Transparent dialog can't recieve mouse move?

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C++

Hi all,

I want to built a transparent dialog over the pixture box,where display CCD video.
I use:

SetLayeredWindowAttributes(this->GetSafeHwnd(),RGB(255,255,255),128,1) //function,
//and on 
paint()
{
CPaintDC dc(this);
CRect rect;
GetClientRect(&rect); 
dc.fillsolidrect(rect,RGB(255,255,255))  }

So I can draw red/Green line or rectangle on transparent dialog, but I want to draw line by mouse's leftbuttondown/up/move.
I find only narrow region can response not whole client region.
How can i do ?
I want to draw lines/rectangle on dynamic video.