Calculating correct position of image in Picture Control, and drawing the background correctly (on MFC dialog)

Question

I have this dialog:

Here is the RC for the dialog:

IDD_DIALOG_SPECIAL_EVENT_VIDEOCONF DIALOGEX 0, 0, 541, 233
STYLE DS_SETFONT | DS_FIXEDSYS | WS_POPUP | WS_VISIBLE | WS_CAPTION | WS_SYSMENU | WS_THICKFRAME
EXSTYLE WS_EX_APPWINDOW
CAPTION "Special Event — Videoconference"
FONT 8, "MS Shell Dlg", 0, 0, 0x1
BEGIN
    GROUPBOX        "Details",IDC_STATIC,7,7,301,201
    LTEXT           "Path to image to display:",IDC_STATIC,15,22,80,8
    CONTROL         "",IDC_BROWSE_SPECIAL_EVENT_VIDEOCONF_PATH_IMAGE,
                    "MfcEditBrowse",WS_BORDER | WS_TABSTOP | 0x880,14,32,288,14
    LTEXT           "50%",IDC_STATIC,13,49,16,8
    CONTROL         "",IDC_SLIDER_SPECIAL_EVENT_VIDEOCONF_IMAGE_WIDTH,
                    "msctls_trackbar32",TBS_AUTOTICKS | TBS_BOTH | WS_TABSTOP,8,58,294,15
    LTEXT           "Width of image:",IDC_STATIC,15,77,52,8
    LTEXT           "%",IDC_STATIC_SPECIAL_EVENT_VIDEOCONF_IMAGE_WIDTH,70,77,19,8
    LTEXT           "Text to display before the image:",IDC_STATIC,15,97,108,8
    EDITTEXT        IDC_EDIT_SPECIAL_EVENT_VIDEOCONF_TEXT_BEFORE_IMAGE,14,109,288,37,ES_MULTILINE | ES_AUTOVSCROLL | WS_VSCROLL
    LTEXT           "Text to display after the image:",IDC_STATIC,16,152,102,8
    EDITTEXT        IDC_EDIT_SPECIAL_EVENT_VIDEOCONF_TEXT_AFTER_IMAGE,14,163,288,37,ES_MULTILINE | ES_AUTOVSCROLL | WS_VSCROLL
    DEFPUSHBUTTON   "OK",IDOK,198,212,50,14
    PUSHBUTTON      "Cancel",IDCANCEL,252,212,50,14
    LTEXT           "100%",IDC_STATIC,287,48,20,8
    GROUPBOX        "Image Preview",IDC_STATIC,314,7,224,201
    CONTROL         "",IDC_STATIC_IMAGE_PREVIEW,"Static",SS_OWNERDRAW,321,18,210,181
END

It has a Picture Control on it. I have set it as Owner Draw . The dialog loads the image into a CImage when the user browses for a file:

void CSpecialEventVideoconferenceInfoDlg::OnEnChangeBrowseSpecialEventVideoconfPathImage()
{
    CString strImagePath;
    GetDlgItemText(IDC_BROWSE_SPECIAL_EVENT_VIDEOCONF_PATH_IMAGE, strImagePath);
    m_imgPreview.Load(strImagePath);
}

And, the rendering is doing like this:

void CSpecialEventVideoconferenceInfoDlg::OnDrawItem(int nIDCtl, LPDRAWITEMSTRUCT lpDrawItemStruct)
{
    if (nIDCtl == IDC_STATIC_IMAGE_PREVIEW && !m_imgPreview.IsNull())
    {
        SetStretchBltMode(lpDrawItemStruct->hDC, HALFTONE);
        CRect dest(lpDrawItemStruct->rcItem);
        float nRatioImage = m_imgPreview.GetWidth() / (float)m_imgPreview.GetHeight();
        float nRatioDest = dest.Width() / (float)dest.Height();
        CRect rectDraw = dest;
        if (nRatioImage > nRatioDest)
            rectDraw.SetRect(0, 0, rectDraw.right, rectDraw.right / nRatioImage);
        else if (nRatioImage < nRatioDest)
            rectDraw.SetRect(0, 0, rectDraw.bottom * nRatioImage, rectDraw.bottom);
        m_imgPreview.Draw(lpDrawItemStruct->hDC, rectDraw);
    }
}

I have two problems here that I can't resolve so far:

1. The image is not being drawn centrally to my container:

2. The picture control is not wiping the background correctly if I decide to use another image:

Update

If I adjust the code like this:

void CSpecialEventVideoconferenceInfoDlg::OnDrawItem(int nIDCtl, LPDRAWITEMSTRUCT lpDrawItemStruct)
{
    if (nIDCtl == IDC_STATIC_IMAGE_PREVIEW && !m_imgPreview.IsNull())
    {
        SetStretchBltMode(lpDrawItemStruct->hDC, HALFTONE);
        FillRect(lpDrawItemStruct->hDC, &lpDrawItemStruct->rcItem, (HBRUSH)GetStockObject(WHITE_BRUSH));
        CRect dest(lpDrawItemStruct->rcItem);
        float nRatioImage = m_imgPreview.GetWidth() / (float)m_imgPreview.GetHeight();
        float nRatioDest = dest.Width() / (float)dest.Height();
        CRect rectDraw = dest;
        if (nRatioImage > nRatioDest)
            rectDraw.SetRect(0, 0, rectDraw.right, rectDraw.right / nRatioImage);
        else if (nRatioImage < nRatioDest)
            rectDraw.SetRect(0, 0, rectDraw.bottom * nRatioImage, rectDraw.bottom);
        m_imgPreview.Draw(lpDrawItemStruct->hDC, rectDraw);
    }
}

... it resolves the second issue. It uses FillRect . So it is just changing my code to centralize the picture to solve.

Answer 1

Here is my solution:

void CSpecialEventVideoconferenceInfoDlg::OnDrawItem(int nIDCtl, LPDRAWITEMSTRUCT lpDrawItemStruct)
{
    if (nIDCtl == IDC_STATIC_IMAGE_PREVIEW && !m_imgPreview.IsNull())
    {
        SetStretchBltMode(lpDrawItemStruct->hDC, HALFTONE);
        FillRect(lpDrawItemStruct->hDC, &lpDrawItemStruct->rcItem, (HBRUSH)GetStockObject(WHITE_BRUSH));
        CRect dest(lpDrawItemStruct->rcItem);
        float nRatioImage = m_imgPreview.GetWidth() / (float)m_imgPreview.GetHeight();
        float nRatioDest = dest.Width() / (float)dest.Height();
        CRect rectDraw = dest;
        if (nRatioImage > nRatioCanvas)
            rectDraw.SetRect(0, 0, rectDraw.right, (int)(rectDraw.right / nRatioImage));
        else if (nRatioImage < nRatioCanvas)
            rectDraw.SetRect(0, 0, (int)(rectDraw.bottom * nRatioImage), rectDraw.bottom);

        rectDraw.DeflateRect(5, 5);
        CSize ptOffset = dest.CenterPoint() - rectDraw.CenterPoint();
        rectDraw.OffsetRect(ptOffset);
        FrameRect(lpDrawItemStruct->hDC, &lpDrawItemStruct->rcItem, (HBRUSH)GetStockObject(BLACK_BRUSH));
        m_imgPreview.Draw(lpDrawItemStruct->hDC, rectDraw);
    }
}

1. I use FillRect to wipe the background.
2. I use CenterPoint and OffsetRect to work out the new center.
3. I use FrameRect to add a nice rectangle around the image.