_PicDC->SelectObject(m_Bitmap);

  m_CanvasDC->BitBlt(m_picture.topleft.x,

    m_picture.topleft.y,

    m_picture.width,

    m_picture.height,

    m_PicDC,

    0, 0, SRCCOPY);

  //blits canvas on screen. Well, only part of it—the

  // “dirty rectangle” part.

  pDC->GetClipBox(cr);

  pDC->BitBlt(cr.left, cr.top, cr.right, cr.bottom, m_CanvasDC,

                 cr.left, cr.top, SRCCOPY);

  //deselects all the bitmaps

  m_CanvasDC->SelectObject(pOldBmp1);

  m_PicDC->SelectObject(pOldBmp2);

}
//—————————————————————

// if user clicks in the small picture, he captures this event

// for consecutive handling.

//—————————————————————

void CXdrawView::OnLButtonDown(UINT nFlags, CPoint point)

{

  CRect PicRc(m_picture.topleft,

    CPoint(m_picture.topleft.x + m_picture.width,

    m_picture.topleft.y + m_picture.height));

  if (PicRc.PtInRect(point))

  {

    SetCapture();

    m_Btndown = true;

    m_offs = point – m_picture.topleft;

    ::SetCursor(::LoadCursor(NULL, IDC_CROSS));

  }

}
//—————————————————————

// Release user input.

//—————————————————————

void CXdrawView::OnLButtonUp(UINT nFlags, CPoint point)

{

  ::ReleaseCapture();

  m_Btndown = false;

}
//—————————————————————

// if we have mouse down captured, then calculate a dirty rect

// that combines the old and new position of the small picture

// and invalidates it

//—————————————————————

void CXdrawView::OnMouseMove(UINT nFlags, CPoint point)

{

  if (m_Btndown)

  {

    CRect DirtyRc = CalcDirtyRect(m_picture.width,

      m_picture.height,

      m_picture.topleft,

      point – m_offs);

    m_picture.topleft = point – m_offs;

    InvalidateRect(DirtyRc, false);

  }
}
//—————————————————————

// calculates dirty rect. that combines 2 rect., expresing old

// and new position of the small picture.

//—————————————————————

CRect CXdrawView::CalcDirtyRect(int pic_w, int pic_h,

                                CPoint p1, CPoint p2)

{

  CSize PicSize(pic_w,pic_h);

  CRect r1(p1,PicSize);

  CRect r2(p2,PicSize);
  return r1 | r2;

}
//—————————————————————

// Try commenting my code and uncommenting the standard one,

// then try to resize the app. window to see the diference.

//—————————————————————

BOOL CXdrawView::OnEraseBkgnd(CDC* pDC)

{

  return false;

// return CView::OnEraseBkgnd(pDC);

// the original code

}

Downloads

Download demo project – 150 Kb

Download source – 82 Kb