VC中如何等比例缩放图像

作者：Future Studio.徐景周
版权所有：徐景周
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　　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);
}