Q:

Transparent bounding box of an ellipse

I'm writing some C++ code to draw ellipses. Sometimes these ellipses could be stacked on top of each other in a grouping. When they are stacked on each other, I'd like to have the bounding box of the ellipse be transparent so that I don't see the white corners of the bounding box. See picture below.

Including the SetBkMode or not doesn't seem to make a difference. If I don't do the FillRect, I get a black background on the bounding box.

HBRUSH brush = CreateSolidBrush(RGB(255, 255, 255));
CDC *pDC = CDC::FromHandle(pSprite->hDCMem);
pDC->SetBkMode(TRANSPARENT);
pDC->FillRect(m_SpriteRect, CBrush::FromHandle(brush));
pDC->SelectObject(m_BackBrush);
pDC->Ellipse(m_SpriteRect);
pDC->SetBkMode(OPAQUE);

DeleteObject(brush);

Is there a way to set a transparent background?

A:

If drawing on memory dc, fill the background with a transparent color, then use TransparentBlt to blit the memory dc on to final HDC. Example:
CDC *pDC = CDC::FromHandle(hDCMem);

//fill the background with transparent color
COLORREF clr_transparent = RGB(255, 255, 255); //<- randomly selected color
CBrush brush(clr_transparent);
pDC->FillRect(m_SpriteRect, &brush);

//any drawing
auto oldbrush = pDC->SelectObject(m_BackBrush);
pDC->Ellipse(m_SpriteRect);
pDC->SelectObject(oldbrush);

//transparent blit
TransparentBlt(final_hdc, x_dest, y_dest, width, height, 
    hDCMem, 0, 0, m_SpriteRect.right, m_SpriteRect.bottom, clr_transparent);

