Why is my stretchblt images losing colours? - visual-c++

Question

I am having some difficulties in correctly populating a CListCtrl with thumbnails of monitor displays.
On the right of my CDialog I have a static control and I render the image on a white canvas like this:
void CCenterCursorOnScreenDlg::OnDrawItem(int nIDCtl, LPDRAWITEMSTRUCT lpDrawItemStruct)
{
if (nIDCtl == IDC_STATIC_MONITOR && !m_imgPreview.IsNull())
{
// Set the mode
SetStretchBltMode(lpDrawItemStruct->hDC, HALFTONE);
// Wipe the canvas
FillRect(lpDrawItemStruct->hDC, &lpDrawItemStruct->rcItem, static_cast<HBRUSH>(GetStockObject(WHITE_BRUSH)));
// Get canvas rectangle
const CRect rectCanvas(lpDrawItemStruct->rcItem);
// Calculate ratio factors
const float nRatioImage = m_imgPreview.GetWidth() / static_cast<float>(m_imgPreview.GetHeight());
const float nRatioCanvas = rectCanvas.Width() / static_cast<float>(rectCanvas.Height());
// Calculate new rectangle size
// Account for portrait images (negative values)
CRect rectDraw = rectCanvas;
if (nRatioImage > nRatioCanvas)
rectDraw.SetRect(0, 0, rectDraw.right, static_cast<int>(rectDraw.right / nRatioImage));
else if (nRatioImage < nRatioCanvas)
rectDraw.SetRect(0, 0, static_cast<int>((rectDraw.bottom * nRatioImage)), rectDraw.bottom);
// Add a margin
rectDraw.DeflateRect(5, 5);
// Move to center
const CSize ptOffset = rectCanvas.CenterPoint() - rectDraw.CenterPoint();
rectDraw.OffsetRect(ptOffset);
// Add a black frame
FrameRect(lpDrawItemStruct->hDC, &lpDrawItemStruct->rcItem, static_cast<HBRUSH>(GetStockObject(BLACK_BRUSH)));
// Draw
m_imgPreview.Draw(lpDrawItemStruct->hDC, rectDraw);
return;
}
CDialogEx::OnDrawItem(nIDCtl, lpDrawItemStruct);
}
The above works beautifully:
But I have problems with the CListCtrl versions of the images. For instance, I am losing the colouring as you can see.
My CImageList is created like this:
m_ImageListThumb.Create(THUMBNAIL_WIDTH, THUMBNAIL_HEIGHT, ILC_COLOR32, 0, 1);
m_ListThumbnail.SetImageList(&m_ImageListThumb, LVSIL_NORMAL);
I then create all the thumbnails by calling DrawThumbnails() in OnInitDialog:
void CCenterCursorOnScreenDlg::DrawThumbnails()
{
int monitorIndex = 0;
m_ListThumbnail.SetRedraw(FALSE);
for (auto& strMonitor : m_monitors.strMonitorNames)
{
CImage img;
CreateMonitorThumbnail(monitorIndex, img, true);
CBitmap* pImage = new CBitmap();
pImage->Attach((HBITMAP)img);
m_ImageListThumb.Add(pImage, nullptr);
CString strMonitorDesc = m_monitors.strMonitorNames.at(monitorIndex);
strMonitorDesc.AppendFormat(L" (Screen %d)", monitorIndex + 1);
m_ListThumbnail.InsertItem(monitorIndex, strMonitorDesc, monitorIndex);
monitorIndex++;
delete pImage;
}
m_ListThumbnail.SetRedraw(TRUE);
}
The CreateMonitorThumbnail function:
BOOL CCenterCursorOnScreenDlg::CreateMonitorThumbnail(const int iMonitorIndex, CImage &rImage, bool bSmall)
{
const CRect rcCapture = m_monitors.rcMonitors.at(iMonitorIndex);
// destroy the currently contained bitmap to create a new one
rImage.Destroy();
auto nWidth = rcCapture.Width();
auto nHeight = rcCapture.Height();
if (bSmall)
{
nWidth = THUMBNAIL_WIDTH;
nHeight = THUMBNAIL_HEIGHT;
}
// create bitmap and attach it to this object
if (!rImage.Create(nWidth, nHeight, 32, 0))
{
AfxMessageBox(L"Cannot create image!", MB_ICONERROR);
return FALSE;
}
// create virtual screen DC
CDC dcScreen;
dcScreen.CreateDC(_T("DISPLAY"), nullptr, nullptr, nullptr);
// copy the contents from the virtual screen DC
BOOL bRet = FALSE;
if (bSmall)
{
CRect rt(0, 0, nWidth, nHeight);
//::FillRect(rImage.GetDC(), rt, static_cast<HBRUSH>(GetStockObject(WHITE_BRUSH)));
bRet = ::StretchBlt(rImage.GetDC(), 0, 0,
nWidth,
nHeight,
dcScreen.m_hDC,
rcCapture.left,
rcCapture.top,
rcCapture.Width(),
rcCapture.Height(), SRCCOPY | CAPTUREBLT);
}
else
{
bRet = ::BitBlt(rImage.GetDC(), 0, 0,
rcCapture.Width(),
rcCapture.Height(),
dcScreen.m_hDC,
rcCapture.left,
rcCapture.top, SRCCOPY | CAPTUREBLT);
}
// do cleanup and return
dcScreen.DeleteDC();
rImage.ReleaseDC();
return bRet;
}
Ideally I want to have exactly the same kind of visual image as on the right, but obviously resized down. How do I fix this?
I simplified the converting from CImage to CBitmap but it made no difference:
void CCenterCursorOnScreenDlg::DrawThumbnails()
{
int monitorIndex = 0;
// Stop redrawing the CListCtrl
m_ListThumbnail.SetRedraw(FALSE);
// Loop monitor info
for (auto& strMonitor : m_monitors.strMonitorNames)
{
// Create the thumbnail image
CImage monitorThumbnail;
CreateMonitorThumbnail(monitorIndex, monitorThumbnail, true);
// Convert it to a CBitmap
CBitmap* pMonitorThumbnailBitmap = CBitmap::FromHandle(monitorThumbnail);
// Add the CBitmap to the CImageList
m_ImageListThumb.Add(pMonitorThumbnailBitmap, nullptr);
// Build the caption description
CString strMonitorDesc = m_monitors.strMonitorNames.at(monitorIndex);
strMonitorDesc.AppendFormat(L" (Screen %d)", monitorIndex + 1);
// Add the item to the CListCtrl
m_ListThumbnail.InsertItem(monitorIndex, strMonitorDesc, monitorIndex);
monitorIndex++;
}
// Start redrawiung the CListCtrl again
m_ListThumbnail.SetRedraw(TRUE);
}
If I change my code to pass false for the last parameter, so that it uses the original captured images without scaling down:
The colours are god there, so it is when I do:
if (bSmall)
{
CRect rt(0, 0, nWidth, nHeight);
//::FillRect(rImage.GetDC(), rt, static_cast<HBRUSH>(GetStockObject(WHITE_BRUSH)));
bRet = ::StretchBlt(rImage.GetDC(), 0, 0,
nWidth,
nHeight,
dcScreen.m_hDC,
rcCapture.left,
rcCapture.top,
rcCapture.Width(),
rcCapture.Height(), SRCCOPY | CAPTUREBLT);
}
that it messes up.

Answer 1

My issue was did not have anything to do with OnDrawItem. I simply included that to indicate how the image on the right was being rendered. I thought it may helped as background information. But it has probably confused the question and I may take it out in the long run!
Based on the comments I was reminded about SetStretchBltMode which was missing from CreateMonitorThumbnail. So, I now have this function:
BOOL CCenterCursorOnScreenDlg::CreateMonitorThumbnail(const int iMonitorIndex, CImage &rImage, bool bResizeAsThumbnail)
{
const CRect rcCapture = m_monitors.rcMonitors.at(iMonitorIndex);
// Destroy the currently contained bitmap to create a new one
rImage.Destroy();
// Massage the dimensions as we want a thumbnail
auto nWidth = rcCapture.Width();
auto nHeight = rcCapture.Height();
if (bResizeAsThumbnail)
{
nWidth = m_iThumbnailWidth;
auto dRatio = rcCapture.Width() / nWidth;
//nHeight = m_iThumbnailHeight;
nHeight = static_cast<int>(rcCapture.Height() / dRatio);
if (nHeight > m_iThumbnailHeight)
{
AfxMessageBox(L"Need to investigate!");
}
}
// Create bitmap and attach it to this object
if (!rImage.Create(nWidth, nHeight, 32, 0))
{
AfxMessageBox(L"Cannot create image!", MB_ICONERROR);
return FALSE;
}
// Create virtual screen DC
CDC dcScreen;
dcScreen.CreateDC(L"DISPLAY", nullptr, nullptr, nullptr);
// Copy (or resize) the contents from the virtual screen DC
BOOL bRet = FALSE;
auto dcImage = rImage.GetDC();
if (bResizeAsThumbnail)
{
// Set the mode first!
SetStretchBltMode(dcImage, COLORONCOLOR);
CPen penBlack;
penBlack.CreatePen(PS_SOLID, 3, RGB(0, 0, 0));
::Rectangle(dcImage, 0, 0, m_iThumbnailWidth, m_iThumbnailHeight);
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);
}
else
{
// Copy
bRet = ::BitBlt(dcImage, 0, 0,
rcCapture.Width(),
rcCapture.Height(),
dcScreen.m_hDC,
rcCapture.left,
rcCapture.top, SRCCOPY | CAPTUREBLT);
}
// Do cleanup and return
dcScreen.DeleteDC();
rImage.ReleaseDC();
return bRet;
}
That was the key to getting the thumbnail showing with the right colours: