GDI+ only draws monochrome on memory DC

Question

I'm trying to do some double buffering in an MFC application and trying to draw on the memory DC with GDI+. However, although I called CreateCompatibleDC(), I'm only getting a monochrome image. Here is the code:

CDC bufferDC;
CBitmap bufferBitmap;
bufferDC.CreateCompatibleDC(&dc);
bufferBitmap.CreateCompatibleBitmap(&bufferDC, 300, 300);
bufferDC.SelectObject(bufferBitmap);
Graphics g(bufferDC);
g.Clear(Color::Green);
dc.BitBlt(0, 0, 300, 300, &bufferDC, 0, 0, SRCCOPY);

Instead of a green patch, I see a rectangle of dithered black and white dots. I even tried to save the bitmap to disk after the g.Clear() call. It is indeed a 1-bit depth file.

Any ideas what went wrong? Thanks.

Answer 1

A common mistake. A memory DC takes on the properties of the bitmap selected into it, no matter what compatibility it was created with. The default bitmap selected into a DC is monochrome. If you create a bitmap compatible with that DC, it will be monochrome too.

Create the bitmap to be compatible with the original DC, not the memory DC.

Answer 2

Both the bitnmap and the bufferDC should be compatible with dc (whatever device it refers to), not the bitmap compatible ... with its own DC.

Try to give &dc to CreateCopmpatibleBitmap.