Log a debug message if a capture backend is generally available but isn't capabable of a capture mode.

modules/videoio/src/cap.cpp

@@ -196,6 +196,17 @@ bool VideoCapture::open(const String& filename, int apiPreference, const std::ve
         }
     }
 
+    if(apiPreference != CAP_ANY)
+    {
+        bool found = cv::videoio_registry::isBackendBuiltIn(static_cast<VideoCaptureAPIs>(apiPreference));
+        if (found)
+        {
+            CV_LOG_WARNING(NULL, cv::format("VIDEOIO(%s): backend is generally available "
+                                            "but can't be used to capture by name",
+                                            cv::videoio_registry::getBackendName(static_cast<VideoCaptureAPIs>(apiPreference)).c_str()));
+        }
+    }
+
     if (throwOnFail)
     {
         CV_Error_(Error::StsError, ("could not open '%s'", filename.c_str()));
@@ -310,6 +321,17 @@ bool VideoCapture::open(int cameraNum, int apiPreference, const std::vector<int>
         }
     }
 
+    if(apiPreference != CAP_ANY)
+    {
+        bool found = cv::videoio_registry::isBackendBuiltIn(static_cast<VideoCaptureAPIs>(apiPreference));
+        if (found)
+        {
+            CV_LOG_WARNING(NULL, cv::format("VIDEOIO(%s): backend is generally available "
+                                            "but can't be used to capture by index",
+                                            cv::videoio_registry::getBackendName(static_cast<VideoCaptureAPIs>(apiPreference)).c_str()));
+        }
+    }
+
     if (throwOnFail)
     {
         CV_Error_(Error::StsError, ("could not open camera %d", cameraNum));