Defining the Clipping Region

When the user selects the Define Clip Region option, Windows issues a WM_COMMAND message. The wParam parameter of this message contains an application-defined constant, IDM_DEFINE, that indicates that the user selected this option from the menu. The application processes this input by setting a Boolean flag, fDefineRegion, as shown in the following code sample.

case WM_COMMAND:

switch (wParam) {

case IDM_DEFINE:

fDefineRegion = TRUE;

break;

After selecting the Define Clipping Region option, the user can begin drawing the rectangle by clicking and dragging the mouse while the cursor is in the application's client area.

When the user presses the left button, Windows issues a WM_LBUTTONDOWN message. The lParam parameter of this message contains the cursor coordinates, which correspond to the upper left corner of a rectangle used to define the clipping region. The application processes the WM_LBUTTONDOWN message, as follows.

/* These variables are required for clipping. */

static POINT ptUpperLeft;

static POINT ptLowerRight;

static POINT aptRect[5];

static POINT ptTmp;

static POINTS ptsTmp;

static BOOL fDefineRegion;

static BOOL fRegionExists;

static HRGN hrgn;

static RECT rctTmp;

int i;

switch (message) {

case WM_LBUTTONDOWN:

if (fDefineRegion) {

/* Retrieve the new upper left corner. */

ptsTmp = MAKEPOINTS(lParam);

ptUpperLeft.x = (LONG) ptsTmp.x;

ptUpperLeft.y = (LONG) ptsTmp.y;

}

if (fRegionExists) {

/* Erase the previous rectangle. */

hdc = GetDC(hwnd);

SetROP2(hdc, R2_NOTXORPEN);

if (!Polyline(hdc, (CONST POINT *) aptRect, 5))

errhandler("Polyline Failed", hwnd);

ReleaseDC(hwnd, hdc);

/* Clear the rectangle coordinates. */

for (i = 0; i < 4; i++) {

aptRect[i].x = 0;

aptRect[i].y = 0;

}

/* Clear the temporary point structure. */

ptTmp.x = 0;

ptTmp.y = 0;

/* Clear the lower right coordinates. */

ptLowerRight.x = 0;

ptLowerRight.y = 0;

/* Reset the flag. */

fRegionExists = FALSE;

fDefineRegion = TRUE;

/* Retrieve the new upper left corner. */

ptsTmp = MAKEPOINTS(lParam);

ptUpperLeft.x = (LONG) ptsTmp.x;

ptUpperLeft.y = (LONG) ptsTmp.y;

}

break;

As the user drags the mouse, Windows issues WM_MOUSEMOVE messages and stores the new cursor coordinates in the lParam parameter. Each time the application receives a new WM_MOUSEMOVE message, it erases the previous rectangle (if one exists) and draws the new rectangle by calling the Polyline function, passing it the coordinates of the four corners of the rectangle. The application performs the following tasks.

/* These variables are required for clipping. */

static POINT ptUpperLeft;

static POINT ptLowerRight;

static POINT aptRect[5];

static POINT ptTmp;

static POINTS ptsTmp;

static BOOL fDefineRegion;

static BOOL fRegionExists;

static HRGN hrgn;

static RECT rctTmp;

int i;

switch (message) {

case WM_MOUSEMOVE:

if (wParam & MK_LBUTTON && fDefineRegion) {

/* Get a window DC. */

hdc = GetDC(hwnd);

if (!SetROP2(hdc, R2_NOTXORPEN))

errhandler("SetROP2 Failed", hwnd);

/*

* If previous mouse movement occurred, store the original

* lower right corner coordinates in a temporary structure.

*/

if (ptLowerRight.x) {

ptTmp.x = ptLowerRight.x;

ptTmp.y = ptLowerRight.y;

}

/*

* Get the new coordinates of the clipping region's lower

* right corner.

*/

ptsTmp = MAKEPOINTS(lParam);

ptLowerRight.x = (LONG) ptsTmp.x;

ptLowerRight.y = (LONG) ptsTmp.y;

/*

* If previous mouse movement occurred, erase the original

* rectangle.

*/

if (ptTmp.x) {

aptRect[0].x = ptUpperLeft.x;

aptRect[0].y = ptUpperLeft.y;

aptRect[1].x = ptTmp.x;

aptRect[1].y = ptUpperLeft.y;

aptRect[2].x = ptTmp.x;

aptRect[2].y = ptTmp.y;

aptRect[3].x = ptUpperLeft.x;

aptRect[3].y = ptTmp.y;

aptRect[4].x = aptRect[0].x;

aptRect[4].y = aptRect[0].y;

if (!Polyline(hdc, (CONST POINT *) aptRect, 5))

errhandler("Polyline Failed", hwnd);

}

aptRect[0].x = ptUpperLeft.x;

aptRect[0].y = ptUpperLeft.y;

aptRect[1].x = ptLowerRight.x;

aptRect[1].y = ptUpperLeft.y;

aptRect[2].x = ptLowerRight.x;

aptRect[2].y = ptLowerRight.y;

aptRect[3].x = ptUpperLeft.x;

aptRect[3].y = ptLowerRight.y;

aptRect[4].x = aptRect[0].x;

aptRect[4].y = aptRect[0].y;

if (!Polyline(hdc, (CONST POINT *) aptRect, 5))

errhandler("Polyline Failed", hwnd);

ReleaseDC(hwnd, hdc);

}

break;