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。

　　涵数中的第一个参数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,

nCenteringFactor,

(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)(dHeight));

}

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,

GetSystemMetrics(SM_CXSCREEN),

GetSystemMetrics(SM_CYSCREEN));

//得到图像尺寸

CSize sizePicture(picture.m_Width, picture.m_Height);

CRect rcNewPictureRect =

SizeRectWithConstantAspectRatio(&rcScreen,

sizePicture,

TRUE);

picture.Show(&dc, rcNewPictureRect);

}