Transforming logical points to device points:Coordinate functions convert client coordinates to screen coordinates (or vice versa). These functions are useful in graphics-intensive applications. Following are the coordinate functions:
| Function | Description |
| DPtoLP | Converts device points (that is, points relative to the window origin) into logical points. |
| GetCurrentPosition | Retrieves the current position, in logical coordinates. |
| GetCurrentPositionEx | Retrieves position in logical units. |
| LPtoDP | Converts logical points into device points. |
GDI uses the following equations to transform logical points to device points and device points to logical points:
Transforming logical points to device points:
Dx = (Lx – xWO) * xVE/xWE + xVO
Dy = (Ly – yWO) * yVE/yWE + yVO
Transforming device points to logical points:
Lx = (Dx – xVO) * xWE/xVE + xWO
Ly = (Dy – yVO) * yWE/yVE + yWO
Following are descriptions of the variables used in these transformation equations:
| Variable | Description |
| xWO | Window origin x-coordinate |
| yWO | Window origin y-coordinate |
| xWE | Window extent x-coordinate |
| yWE | Window extent y-coordinate |
| xVO | Viewport origin x-coordinate |
| yVO | Viewport origin y-coordinate |
| xVE | Viewport extent x-coordinate |
| yVE | Viewport extent y-coordinate |
| Lx | Logical-coordinate system x-coordinate |
| Ly | Logical-coordinate system y-coordinate |
| Dx | Device x-coordinate |
| Dy | Device y-coordinate |
The following four ratios are scaling factors used to determine the necessary stretching or compressing of logical units: xVE/xWE, yVE/yWE, xWE/xVE, and yWE/yVE.
The subtraction and addition of viewport and window origins is referred to as the translational component of the equation.
In addition, applications can use the following functions from the USER API to convert coordinates from one system to another:
| Function | Description |
| ChildWindowFromPoint | Determines which, if any, of the child windows belonging to a given parent window contains a specified point. |
| ClientToScreen | Converts the client coordinates of a given point on the display to screen coordinates. |
| ScreenToClient | Converts the screen coordinates of a given point on the display to client coordinates. |
| WindowFromPoint | Retrieves the handle of the window that contains a given point. |
For more information about these USER functions, see Chapter 1, “Window Management.”