MapWindowPoints function (winuser.h)

Article
11/19/2022

The MapWindowPoints function converts (maps) a set of points from a coordinate space relative to one window to a coordinate space relative to another window.

Syntax

int MapWindowPoints(
  [in]      HWND    hWndFrom,
  [in]      HWND    hWndTo,
  [in, out] LPPOINT lpPoints,
  [in]      UINT    cPoints
);

Parameters

[in] hWndFrom

A handle to the window from which points are converted. If this parameter is NULL or HWND_DESKTOP, the points are presumed to be in screen coordinates.

[in] hWndTo

A handle to the window to which points are converted. If this parameter is NULL or HWND_DESKTOP, the points are converted to screen coordinates.

[in, out] lpPoints

A pointer to an array of POINT structures that contain the set of points to be converted. The points are in device units. This parameter can also point to a RECT structure, in which case the cPoints parameter should be set to 2.

[in] cPoints

The number of POINT structures in the array pointed to by the lpPoints parameter.

Return value

If the function succeeds, the low-order word of the return value is the number of pixels added to the horizontal coordinate of each source point in order to compute the horizontal coordinate of each destination point. (In addition to that, if precisely one of hWndFrom and hWndTo is mirrored, then each resulting horizontal coordinate is multiplied by -1.) The high-order word is the number of pixels added to the vertical coordinate of each source point in order to compute the vertical coordinate of each destination point.

If the function fails, the return value is zero. Call SetLastError prior to calling this method to differentiate an error return value from a legitimate "0" return value.

Remarks

If hWndFrom or hWndTo (or both) are mirrored windows (that is, have WS_EX_LAYOUTRTL extended style) and precisely two points are passed in lpPoints, MapWindowPoints will interpret those two points as a RECT and possibly automatically swap the left and right fields of that rectangle to ensure that left is not greater than right. If any number of points other than 2 is passed in lpPoints, then MapWindowPoints will correctly map the coordinates of each of those points separately, so if you pass in a pointer to an array of more than one rectangle in lpPoints, the new rectangles may get their left field greater than right. Thus, to guarantee the correct transformation of rectangle coordinates, you must call MapWindowPoints with one RECT pointer at a time, as shown in the following example:


   RECT        rc[10];

   for(int i = 0; i < (sizeof(rc)/sizeof(rc[0])); i++)
   {
       MapWindowPoints(hWnd1, hWnd2, (LPPOINT)(&rc[i]), (sizeof(RECT)/sizeof(POINT)) );
   }

Also, if you need to map precisely two independent points and don't want the RECT logic applied to them by MapWindowPoints, to guarantee the correct result you must call MapWindowPoints with one POINT pointer at a time, as shown in the following example:


   POINT pt[2];

   MapWindowPoints(hWnd1, hWnd2, &pt[0], 1);
   MapWindowPoints(hWnd1, hWnd2, &pt[1], 1);

Requirements

Requirement	Value
Minimum supported client	Windows 2000 Professional [desktop apps only]
Minimum supported server	Windows 2000 Server [desktop apps only]
Target Platform	Windows
Header	winuser.h (include Windows.h)
Library	User32.lib
DLL	User32.dll
API set	ext-ms-win-ntuser-window-l1-1-2 (introduced in Windows 10, version 10.0.10240)