highgui: add property WND_PROP_VISIBLE

checks whether the window exists and is visible. On QT closing a window
merley hides it, so the common hack for checking whether a window exists

exists = cv2.getWindowProperty(.., 0) >= 0

does not work.

modules/highgui/include/opencv2/highgui.hpp

@@ -196,7 +196,8 @@ enum WindowPropertyFlags {
        WND_PROP_FULLSCREEN   = 0, //!< fullscreen property    (can be WINDOW_NORMAL or WINDOW_FULLSCREEN).
        WND_PROP_AUTOSIZE     = 1, //!< autosize property      (can be WINDOW_NORMAL or WINDOW_AUTOSIZE).
        WND_PROP_ASPECT_RATIO = 2, //!< window's aspect ration (can be set to WINDOW_FREERATIO or WINDOW_KEEPRATIO).
-       WND_PROP_OPENGL       = 3  //!< opengl support.
+       WND_PROP_OPENGL       = 3, //!< opengl support.
+       WND_PROP_VISIBLE      = 4  //!< checks whether the window exists and is visible
      };
 
 //! Mouse Events see cv::MouseCallback

modules/highgui/include/opencv2/highgui/highgui_c.h

@@ -111,6 +111,7 @@ enum
     CV_WND_PROP_AUTOSIZE   = 1, //to change/get window's autosize property
     CV_WND_PROP_ASPECTRATIO= 2, //to change/get window's aspectratio property
     CV_WND_PROP_OPENGL     = 3, //to change/get window's opengl support
+    CV_WND_PROP_VISIBLE    = 4,
 
     //These 2 flags are used by cvNamedWindow and cvSet/GetWindowProperty
     CV_WINDOW_NORMAL       = 0x00000000, //the user can resize the window (no constraint)  / also use to switch a fullscreen window to a normal size

modules/highgui/src/precomp.hpp

@@ -123,6 +123,7 @@ double cvGetRatioWindow_QT(const char* name);
 void cvSetRatioWindow_QT(const char* name,double prop_value);
 
 double cvGetOpenGlProp_QT(const char* name);
+double cvGetPropVisible_QT(const char* name);
 #endif
 
 #endif /* __HIGHGUI_H_ */

modules/highgui/src/window.cpp

@@ -153,6 +153,14 @@ CV_IMPL double cvGetWindowProperty(const char* name, int prop_id)
         #endif
     break;
 
+    case CV_WND_PROP_VISIBLE:
+        #if defined (HAVE_QT)
+            return cvGetPropVisible_QT(name);
+        #else
+            return -1;
+        #endif
+    break;
+
     default:
         return -1;
     }

modules/highgui/src/window_QT.cpp

@@ -138,6 +138,20 @@ double cvGetRatioWindow_QT(const char* name)
     return result;
 }
 
+double cvGetPropVisible_QT(const char* name) {
+    if (!guiMainThread)
+        CV_Error( CV_StsNullPtr, "NULL guiReceiver (please create a window)" );
+
+    double result = 0;
+
+    QMetaObject::invokeMethod(guiMainThread,
+        "getWindowVisible",
+        autoBlockingConnection(),
+        Q_RETURN_ARG(double, result),
+        Q_ARG(QString, QString(name)));
+
+    return result;
+}
 
 void cvSetRatioWindow_QT(const char* name,double prop_value)
 {
@@ -903,6 +917,16 @@ double GuiReceiver::getPropWindow(QString name)
     return (double) w->getPropWindow();
 }
 
+double GuiReceiver::getWindowVisible(QString name)
+{
+    QPointer<CvWindow> w = icvFindWindowByName(name);
+
+    if (!w)
+        return 0;
+
+    return (double) w->isVisible();
+}
+
 
 void GuiReceiver::setPropWindow(QString name, double arg2)
 {

modules/highgui/src/window_QT.h

@@ -132,6 +132,7 @@ public slots:
     double getPropWindow(QString name);
     void setPropWindow(QString name, double flags );
     void setWindowTitle(QString name, QString title);
+    double getWindowVisible(QString name);
     double getRatioWindow(QString name);
     void setRatioWindow(QString name, double arg2 );
     void saveWindowParameters(QString name);