等比例缩放图像

在放大或缩小图像时，如何能最好的保持图像原态比例来显示呢？下面提供一个涵数可帮你来实现，调用它后，返回的矩形区域既为最佳显示图像大小。
    函数中的第一个参数rcScreen为图像要被显示的矩形大小，第二个参数sizePicture为图像自身大小，第三个参数bCenter为是否居中显示，返回值CRect既为图像最佳显示大小。
涵数如下所示：
CRect RectSizeWithConstantRatio( CRect* rcScreen,
                                          CSize sizePicture,
                                          BOOL bCenter)
{
  CRect rect(rcScreen);
  double dWidth = rcScreen->Width();
  double dHeight = rcScreen->Height();
  double dAspectRatio = dWidth/dHeight;

  double dPictureWidth = sizePicture.cx;
  double dPictureHeight = sizePicture.cy;
  double dPictureAspectRatio = dPictureWidth/dPictureHeight;

  //If the aspect ratios are the same then the screen rectangle
  // will do, otherwise we need to calculate the new rectangle

  if (dPictureAspectRatio > dAspectRatio)
  {
    int nNewHeight = (int)(dWidth/dPictureWidth*dPictureHeight);
    int nCenteringFactor = (rcScreen->Height() – nNewHeight) / 2;
    rect.SetRect( 0,
                   nCenteringFa ctor,
                   (int)dWidth,
                   nNewHeight + nCenteringFactor);

  }
  else if (dPictureAspectRatio < dAspectRatio)
  {
    int nNewWidth =  (int)(dHeight/dPictureHeight*dPictureWidth);
    int nCenteringFactor = (rcScreen->Width() – nNewWidth) / 2;
    rect.SetRect( nCenteringFactor,
                   0,
                   nNewWidth + nCenteringFactor,
                   (int)(dHeigh t));
  }

  return rect;
};
此涵数的使用方法如下：
{
……(部分省略)
//得到屏幕大小
  CRect rcScreen( 0, 0,
                   GetSystemMet rics(SM_CXSCREEN),
                   GetSystemMet rics(SM_CYSCREEN));

  //得到图像尺寸
  CSize sizePicture(picture.m_Width, picture.m_Height);

  CRect rcNewPictureRect =
        SizeRectWith ConstantAspectRatio(&rcScreen,
                                            sizePicture,
                                            TRUE);
  picture.Show(&dc, rcNewPictureRect);
}