Get Ready to Unleash the Power of UCanCode .NET

Coordinate mapping.

For those who prefer to think in terms of formulae, here is how device coordinates (Dx and Dy) are derived from logical coordinates (Lx and Ly) and vice versa, using the window and viewport origin (xWo and yWO, xVO and yVO), and window and viewport extent (xWE and yWE, xVE and yVE) values:

Dx = (Lx Ð xWO) * xVE/xWE + xVO

Dy = (Ly Ð yWO) * yVE/yWE + yVO

Lx = (Dx Ð xVO) * xWE/xVE + xWO

Ly = (Dy Ð yVO) * yWE/yVE + yWO

The CDC class does not directly support world coordinate transformations that are available in Windows NT. To use world coordinate transforms, applications may need to call the Windows function SetWorldTransform directly.

The CDC class provides a set of coordinate transformation functions that can be used to obtain logical coordinates from physical coordinates or vice versa. These functions are DPtoLP and LPtoDP; both of these functions have several overloaded versions that enable them to be used on points, rectangles, and SIZE objects, or MFC classes that encapsulate these objects (CPoint, CRect, CSize).

Additional transformation functions include DPtoHIMETRIC, HIMETRICtoDP, LPtoHIMETRIC, and HIMETRICtoLP. These functions are particularly useful for OLE applications. OLE objects are usually measured in HIMETRIC units; these functions provide a direct means of transforming those units directly into physical or logical coordinates or vice versa.

Coordinate mapping is used extensively in views (that is, classes derived from CView). In these classes, the member function OnPrepareDC is used to set up coordinate mapping that appropriately reflects the view and its current state. For example, in scroll views, OnPrepareDC is used by the framework to displace the window and/or viewport origin to reflect the amount by which the view client area is scrolled. Applications that wish to implement features such as zooming can do so, for example, by overriding CView::OnPrepareDC and changing the window or viewport extent.