VideoCapture: add exception error mode similar to fstream + test

modules/videoio/include/opencv2/videoio.hpp

@@ -782,9 +782,18 @@ public:
      */
     CV_WRAP String getBackendName() const;
 
+    /** Switches exceptions mode
+     *
+     * methods raise exceptions if not successful instead of returning an error code
+     */
+    CV_WRAP void setExceptionMode(bool enable) { throwOnFail = enable; }
+
+    /// query if exception mode is active
+    CV_WRAP bool getExceptionMode() { return throwOnFail; }
 protected:
     Ptr<CvCapture> cap;
     Ptr<IVideoCapture> icap;
+    bool throwOnFail;
 };
 
 class IVideoWriter;

modules/videoio/src/cap.cpp

@@ -55,16 +55,16 @@ void DefaultDeleter<CvCapture>::operator ()(CvCapture* obj) const { cvReleaseCap
 void DefaultDeleter<CvVideoWriter>::operator ()(CvVideoWriter* obj) const { cvReleaseVideoWriter(&obj); }
 
 
-VideoCapture::VideoCapture()
+VideoCapture::VideoCapture() : throwOnFail(false)
 {}
 
-VideoCapture::VideoCapture(const String& filename, int apiPreference)
+VideoCapture::VideoCapture(const String& filename, int apiPreference) : throwOnFail(false)
 {
     CV_TRACE_FUNCTION();
     open(filename, apiPreference);
 }
 
-VideoCapture::VideoCapture(int index, int apiPreference)
+VideoCapture::VideoCapture(int index, int apiPreference) : throwOnFail(false)
 {
     CV_TRACE_FUNCTION();
     open(index, apiPreference);
@@ -112,10 +112,13 @@ bool VideoCapture::open(const String& filename, int apiPreference)
                             CV_LOG_WARNING(NULL, cv::format("VIDEOIO(%s): can't create capture", info.name));
                     }
                 } catch(const cv::Exception& e) {
+                    if(throwOnFail && apiPreference != CAP_ANY) throw;
                     CV_LOG_ERROR(NULL, cv::format("VIDEOIO(%s): raised OpenCV exception:\n\n%s\n", info.name, e.what()));
                 } catch (const std::exception& e) {
+                    if(throwOnFail && apiPreference != CAP_ANY) throw;
                     CV_LOG_ERROR(NULL, cv::format("VIDEOIO(%s): raised C++ exception:\n\n%s\n", info.name, e.what()));
                 } catch(...) {
+                    if(throwOnFail && apiPreference != CAP_ANY) throw;
                     CV_LOG_ERROR(NULL, cv::format("VIDEOIO(%s): raised unknown C++ exception!\n\n", info.name));
                 }
             }
@@ -126,6 +129,10 @@ bool VideoCapture::open(const String& filename, int apiPreference)
             }
         }
     }
+
+    if (throwOnFail)
+        CV_Error_(Error::StsError, ("could not open '%s'", filename.c_str()));
+
     return false;
 }
 
@@ -176,10 +183,13 @@ bool  VideoCapture::open(int cameraNum, int apiPreference)
                             CV_LOG_WARNING(NULL, cv::format("VIDEOIO(%s): can't create capture", info.name));
                     }
                 } catch(const cv::Exception& e) {
+                    if(throwOnFail && apiPreference != CAP_ANY) throw;
                     CV_LOG_ERROR(NULL, cv::format("VIDEOIO(%s): raised OpenCV exception:\n\n%s\n", info.name, e.what()));
                 } catch (const std::exception& e) {
+                    if(throwOnFail && apiPreference != CAP_ANY) throw;
                     CV_LOG_ERROR(NULL, cv::format("VIDEOIO(%s): raised C++ exception:\n\n%s\n", info.name, e.what()));
                 } catch(...) {
+                    if(throwOnFail && apiPreference != CAP_ANY) throw;
                     CV_LOG_ERROR(NULL, cv::format("VIDEOIO(%s): raised unknown C++ exception!\n\n", info.name));
                 }
             }
@@ -190,6 +200,10 @@ bool  VideoCapture::open(int cameraNum, int apiPreference)
             }
         }
     }
+
+    if(throwOnFail)
+        CV_Error_(Error::StsError, ("could not open camera %d", cameraNum));
+
     return false;
 }
 
@@ -216,15 +230,22 @@ void VideoCapture::release()
 bool VideoCapture::grab()
 {
     CV_INSTRUMENT_REGION();
-    return !icap.empty() ? icap->grabFrame() : false;
+    bool ret = !icap.empty() ? icap->grabFrame() : false;
+    if (!ret && throwOnFail)
+        CV_Error(Error::StsError, "");
+    return ret;
 }
 
 bool VideoCapture::retrieve(OutputArray image, int channel)
 {
     CV_INSTRUMENT_REGION();
+
+    bool ret = false;
     if (!icap.empty())
-        return icap->retrieveFrame(channel, image);
+        ret = icap->retrieveFrame(channel, image);
-    return false;
+    if (!ret && throwOnFail)
+        CV_Error_(Error::StsError, ("could not retrieve channel %d", channel));
+    return ret;
 }
 
 bool VideoCapture::read(OutputArray image)
@@ -277,7 +298,10 @@ VideoCapture& VideoCapture::operator >> (UMat& image)
 bool VideoCapture::set(int propId, double value)
 {
     CV_CheckNE(propId, (int)CAP_PROP_BACKEND, "Can't set read-only property");
-    return !icap.empty() ? icap->setProperty(propId, value) : false;
+    bool ret = !icap.empty() ? icap->setProperty(propId, value) : false;
+    if (!ret && throwOnFail)
+        CV_Error_(Error::StsError, ("could not set prop %d = %f", propId, value));
+    return ret;
 }
 
 double VideoCapture::get(int propId) const

modules/videoio/test/test_video_io.cpp

@@ -416,4 +416,23 @@ INSTANTIATE_TEST_CASE_P(videoio, videoio_synthetic,
                             testing::ValuesIn(all_sizes),
                             testing::ValuesIn(synthetic_params)));
 
+TEST(Videoio, exceptions)
+{
+    VideoCapture cap;
+
+    Mat mat;
+
+    EXPECT_FALSE(cap.grab());
+    EXPECT_FALSE(cap.retrieve(mat));
+    EXPECT_FALSE(cap.set(CAP_PROP_POS_FRAMES, 1));
+    EXPECT_FALSE(cap.open("this_does_not_exist.avi", CAP_OPENCV_MJPEG));
+
+    cap.setExceptionMode(true);
+
+    EXPECT_THROW(cap.grab(), Exception);
+    EXPECT_THROW(cap.retrieve(mat), Exception);
+    EXPECT_THROW(cap.set(CAP_PROP_POS_FRAMES, 1), Exception);
+    EXPECT_THROW(cap.open("this_does_not_exist.avi", CAP_OPENCV_MJPEG), Exception);
+}
+
 } // namespace