定义剪辑区域

当用户单击“定义剪辑区域”时，系统会发出 WM_COMMAND 消息。 此消息的 wParam 参数包含应用程序定义的常量（IDM_DEFINE），该常量指示用户从菜单中选择了此选项。 应用程序通过设置布尔标志 fDefineRegion 来处理此输入，如以下代码示例所示。

case WM_COMMAND: 
    switch (wParam) 
    { 
 
        case IDM_DEFINE: 
            fDefineRegion = TRUE; 
            break; 

单击“ 定义剪辑区域 ”后，用户可以通过在光标位于应用程序的工作区时单击并拖动鼠标来开始绘制矩形。

当用户按下左键时，系统会发出 WM_LBUTTONDOWN 消息。 此消息的 lParam 参数包含光标坐标，这些坐标对应于用于定义剪裁区域的矩形的左上角。 应用程序处理 WM_LBUTTONDOWN 消息，如下所示。

// 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; 
}

当用户拖动鼠标时，系统会发出 WM_MOUSEMOVE 消息，并将新的光标坐标存储在 lParam 参数中。 每次应用程序收到新的 WM_MOUSEMOVE 消息时，都会清除上一个矩形 (如果存在) ，并通过调用 Polyline 函数，向其传递矩形四角的坐标来绘制新矩形。 应用程序执行以下任务。

// 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;