allow to retrieve videocapture properties before first frame reading

modules/highgui/src/cap_images.cpp

@@ -67,10 +67,11 @@ class CvCapture_Images : public CvCapture
 public:
     CvCapture_Images()
     {
-        filename = 0;
+        filename = NULL;
         currentframe = firstframe = 0;
         length = 0;
-        frame = 0;
+        frame = NULL;
+        grabbedInOpen = false;
     }
 
     virtual ~CvCapture_Images()
@@ -92,6 +93,7 @@ protected:
     unsigned length; // length of sequence
 
     IplImage* frame;
+    bool grabbedInOpen;
 };
 
 
@@ -100,7 +102,7 @@ void CvCapture_Images::close()
     if( filename )
     {
         free(filename);
-        filename = 0;
+        filename = NULL;
     }
     currentframe = firstframe = 0;
     length = 0;
@@ -113,6 +115,14 @@ bool CvCapture_Images::grabFrame()
     char str[_MAX_PATH];
     sprintf(str, filename, firstframe + currentframe);
 
+    if (grabbedInOpen)
+    {
+        grabbedInOpen = false;
+        ++currentframe;
+
+        return true;
+    }
+
     cvReleaseImage(&frame);
     frame = cvLoadImage(str, CV_LOAD_IMAGE_ANYDEPTH | CV_LOAD_IMAGE_ANYCOLOR);
     if( frame )
@@ -123,7 +133,7 @@ bool CvCapture_Images::grabFrame()
 
 IplImage* CvCapture_Images::retrieveFrame(int)
 {
-    return frame;
+    return grabbedInOpen ? NULL : frame;
 }
 
 double CvCapture_Images::getProperty(int id)
@@ -166,6 +176,8 @@ bool CvCapture_Images::setProperty(int id, double value)
             value = length - 1;
         }
         currentframe = cvRound(value);
+        if (currentframe != 0)
+            grabbedInOpen = false; // grabbed frame is not valid anymore
         return true;
     case CV_CAP_PROP_POS_AVI_RATIO:
         if(value > 1) {
@@ -176,6 +188,8 @@ bool CvCapture_Images::setProperty(int id, double value)
             value = 0;
         }
         currentframe = cvRound((length - 1) * value);
+        if (currentframe != 0)
+            grabbedInOpen = false; // grabbed frame is not valid anymore
         return true;
     }
     CV_WARN("unknown/unhandled property\n");
@@ -278,6 +292,12 @@ bool CvCapture_Images::open(const char * _filename)
     }
 
     firstframe = offset;
+
+    // grab frame to enable properties retrieval
+    grabFrame();
+    grabbedInOpen = true;
+    currentframe = 0;
+
     return true;
 }
 
@@ -290,7 +310,7 @@ CvCapture* cvCreateFileCapture_Images(const char * filename)
         return capture;
 
     delete capture;
-    return 0;
+    return NULL;
 }
 
 //