How to draw text using CDC with transparent background on CBitmap? - c++

Question

I have the following code which sort of works provided you mask out the pink pixels however what I actually want is transparent bits like a PNG file so that I can avoid alpha blending issues and the need mask out a specific color everywhere the bitmap will be used.
CClientDC dc(pWnd);
CDC memDC;
if(!memDC.CreateCompatibleDC(&dc))
return NULL;
CRect bitmapRect;
bitmapRect.SetRectEmpty();
CFont* pOldFont = memDC.SelectObject(pWnd->GetFont());
CSize fontSize = memDC.GetTextExtent(imageText);
bitmapRect.right = fontSize.cx;
bitmapRect.bottom = fontSize.cy;
CBitmap bitmap;
if(!bitmap.CreateCompatibleBitmap(&dc, bitmapRect.Width(), bitmapRect.Height()))
return NULL;
CBitmap* pOldMemDCBitmap = memDC.SelectObject(&bitmap);
memDC.FillSolidRect(&bitmapRect, RGB(255,0,255));
//memDC.SetBkMode(TRANSPARENT); // doesn't work
//memDC.SetBkColor(TRANSPARENT); // doesn't work
memDC.SetTextColor(GetSysColor(COLOR_WINDOWTEXT));
//memDC.DrawText(imageText, bitmapRect, DT_TOP|DT_LEFT|DT_NOCLIP); // same difference
memDC.TextOut(0, 0, imageText);
memDC.SelectObject(pOldMemDCBitmap);
memDC.SelectObject(pOldFont);
memDC.DeleteDC();
CImageList bmImage;
bmImage.Create(bitmapRect.Width(), bitmapRect.Height(), ILC_COLOR32|ILC_MASK, 0, 1);
// this masks out the pink but for some windows blends edges of text causing pink text instead of black!
bmImage.Add(&bitmap, RGB(255,0,255));
Is just the bug filled beast that is MFC misbehaving or am I missing something?

Answer 1

Simple DrawText() with transparent background without MFC:
// in my case a user drawn button:
_windowHandle = CreateWindowEx(...);
SendMessage(_windowHandle, WM_SETFONT, (WPARAM)font, (LPARAM)NULL);
...
// WM_DRAWITEM
SetTextColor(hDC, RGB(216, 27, 27));
SetBkMode(hDC, TRANSPARENT);
RECT rect = { 0, 0, backgroundBitmap.bmWidth, backgroundBitmap.bmHeight };
DrawText(hDC, _text.c_str(), -1, &rect, DT_CENTER | DT_WORDBREAK);
--hfrmobile

Answer 2

About 10 minutes after asking this I read my own comment and realized that "some windows" means it was related to the windows being passed in. Specifically the font being used from said window. Fonts with default properties were exhibiting the strange blending.
At the end of the day I determined I needed to modify the font to turn off the things messing up my drawing code. I eventually narrowed it down to the one culprit causing the problem:
CClientDC dc(pWnd);
CDC memDC;
if(!memDC.CreateCompatibleDC(&dc))
return NULL;
LOGFONT tempFont;
CFont* winFont = pWnd->GetFont();
if (winFont)
winFont->GetLogFont(&tempFont);
else
{
// generate a likely font
SecureZeroMemory(&tempFont, sizeof(LOGFONT));
//TEXTMETRIC txt;
//GetTextMetrics(memDC, &txt);
//tempFont.lfHeight = txt.tmHeight * -1; // 16 is too big looking
tempFont.lfHeight = -12;
tempFont.lfWeight = FW_NORMAL;
tempFont.lfCharSet = DEFAULT_CHARSET;
wcscpy_s(tempFont.lfFaceName, L"Segoe UI"); // Win7 control default
}
tempFont.lfQuality = NONANTIALIASED_QUALITY; // this is the fix!!!
CFont newFont;
newFont.CreateFontIndirect(&tempFont);
CFont* pOldFont = memDC.SelectObject(&newFont);
// ... other stuff same as before ...
So I still FillSolidRect pink then draw my icons, text, whatever I want, etc. Then mask out the pink pixels. With the font quality adjustment it no longer blends pink into the font text and it looks good. The else case above creates a default font to use in case the CWnd* passed in doesn't have a valid one specified.