Windows GDI: horizontal/vertical DPI

Question

When obtaining the DPI for the screen under Windows (by using ::GetDeviceCaps) will the horizontal value always be the same as the vertical? For example:

HDC dc = ::GetDC(NULL);
const int xDPI = ::GetDeviceCaps(dc, LOGPIXELSX);
const int yDPI - ::GetDeviceCaps(dc, LOGPIXELSY);
assert(xDPI == yDPI);
::ReleaseDC(NULL, dc);

Are these values ever different?

Answer 1

I have never seen them be different, but on this MSDN page I see a comment that suggests that they might be:

   int nHorz = dc.GetDeviceCaps(LOGPIXELSX);
   int nVert = dc.GetDeviceCaps(LOGPIXELSY);

   // almost always the same in both directions, but sometimes not!

Answer 2

I've never seen a case where they're different, but the fact that there are two separate calls for it strongly suggests that they might be sometimes.

Answer 3

It's possible for it to be different, but that generally only applies to printers. It can be safely assumed that the screen will always have identical horizontal and vertical DPIs.