Using CreateCompatibleDC with mapping modes other than MM_TEXT

Question

I have a visual C++ application that uses a CView derived class to render its display, which is primarily 3d vector data and true type text. The mapping mode used is either MM_ANISOTROPIC or MM_LOMETRIC. I can't use MM_TEXT as I use the same code for printing and plotting of the data and also have to overcome non-square screen pixel issues.

The drawing code currently draws directly onto the screen using the CViews OnDraw method and the CDC object provided. I am trying to replace this with drawing to a bitmap and blitting the bitmap to screen, using a CreateCompatibleDC / CreateCompatibleBitmap combination, as described in the MS documentation and elsewhere.

The problem is that the DCs are not compatible for mapping modes other than MM_TEXT, such that my view is rendered upside down and at the wrong scale. Investigation shows the following;

void CMyView::OnDraw(CDC *pDC)
{
    CDC MyDC = CreateCompatibleDC(pDC); // Create a new memory DC;
    int a = pDC->GetMapMode(),b = MyDC.GetMapMode();
'
'
'
}

a = 2
b = 1

Calling a SetMapMode on MyDC causes the display to be drawn entirely in black. Do I have to rewrite my code to suit MM_TEXT for drawing to a bitmap, or is there another way to overcome this problem.

Answer 1

You probably need to also call SetWindowExt and SetViewportExt. I have definitely used MM_ISOTROPIC with bitmap DCs before and it worked OK (don't have the code to hand as it was since ported to GDI+)