Calculating the correct width to resize image to display in a CListCtrl

Question

At the moment I am having to fudge my code like this:

CRect rcList;
m_ListThumbnail.GetClientRect(rcList);
rcList.DeflateRect(25, 25);

// Use monitor 1 to work out the thumbnail sizes
CRect rcMonitor = m_monitors.rcMonitors.at(0);

m_iThumbnailWidth = rcList.Width();

double dHt = ((double)rcMonitor.Height() / (double)rcMonitor.Width()) * (double)m_iThumbnailWidth;
m_iThumbnailHeight = (int)dHt;

I fudge it by deflating the rectangle by 25. m_ListThumbnail is a CListCtrl and I am trying to render my thumbnails so that I do not need a horizontal scroll bar.

When I render the thumbnails of the monitors, I attempt to massage these thumbnail values (incomplete):

nWidth = m_iThumbnailWidth;

double dHt = ((double)rcCapture.Height() / (double)rcCapture.Width()) * (double)m_iThumbnailWidth;
nHeight = (int)dHt;

if (nHeight > m_iThumbnailHeight)
{
    AfxMessageBox(L"Need to investigate!");
}

Where rcCapture is the size of the monitor.

If I remove the DeflateRect line, my window displays like this:

As you can see, it is note as I expected. There is a horizontal scroll bar and I have to resize quite a bit to see the thumbnail:

All I want to compute is a set of thumbnail dimensions so that the scaled down monitor image is going to fit in the CListCtrl. I don't really want the user to have to resize the window.

---

Update

With my adjusted code which now uses the primary monitor aspect ratio to work out the thumbnail sizes (as added above) renders the app with better whitespace:

That was the reason of the extra space at the bottom because the monitors were 16:9 and I was squishing into 4:3. So that is fixed.

But as you can see, using the CListCtrl client width is not sufficient.

---

Update

This is the rendering code:

// Set the mode first!
SetStretchBltMode(dcImage, COLORONCOLOR);

int iTop = (m_iThumbnailHeight - nHeight) / 2;

// Copy (and resize)
bRet = ::StretchBlt(dcImage, 0, iTop,
    nWidth, 
    nHeight, 
    dcScreen.m_hDC, 
    rcCapture.left, 
    rcCapture.top, 
    rcCapture.Width(), 
    rcCapture.Height(), SRCCOPY | CAPTUREBLT);

Answer 1

The control renders the icon with a margin. When I used GetItemPosition function it indicated that the x value was 21. This is why my DeflateRect(25, 25) worked. But I don;t know how the CListCtrl computed that margin value.

---

Eventually I found out how to do this without deflating, by using the SetIconSpacing function. Like this:

m_ListThumbnail.SetIconSpacing(
    CSize(m_iThumbnailWidth, ::GetSystemMetrics(SM_CXHSCROLL)));

Once the above has been done the window looks like this:

Things might be a little different when there are 3 or 4 monitor thumbnails, thus causing a vertical scroll bar. But I only have two monitors to test with.