allow specifying apiPreference in VideoCapture when opening a file. Add

a separate function instead of an overload not to change the ABI.

rename VideoCapture paramter 'device' to 'index' in CPP to reflect that
it allows specifying the API.
update comments to explain how to specify the API.

modules/videoio/include/opencv2/videoio.hpp

@@ -90,7 +90,10 @@ enum { CAP_ANY          = 0,     // autodetect
        CAP_INTELPERC    = 1500,  // Intel Perceptual Computing SDK
        CAP_OPENNI2      = 1600,  // OpenNI2 (for Kinect)
        CAP_OPENNI2_ASUS = 1610,  // OpenNI2 (for Asus Xtion and Occipital Structure sensors)
-       CAP_GPHOTO2      = 1700   // gPhoto2 connection
+       CAP_GPHOTO2      = 1700,  // gPhoto2 connection
+       CAP_GSTREAMER    = 1800,  // GStreamer
+       CAP_FFMPEG       = 1900,  // FFMPEG
+       CAP_IMAGES       = 2000   // OpenCV Image Sequence (e.g. img_%02d.jpg)
      };
 
 // generic properties (based on DC1394 properties)
@@ -398,10 +401,19 @@ public:
     CV_WRAP VideoCapture(const String& filename);
 
     /** @overload
-    @param device id of the opened video capturing device (i.e. a camera index). If there is a single
-    camera connected, just pass 0.
+    @param filename name of the opened video file (eg. video.avi) or image sequence (eg.
+    img_%02d.jpg, which will read samples like img_00.jpg, img_01.jpg, img_02.jpg, ...)
+
+    @param apiPreference preferred Capture API to use. Can be used to enforce a specific reader
+    implementation if multiple are available: e.g. CAP_FFMPEG or CAP_IMAGES
+    */
+    CV_WRAP VideoCapture(const String& filename, int apiPreference);
+
+    /** @overload
+    @param index = camera_id + domain_offset (CAP_*). id of the video capturing device to open. If there is a single
+    camera connected, just pass 0. Advanced Usage: to open Camera 1 using the MS Media Foundation API: index = 1 + CAP_MSMF
     */
-    CV_WRAP VideoCapture(int device);
+    CV_WRAP VideoCapture(int index);
 
     virtual ~VideoCapture();
 
@@ -415,9 +427,10 @@ public:
     CV_WRAP virtual bool open(const String& filename);
 
     /** @overload
-    @param device id of the opened video capturing device (i.e. a camera index).
+    @param index = camera_id + domain_offset (CAP_*). id of the video capturing device to open. If there is a single
+    camera connected, just pass 0. Advanced Usage: to open Camera 1 using the MS Media Foundation API: index = 1 + CAP_MSMF
     */
-    CV_WRAP virtual bool open(int device);
+    CV_WRAP virtual bool open(int index);
 
     /** @brief Returns true if video capturing has been initialized already.
 
@@ -541,6 +554,18 @@ public:
      */
     CV_WRAP virtual double get(int propId) const;
 
+    /** @overload
+
+    @param filename name of the opened video file (eg. video.avi) or image sequence (eg.
+    img_%02d.jpg, which will read samples like img_00.jpg, img_01.jpg, img_02.jpg, ...)
+
+    @param apiPreference preferred Capture API to use. Can be used to enforce a specific reader
+    implementation if multiple are available: e.g. CAP_FFMPEG or CAP_IMAGES
+
+    The methods first call VideoCapture::release to close the already opened file or camera.
+     */
+    CV_WRAP virtual bool open(const String& filename, int apiPreference);
+
 protected:
     Ptr<CvCapture> cap;
     Ptr<IVideoCapture> icap;

modules/videoio/include/opencv2/videoio/videoio_c.h

@@ -63,6 +63,9 @@ typedef struct CvCapture CvCapture;
 /* start capturing frames from video file */
 CVAPI(CvCapture*) cvCreateFileCapture( const char* filename );
 
+/* start capturing frames from video file. allows specifying a preferred API to use */
+CVAPI(CvCapture*) cvCreateFileCaptureWithPreference( const char* filename , int apiPreference);
+
 enum
 {
     CV_CAP_ANY      =0,     // autodetect
@@ -111,8 +114,10 @@ enum
     CV_CAP_INTELPERC = 1500, // Intel Perceptual Computing
 
     CV_CAP_OPENNI2 = 1600,   // OpenNI2 (for Kinect)
-
-    CV_CAP_GPHOTO2 = 1700
+    CV_CAP_GPHOTO2 = 1700,
+    CV_CAP_GSTREAMER = 1800, // GStreamer
+    CV_CAP_FFMPEG = 1900,    // FFMPEG
+    CV_CAP_IMAGES = 2000     // OpenCV Image Sequence (e.g. img_%02d.jpg)
 };
 
 /* start capturing frames from camera: index = camera_index + domain_offset (CV_CAP_*) */

modules/videoio/src/cap.cpp

@@ -359,56 +359,64 @@ CV_IMPL CvCapture * cvCreateCameraCapture (int index)
  * Videoreader dispatching method: it tries to find the first
  * API that can access a given filename.
  */
-CV_IMPL CvCapture * cvCreateFileCapture (const char * filename)
+CV_IMPL CvCapture * cvCreateFileCaptureWithPreference (const char * filename, int apiPreference)
 {
     CvCapture * result = 0;
 
+    switch(apiPreference) {
+    default:
+    case CV_CAP_FFMPEG:
 #ifdef HAVE_FFMPEG
         if (! result)
             result = cvCreateFileCapture_FFMPEG_proxy (filename);
 #endif
-
+    case CV_CAP_VFW:
 #ifdef HAVE_VFW
         if (! result)
             result = cvCreateFileCapture_VFW (filename);
 #endif
-
+    case CV_CAP_MSMF:
 #ifdef HAVE_MSMF
         if (! result)
             result = cvCreateFileCapture_MSMF (filename);
 #endif
-
 #ifdef HAVE_XINE
         if (! result)
             result = cvCreateFileCapture_XINE (filename);
 #endif
-
+    case CV_CAP_GSTREAMER:
 #ifdef HAVE_GSTREAMER
         if (! result)
             result = cvCreateCapture_GStreamer (CV_CAP_GSTREAMER_FILE, filename);
 #endif
-
+    case CV_CAP_QT:
 #if defined(HAVE_QUICKTIME) || defined(HAVE_QTKIT)
         if (! result)
             result = cvCreateFileCapture_QT (filename);
 #endif
-
+    case CV_CAP_AVFOUNDATION:
 #ifdef HAVE_AVFOUNDATION
         if (! result)
             result = cvCreateFileCapture_AVFoundation (filename);
 #endif
-
+    case CV_CAP_OPENNI:
 #ifdef HAVE_OPENNI
         if (! result)
             result = cvCreateFileCapture_OpenNI (filename);
 #endif
-
+    case CV_CAP_IMAGES:
         if (! result)
             result = cvCreateFileCapture_Images (filename);
+    }
 
     return result;
 }
 
+CV_IMPL CvCapture * cvCreateFileCapture (const char * filename)
+{
+    return cvCreateFileCaptureWithPreference(filename, CV_CAP_ANY);
+}
+
 /**
  * Videowriter dispatching method: it tries to find the first
  * API that can write a given stream.
@@ -615,14 +623,19 @@ static Ptr<IVideoWriter> IVideoWriter_create(const String& filename, int _fourcc
 VideoCapture::VideoCapture()
 {}
 
+VideoCapture::VideoCapture(const String& filename, int apiPreference)
+{
+    open(filename, apiPreference);
+}
+
 VideoCapture::VideoCapture(const String& filename)
 {
-    open(filename);
+    open(filename, CAP_ANY);
 }
 
-VideoCapture::VideoCapture(int device)
+VideoCapture::VideoCapture(int index)
 {
-    open(device);
+    open(index);
 }
 
 VideoCapture::~VideoCapture()
@@ -631,24 +644,29 @@ VideoCapture::~VideoCapture()
     cap.release();
 }
 
-bool VideoCapture::open(const String& filename)
+bool VideoCapture::open(const String& filename, int apiPreference)
 {
     if (isOpened()) release();
     icap = IVideoCapture_create(filename);
     if (!icap.empty())
         return true;
 
-    cap.reset(cvCreateFileCapture(filename.c_str()));
+    cap.reset(cvCreateFileCaptureWithPreference(filename.c_str(), apiPreference));
     return isOpened();
 }
 
-bool VideoCapture::open(int device)
+bool VideoCapture::open(const String& filename)
+{
+    return open(filename, CAP_ANY);
+}
+
+bool VideoCapture::open(int index)
 {
     if (isOpened()) release();
-    icap = IVideoCapture_create(device);
+    icap = IVideoCapture_create(index);
     if (!icap.empty())
         return true;
-    cap.reset(cvCreateCameraCapture(device));
+    cap.reset(cvCreateCameraCapture(index));
     return isOpened();
 }