mouse-draw rect map to img's rgn unprecise problem.

I draw a rect by mouse,map this rect to piscure's rgn.and then draw the rgn.but they show not completely at the same pos. in attachement sample, the dotrect is the rng,the black sold is the original.and i find that, the bigger of image ,the bigger measurement error .

my code is this:

RECT rcScaled;
rcScaled.left=(int)((m_rcCLoneRect.left-m_rcDst.left)/(GetZoomPercent()/100.0)+0.5);

rcScaled.top=(int)((m_rcCLoneRect.top-m_rcDst.top)/(GetZoomPercent()/100.0)+0.5);

rcScaled.right=rcScaled.left+(int)((m_rcCLoneRect.Width()*100/GetZoomPercent()));
rcScaled.bottom=rcScaled.top+(int)((m_rcCLoneRect.Height()*100/GetZoomPercent()));

SetAutoRgnToFloater(TRUE);
LBitmapRgn Region(this);
Region.SetRgnRect(&rcScaled);
this->Repaint();

The most likely cause of the difference is rounding errors. The GetZoomPercent() function returns integer, and all the calculations in your code are integer based.
This means you will not get accurate results this way.

A better way to convert from client coordinates to bitmap coordinates, is to use the formula in this forum post:
http://support.leadtools.com/cs/forums/34411/ShowPost.aspx

You can also use the same rectangles to do the inverse calculation from bitmap to client coordinates if you need it.