Question

I’ve been struggling with a clipping problem with a window with non-zero window origin because I didn’t realize SelectClipRgn required device units.

So given that I have determined the viewport origin and the window origin as follows…

POINT VP_Org, WN_Org;
GetViewportOrgEx (dc, VP_Org);
GerWindowOrgEx (dc, WM_Org);

and I have a clipping region R in logical coordinates, how does one convert R to device units for use in defining a clipping region?

In other words, how do I transform R so that the following works.

RECT R;
ClipRgn = CreateRectRgn (R.left, R.top, R.right, R.bottom)
SelectClipRgn (dc, ClipRgn)

I know that it looks something like

R.top = R.top - WN_Org.y +- VP_Org.y;

from experiment but don’t know what sign to use for the viewport.

Answer 1

You can use the LPtoDP function:

The LPtoDP function converts logical coordinates into device
coordinates. The conversion depends on the mapping mode of the device
context, the settings of the origins and extents for the window and
viewport, and the world transformation.